Houston, T exas: Johnson Space Center where astronauts train, testing of ...... If we call the hemispherical radius of the crater R, we let the height needed to ...
GEOLOGY OF METEORITE IMPACTS
by Michael Issigonis
COURSE OUTLINE Ch.1 Introduction
Ch.2 Detection of Asteroids in Space Ch.3 Inventory of World Meteorite Craters Ch.4 How to identify a Meteorite Impact Ch.5 Mechanics of Impacts Ch.6 Classification of Terrestrial Meteorites
Ch.7 Analytical Data Derived from Meteorites / Comets Ch.8 Meteorites and Extinctions Ch.9 Meteorites and Craters in Manitoba and Canada Ch.10 Survey of Notable World Craters and Meteorites Ch.11 Threat of Asteroids / Comets in the Future
MATERIALS • Meteorites: ed. Zanda-Rotaru, Cambridge Univ. Press, 2001 • Impact Structures in Canada, Grieve, GAC, 2006 with Reference Texts - Field Guide to Meteors & Meteorites, Norton- Chitwood, Springer, 2008 - Meteorites, Bevan-De Laeter, Smithsonian Inst. Press, 2002 - Plus several videos, numerous pictures & rock samples
CHAPTER
1
INTRODUCTION
--- A UBIQUITOUS GEOLOGICAL PROCESS - dominant landform on the moon
--- THE TERRESTRIAL CRATERING RECORD
--- SPATIAL, TEMPORAL, SIZE DISTRIBUTION
--- CRITERIA NEEDED TO IDENTIFY IMPACT STRUCTURES - crater, breccias, melt rocks, shatter cones, planar deformation features, etc
THE GEOLOGICAL RECORD OF METEORITE IMPACTS - 100 years on - suspected earth impact sites (SEIS) - recognition of meteorite impact craters
THE CATACLYSMIC LATE HEAVY BOMBARDMENT (LHB) - ~ 700 m.y. after the planets formed - planetesimal disk destabilized - a mixture of comet and asteroid impactors
ORBITS OF METEORITES - only 5 orbits known so far
Now recognized as a ubiquit ous geological process that affects all planetary objects with a solid surface. We see it as a dominant landform on the Moon. On Earth, however, erosion, volcanic and tect onic activities are continually erasing impact craters from the rock record. Despite this, 174 confirmed impact st ructures have been documented with several new sites recognized each year. Several criteria may be used to identify hypervelocity impact structures, including the presence of a crater form and/or unusual rocks such as breccias, melt rocks and pseudotachylyte. However, these are not definitive evidence of a meteorite impact structure. There should also be evidence for hypervelocity impact in the form of shock metamorphic indicat ors, either megascopic (shat ter cones) or microscopic (planar deformation feat ures ) and the presence of high-pressure polymorphs (coesite, stishovite). Unfortunately, this requires investigation and preservation of suitable rock. However, this is not often possible due t o erosion, even though there is strong evidence for an impact origin.
The impact cratering record is incomplete. The majority of impact struct ures occur on land and there are 27 marine impact structures, of which 7 are still currently in the marine environment. There is a clear bias in the ages of terrestrial impact structures with over half being less than 200 Ma.Is the cratering record falling off smoothly since the end of the Late Heavy Bombardment or are there periods of enhanced flux? However, it is interesting t o not e the large number of Ordovician craters and an increased flux during the late Eocene (one around to 35 Ma the other 39 Ma). As for morphology craters are subdivided into simple and complex. Simple craters comprise a bowl-shaped depression that is similar in shape t o the initial transient cavity. Complex craters generally have a struct urally complicated rim, a down-faulted annular trough and an uplifted central area. The features form as a result of gravitational adjustments of the initial crater during the modification stage of impact crater formation. The transition from simple to complex in a diameter range from 3 to 5 km with struct ures in sedimentary rocks being relatively smaller than those in crystalline rocks. The study of meteorit e impact structures has progressed substantially during the past century since D. Barringer proposed the impact origin of Meteor Crater, Arizona.However, there are still significant gaps within geological record of meteorit e impacts.
The prospect of finding new ones is considered low. The cataclysmic Late Heavy Bombardment period The petrology record on the Moon suggests that a cataclysmic spike in the cratering rate occurred about 700 million years after the planets formed. Several models have been proposed t o explain a late impact spike. One of them involves a rapid migration of the giant planets which occurred after a long quiescent period. During this burst of migration the planetesimal disk outside the orbits of the planets was destabilized causing a sudden massive delivery of planetesimals t o the inner Solar System. The asteroid belt was also strongly perturbed with these objects supplying a significant fraction of the LHB impact ors in accordance with recent geochemical evidence. The passage of only 6 meteorites through the atmosphere has been recorded phot ographically by cameras at more than one place. The accurate plot ting of the orbits can also allow the likely area of fall on the ground t o be predicted.
MODERN IMPACT EVENT S T unguska, 1908 An airburst , killed 80 million trees,
2,150 km2 flat tened forest
OBSERVED Sikhote Ğ Alin, P rimorye, Siberia, 1947 Revelst oke Fireball, 1965, BC Fireball over Dubbo, Australia, 1993 ?
OBSERVED WITH AUT OMAT ED CAMERAS & RECOVERED FOLLOWING CALCULAT ION OF IMPACT POINT P ribram meteorite, Czech R., 1959 Lost City chondrit e, USA Innisfree, 1977 Neuschwanstein, Germany, 2002 HUMANS HIT BY MET EOR Alabama, 1954 4 kg hit Ann Hodges, bounced off her radio
FAMOUS FLY-BYs Aug. 1972 :over Wyoming, Alberta
higher than 60 km
grazed for 100 seconds
RECENT FALLS Jan. 18, 2000 meteor 4.6 m
180 t ons
June 7, 2007 Norway Fireball Between 1975 Ğ 1992 : Missile Warning System picked up 136 major explosions in the upper atmosphere with 300 flashes caused by 1 m to 10 m sized meteors
RECENT
P RE- HIST ORIC
IMPACT EVENT S
Barringer Rio Cuatro craters, at a very low angle, ~ 10,000 years ago Henbury crater, Australia 4,000 Ğ 5,000 years ago
also Kaali, Est onia by objects that broke up before impact
Wabar craters, a few hundred years ago Off Madagascar:evidence points t o a spot in the middle of the Indian Ocean, Burckle crater 18 miles big lies 12,500 ft below surface. 4,000 Ğ 5,000 years ago it produced a tsunami 600 ft high
NEAR MISSES AND FUT URE T HREAT S May 1996 : 400 m
1997 JAI
Summer of 2001 : over Mediterranean detected a flash of energy similar to a nuclear weapon Mar. 2004: 30 m
2004 FH
2004 Apophis (previously known as 2004 MN4) had highest probability for Fri. 13th, Apr. 2029, then 13-14 Apr. 2036 2004 VD17, 580 m 4 May 2102 1950 DA
1.1 km
Mar. 16, 2080
Known orbits of fallen meteorites
VIDEO
KILLERS IN SPACE
Arecibo, P uerto Rico : biggest radio-telescope dish. Like a radar gun Measures distance t o asteroids Gets information about the size and speed of objects in space First asteroid identified in P alermo, Italy in 1801, Jan. 1 T elescope is the Rumsden Circle : very accurate for its day Astronomer (Piazzi) thought it was a new star. But next night it had moved. So, it wasnÕt He found the missing planet between Mars and Jupiter : Ceres ( Demetra) All asteroids could form a planet 4 X the size of the Earth Craters on the moon were thought t o be extinct volcanoes: in fact there are more than 300,000impacts on the moon Therefore, the Earth has been hit as well Gravity from Jupiter threw asteroids away to orbit the sun, in this way made to be earthcrossers Erosion has obliterated evidence of impacts, except in deserts or frozen deserts Meteor crater was positively identified in the 1950Õs Asteroid 30 m big exploded with a 20 Mt impact Sud bury: a huge impact crat er by an asteroid 10-20 km big altered the crust down t o the mantle. Numerous shatter cones Shat ter waves over the entire planet, crater walls over a mile high, molten rock rushed to surface from the core. Devastating t o life No plan t o deflect approaching asteroids Jay Melosh: focus sunlight on rock to vaporize it or mine a small piece of it and throw it away, changing its orbit in the process NASAÕs mission t o Eros (Bob Fakua) launched in 1996 and landed in 2001. More than 1 million images of the asteroid Comets : Huakutake, 1996, Haley-Bob in 1997 1066 T apes try show Halleys comet First space probe went to the comet in 1996 : First picture of the nucleus so f a comet Comets are un-contaminated since the formation of the solar system Comet started its trip 2 million years ago. 1 million years ago it reached Neptune, still invisible. By the time it reached Jupiter vaporization had started, cracks appearedas it was hit by the solar wind. Stretched int o a tail millions of miles long Space dust falls down everyday, we eat it breath it Ingredients of life brought to Earth by cosmic dust . It has been collected in Antarctica Observed by electron microscope. Dust is rich in carbon with aminoacids The Stardust project will collect cosmic dust from the tail of the comet Early earth was fragile but rich in hydrothermal fluids
VIDEO
AST EROIDS
T unguska explosion with shock waves. Object from 50 to 100 m big (could have levelled London) Near Earth Objects (UK) T ask Force 1980 :Iridium discovered by Alvarez in a unique clay layer around the world dated at 65 m.y. New theory about the demise of the dinosaurs. Lat er, crater was discovered with gravity anomaly, etc Most of the crater is underwater Hilderbrand (Calgary) : the shock wave st opped 30 km inside the earth. Plume rose 10,000 km up in the air & the forests burned. The rock was the size of Mt . Everest Houst on, T exas: Johnson Space Center where ast ronauts train, testing of rockets, etc. Diaz tests space shut tle that was hit by micro-met eorites New Mexico desert: telescopes part of the Star Wars missile, LINEAR system t o find asteroids : one can find 10,000Ğ 12,000 objects in 1 night . T akepictures of the sky Data sent to Marsden, Harvard University, Cambridge, Mass. for initial processing. He will decide if there are any FAST MOVERS. This is the first preliminary orbit calculation The fast movers are sent to NASAÓsP asadena, Calif. Lab where the Orbital Mechanics T eam under P aul Chodas will make a detailed orbit animation. Chodas predicted the 1994 comet collision with Jupiter in very precise manner. Also, Hubble aimed at the collision, so we have amazing pictures of the impacts
1997XF11 : First seen in Arizona. Marsden sees a problem in Oct. 2028, especially for Europe, but Chodas said no problem even then. It is 1 mile large asteroid and was seen during the 1998 fly-by NASA was t old in 1998 to find all asteroids in 10 years Steve Ostra : Radar telescopes in Arizona (power of 400,000 wat ts) Toutatis, has a wobbly spin 4.3 km a eD model has been made Cleopatra: like a dog-bone metal 1950 DA 1.1 km like a big rock. Most dangerous for 2080. Computer model of impact off NewYork creating a monster tsunami Castalea: made up of 2 rocks held t ogether by weak gravity. A blast on Castalea would not affect it !!! (from experiments) Dave Ostra: radar makes pictures of asteroids & calculates their distance
VIDEO
KILLER ASTEROIDS
SCIENCE VERSU S EXT INCT ION
T agish lake, Jan. 2000 near Atlin, BC 8.30 am Big noise, incredible white light, second light, fireball, ashes running down, felt like a bomb, brilliant flash, luminescent cloud (thought airplane blown up) 200 t on asteroid exploded in the air if it hit the earth, a nuclear bomb. Disintegrated in the air We thought we were safe 2001 over the P acific, 2002 over the Mediterranean David Levy saw a genuine threat . Look at the moon. It has the record On earth, erosion has obliterated traces of impact s Craters thought to be volcanoes Skeptics changed their minds 1994:First time in civilization to witness a collision (most destructive event) Now earth is vulnerable After 10 years of searching, NASA found one with our name on it 1950 DA enemy # 1 2080 Energy of 100,000 MT (10 Mt is tremendous) Up t o 600 of that size remain undiscovered. Enormous task Jay Melosh: part of the team Movies ignore the scale Weapons will make it worse, a cluster bomb! Find a bet ter way, such as vaporizing it Det onate weapon next to it t o deflect its course ( a stand-off blast)
T agish lake: Jim Brook saw dark objects in the frozen lake, thought they were animal droppings first , later, he knew were meteorites, never t ouched them. Very light like charcoal, low density (full of air) The lightest meteorite ever found Not all meteorites are heavy metal or rock T est black meteorites with bullets. Then test T agish lake: nothing happened!! Porous meteorites absorb the force. CanÕt be deflected Spin of asteroids: Fast if solid, slow if porous 1950 DA spins very fast. Should be diverted by st and off blast many are slow spinners Sun: huge supplier of energy in space Giant magnifying glass t o vaporize meteors A solar collect or vaporizes rock on surface, jets of energy will change its course Hostile reception of the idea Collect ors already making such equipment in orbit How about comets ? Hale-Bopp. A long period comet donÕt show until they arrive at the sun and show-off their tail. A rare event
CHAPTER
2
Detection of Asteroids in Space
THE FIRST ASTEROIDS
--- Bode’s Law --- Gap between Mars & Jupiter --- Jan. 1, 1801 : First mini planet Ceres 910 km across, then Pallas, Juno, Vesta�.
--- Many, many more …. called Minor planets TOO CLOSE FOR COMFORT --- Laplace (black holes)
--- Georges Cuvier --- Count Buffon --- First near-earth asteroid in 1873 : Eros
--- Amor
DET ECTION 1801:First asteroid detected : Ceres Methods in use: radio telescopes using radar LINEAR program in New Mexico IRAS satellite (Infra-red), Univ. of Arizona, Spacewatch survey Satellite Hipparchos Hubble Space T elescope Eros landing, 2001 Individuals with backyard telescopes (one in North Dakota) KeplerÕs Laws Orbits are elliptical Velocity depends on distance from the sun Still have some relationship t o Jupiter ( 3 revolutions of asteroids t o 2 revolutions of Jupiter : 2/3 resonance) 20 asteroid families : from fragmentation of larger ones if collided, ejected away from belt or t owards the sun if at tracted by the earth (largest planet in the Inner solar system) become a N.E.A.s P rimary interest on asteroids
if > 140 m (Barringer was 40 m ) 1 km can wipe out most life
AST EROIDS Today, 378,546 registered with t otal mass = 4 X our Moon 160,508 have known orbits 13,889 have official names there are an estimated 1.1 Ğ 1.9 million asteroids with size > 1 km VISIBLE to the naked eye : just Apophis, Vesta, maybe P allas CLASSI FICATION was based on spectra, but it is not accurate Carbonaceous C-type are ~ 75 % Silicaceous
S-type
~ 17 %
Metallic
M-type
~ 8%
LINEAR system has discovered 85,000asteroids, 4,711 N.E.A.s (600 have > 1 km) Eros, Apollo, Adonis, Hermes, Amor have already missed Earth once Most names from Greek mythology Some names non-mythological (Mr. Spock, from a cat, James Bond, Misterogers, et c) Comets are named after the discoverer (not asteroids, or small meteors) During last 600 m.y. Earth was hit by 60 objects with diameter > 5 km The smallest would produce 10 million Mt of TNT and leave a crater 95 km across
Cross-section of Solar System showing density of orbits of known asteroids ( Earth is in the red zone)
EARTH-CROSSING ASTEROIDS -- First asteroid known : Apollo --- Adonis, Hermes, etc.
--- Film exposure for at least an hour --- Waiting for us to find them before they find us ! --- Probable number of asteroids --- A 1.6 km big will release a million megatons of TNT
The map of orbits of known near-earth asteroids as of year 2000
COMETS ALL OVER Asteroids have orbits of a few years around the sun, but comets into thousands and millions of years around the sun Halley has an orbit of 76 years Hale Bopp about 4,000 years To look for asteroids point your telescope close to the planetary plane because that is where most of the potential targets lie
But to look for comets point to any part of the sky
VIDEO
MENACE FROM T HE SKY
(UNIVERSE Ğ METEORITES)
Galileo : picture of asteroid in 1993 Aida Killer asteroids coming Tootatis 1992 missing by 2.2 million miles Meteor crater, Barringer Yucatan, 180 km crater environmental catastrophy 65 m.y. Dinosaus had been alive for 150 m.y. Utah : Dino Nat. Monument : Too many fossils (Graveyard), thousands, 11 species Plant & predat or dinosaurs Abundant forests Extinction : big mystery also 70 % of of life asteroid or volcano Italy, Denmark : thin clay bombardment with neutrons, rich in iridium, origin in space N. Mexico : only under microscope, shocked quartz, shock doomsday 10 km 180 km on fire Smoke, dust , vapor, temp. drops, no plants, froze to death. Then, CO2 heat wave killed plankt on Small meteor in T unguska Supernova solar nebula solid bodies Sun with nuclear reactions, solar wind, through meteorite impacts, planets increased in size 9 planets Differentiated, non-differentiated (chondrites): most abundant, molten droplets, drops of fiery rain : never melted, info. On birth of solar system High Ğ P minerals Lab can simulat e impacts 1922 T ut t omb had 143 objects, dagger with meteorite iron ( 4,000 yrears old)
CeasarÕs death announced by a meteorite Birth of Jesus : Jupiter + Saturn t ogether Comets were bad omen Halley ridiculed, 1986 : 5 spacecraft went t o meet it. First image of nucleus Extinctions every 26 m.y. Nemsisi star ? Austria 1932 : fireball, bang SW Africa 80,000 years ago, fireball, thunder terrorized animals Hoba, Namibia : no crater, also Gibern: broken up, largest shower Total 21 t ons P rairies : Anasazi worshipped meteorite. Also in their drawings 1492 :Einishein : oldest observed still in Town Hall loud bang, 280 lbs, 2 witnessed the fall sign of God, Maximilian against the French Goethe owned 2 pieces Quarry for church (St. GeorgeÕs) at Reis : impact glass
15 m.y. ago
No miracles but Chladni : first father of research 1803 France , real event U. of P enn : Accelerat or, measure rare isot opes inside meteorites Studies : fragmentation, how long has been sit ting on earth, where it formed in space T race elements in P. T and Temp during early history (parent isot opes) Vienna : largest meteorit e museum
5,000 pieces
Schribe-it e mineral
Java meteorite from T emple of Borobudur (ONTONG) daggers that Òfly through the air Ò
ÒHeavenly IronÓ: used for
Mecca Ğ Kaaba Copenhagen : collection 20 t ons from 200 t ons broke up into , 58 t ons recovered 3,000 years ago Cap York Group Aniguit o, ÒThe T ent Ó : t ook 4 years to bring it t o NY. Largest piece is 34 t ons Antarctica : Japanese discovered in 1970 thousands discovered ÒBlue IcefieldsÓ Johnson Space Center : kept in lab under special conditions From Mars meteors contain same mixture of gases Also, young in age, fits Mars very well Sur face data indicates only recently st opped geological activity Lunar fragment in Japan : Lunar seas 17 Apollo missions but only 6 manned missions Religion : Shint oism, many gods Oldest observed meteorite : 861 AD fell in the templeÕs floor Forgot ten, surfaced 1,000 years later ! 1979 a priest allowed t o be examined (Nogata) Fukuoka micro-meteorites 1990 : SIC, pre-solar, diamonds
Ch.2
VOYAGE T O T HE GIANT ASTEROIDS : lift off Sept.27, 2007
Pieces of Vesta have fallen to earth as meteorites Much is still unknown about the solar systemÕs beginnings Dawn spacecraft intends t o fly t o 2 giant asteroids, Ceres and Vesta and explore them closely Spacecraft receives power from its massive solar arrays Fist st op is Vesta: asteroid that may implicate ancient supernovas in the solar systemÕs birth Maybe it was partially molten early in its hist ory, allowing heavy elements like iron to sink and form a dense core with a lighter crust on top Melting requires a source of heat such as gravitational energy released when materials come t ogether to make an asteroid But Vesta is a small world, only 530 km across There would have been not enough gravitational energy t o melt the asteroid when it formed A supernova would provide the explanation: when Vesta formed it was Òspiced upÓby Aluminum Ğ 26 and iron Ğ 60 created in possibly 2 supernovas that exploded around the time of the solar systemÕs birth These forms of iron & aluminum are radioactive isotopes that could have provided the extra heat needed t o melt Vesta
This would explain why VestaÕs surface appears to bear the marks of ancient basaltic lava flows and magma oceans, much as the Moon does If chunks of rock the size of Vesta could melt and form dense cores, it would affect the way planets and their cores grew and evolved Dawn will orbit Vesta for 7 months in 2011 VestaÕs gravitational field would be mapped as the probe orbits the asteroid This would set tle whether Vesta has an iron core On t o Ceres Dawn will at tempt to leave the orbit of one distant body and fly out and orbit another This has not be done before Asteroid hopping would be impossible if it used conventional rocket fuel It uses ion propulsion which requires 1 tenth as much propellant Dawn will arrive in 2015 950 km in diameter: by far the largest body in the asteroid belt not a rocky world but covered in water ice it appearst o harbor a layer of ice 60 t o 120 km thick surface probably has changed a lot over time ice may be obscuring its earlier hist ory it would teach scientist the role of water has played since it started why some worlds like Earth and Ceres have large amounts of water while others like Vesta are bone dry? Vesta will tell us about the earliest epoch and Ceres about what happened later
CHAPTER
3
Inventory of World Meteorite Craters
WWW UNB.CA/PASSC/IMPACT DATABASE/CIDIAMETERSORT2.HTM maintained by Planetary Science Centre, UNB sorted by Age / Diameter / Name - Maps of all continents
CHAPTER
4
How to Identify a Meteorite Impact
6-page Abstract
Ch.4
HOW TO IDENT IFY A METEORIT E IMPACT
IMPACT S Major process in origin & evolution of all parts of the solar system Impacts actually created the planets. Bit by bit EFFECTS Shock metamorphic effects produced in rocks & minerals due t o intense shock waves About 174 on surface, others buried inside the Earth, many others eroded alt ogether with weathering Many contain resources, such as Ni (Sudbury), diamonds (Popigai), building st one (Reis), gypsum (St . Martin), oil / gas (N. Dakota) P rinciple: older surfaces accumulate more crat ers (Shoemaker) example the Moon
Impact events, especially, large ones, have had a major role in the formation & early hist ory of the solar system Formation of planetesimals in original solar nebula Intense bombardment until about 3.5 b.y. Mars-sized body collided with Prot o-Earth at ! 4.5 b.y. t o form the moon Impact events are different than other geological processes We donÕt know much about them, scientist & the public alike Lit tle experience: 1994 comet on Jupiter
IMMENSE ENERGY released from the kinetic energy of a bolide (=1/2 mv2) Earthquakes & volcanoes have upper limits, impacts donÕt INSTANT EFFECT S Kinetic energy converted to high pressure shock wave through rocks at km / sec Shat tered, deformed, melted, vaporized in a few sec Barringer crater formed in seconds
Sud bury crater formed in less than 10 minutes with subsequent adjustments for years after A small impact forms a big crater Near-surface release of energy transfers into the biosphere causing extinctions. Some is converted into heat , vaporizes target rocks cooling the air Impact velocities going through t arget rocks faster than the speed of sound (shock waves) causing tremendous stresses in the rocks Shock-produced temperatures exceed 2,000Õ C causing melting
UNIQUE METAMORPHICEFFECTS Shock-metamorphic effects are unique for impacts Remains of original meteorite is only found around ~ 12 craters Barringer is the largest of this kind Absence of original meteorite in the rest suggests that it t oo is subjected to intense shock waves, melted and vaporized
HOW T O FIND Circular structure in t opography or geology (lakes, hills central peak) Places of unusual volcanic activity By air, satellite Verification by shock metamorphic effects and geochemistry Unusual rocks of impact breccias or melt rocks Gravity anomaly compared t o surrounding rocks Shat ter cones P DFs in quart z and feldspar (Planar deformation features) High iridium Coesite and stishovite Diaplectic glass in feldspar T ektites (glass) GEOCHEMIST RY of melt will indicate composition of original meteorite
HYPERVELOCITY IMPACTS
Instantaneous transfer of the considerable kinetic energy craterform internal energy leading to shock metamorpshism Not amenable to exact duplication by experiment Post-impact endogenic geologic processes Ries coesite also at the same time Meteor crater, Arizona Until the so-called shock metamorphic effects were recognized, then become more acceptable shatter cones
40 % have been dated isotopically insufficient shock pressures and post-shock temperatures to significantly disturb isotopic dating systems K-Ar or 40Ar Ğ39 Ar U-Pb dates from shocked zircons Diameters less than approximately a km formed by iron or stony-irons
MORPHOLOGY mathematical relationships between apparent depth, true depth, diameter & structural uplift for simple and complex structures
Upraised rim
da = 0.13 D and Dt = 0.28 D Moon : da = 0.196 D Simple structures up t o a diameter of ~ 4 km Structural uplift (SU) SU = 0.1 D for complex structures da = 0.12 D and da = 0.15 D for sedimentary & crystalline targets respectively data from 44 complex impact structures D su = 0.31 D Multi-rings only at Chicxulub, Sud bury, & Vredefort, also the moon
FORMATION OF IMPACT STRUCTURES
Kinetic energy, shock wave, cratering flow-field in the target, ejection, excavation stage,
transient cavity, modification & collapse Simple & complex structures Sedimentary & crystalline targets Central peak and ring formation in complex structures
Kinetic energy shock metamorphism Shock wave vaporization & melting meteor ceases t o exist as a physical entity Supersonic speed compresses the target rocks downward & outward in a hemispherical direction seismic wave cratering flow-field in the target upwards & outwards, leading t o the ejection of target material end of the excavation stage transient cavity partly by displacement and partly by the ejection of target materials With a depth that approximates 1/3 of the diameter Represents the limit of motions T ransient cavity is not stable geometric entity and in all but the smallest of impact events, it rapidly undergoes modification to a more stable landform Are mixed with the collapsing wall rocks t o form am interior breccia lens A mix of shocked including melted and unshocked material A depth of approximately half that of the original transient cavity depth Complex considerably more modification
Sedimentary cover overlying crystalline basement, the sedimentary cover rocks were preserved in the annular troughs Additional processes were occurring at complex impact structures compared to simple impact structures Deepening transient cavity Structural uplift displacement was initially downwards and outwards, during cavity formation, and then upwards and inwards during transient cavity modification This structurally uplifted material came from a maximum depth of approximately one / tenth 1/10 th of the final rim diameter at complex impact structures T ransient cavity diameter of complex impact structures is some 0.5 Ğ 0.65 of the modified final rim diameter The deepest non-excavated material of the struct ural uplift at an original depth of 1/5 to 1/6 of the estimated diameter of the transient cavity A diameter of excavation of ~ 10 km at Haught on depth Ğ diameter ratio of 1/6 for the portion of the transient cavity that was due t o excavation
Acoustic fluidization Sedimentary targets evidence of thrusting and faulting in the structural uplift Reflection seismic data over structural uplifts indicate a loss of coherent reflections and a reduction in seismic velocity Discrete blocks in the struct ural uplift Central peak max. of ~ 2 km Ring formation being an extension of the process of structural uplift in complex impact structures A central structural uplift with lithologies progressively decreasing in age outwards Sud bury: post-impact tect onic movements Discrete block rotations in the uplifted centre of Vredefort Lubrication by the pervasive network of pseudotachylite veins An over-heightened central peak and its subsequent collapse at very large impact events Chicxulub is buried by ~1 km of post-impact sediments As event size increases so-called cratering efficiency decreases Gravity is an acceleration
SUMMARY GEOLOGY OF IMPACT STRUCTURES: SHOCK METAMORPHISM The burden of proof lies in the shock metamorphic effects THE FORMATION OF SHATTER CONES Mechanism for the formation of shatter cones Shock waves in spherical symmetry Description of the model Structure of shatter cones Aggregates of multiple shatter cones
Shocked quartz
SUB-SOLIDUS SHOCK EFFECTS Planar fractures (PFs) and Planar Deformation Features (PDFs) Relationship with pressure
FORMAT ION OF SHATT ER CONES The conical fractures initiated after the passage of the main plastic compression pulse, not before A restricted range of pressure allowing the formation of shat ter cones From a few cm t o 12 meters The shock front is scat tered by a heterogeneity in the rock Are tensional fractures Conical striated fracture surfaces Ridge and groove striations diverging from an apex Blast fractures
percussion marks
Wind abrasion fractures would be restricted t o surface only Planar deformation features (P DFs) in the vicinity of the cone surfaces Also spherules (vapor condensates) Deformed rocks characterized by hierarchial st riated features
Shat ter cone striations, nonlinear waves (front waves) that propagate along a fracture front The striation angles increase with the distance from the impact Bifurcations of the fracture front Are close t o impact P ropagate at nearly the Rayleigh wave speed of the host rocks Furthest cones were 40 km from the impact Beyond the near-impact region, rock evaporation & melting prevail, shock induced structures are dominant Shat ter cones only at large impact sites Horse-tail structures
Characteristic striations are observed on nearly planar surfaces Rarity of complet e cones Indiana: carbonate and clastic rocks Cone axes fractures
multiple curved, striated surfaces
parent fracture + secondary branched
Radially propagating shock waves produce tensile stresses normal t o the compressive (radial) direction Shat ter cone axes generally point t owards the impact center Striated surface features that are the hallmark of shat ter cones : striations are restricted solely t o shat ter cone surfaces
Vredefort: a distance from 14 km t o 37.5 km from its center Well-developed in the fine-grain rocks (slates & quartzites) and less developed in the coarse-grained rocks like granite Striations are formed by front waves Shat ter cones are branched tensile fractures Conic shape is the consequence of the initially curved shape observed in branched fractures that are generated by rapidly propagating fractures Striations are the preserved tracks of the fracture front waves
Eugene Shoemaker
Meteor Crater, Arizona
Shatter cone, Haughton Crater, Nunavut
SUB-SOLIDUS SHOCK EFFECTS Planar fractures (PFs) and Planar Deformation Features (PDFs) Relationship with pressure
Planar micro-structures in quartz relative ease by which they can be documented Planar fractures (PFs) and planar deformation features (PDFs) 5 Ğ 10 GPA to ~ 35 GPa in sedimentary rocks more waste heat is trapped melting 30 to 35 GPa are sufficient to render feldspar and quartz respectively to glasses diaplectic glass
IMPACT MELTING Above 60 GPa rock melts
Melt glasses & Impact melt sheets Diaplectic glass ~ 50 GP a rock melts above 60 GP a waste heat melts whole rock initial 87 Sr / 86 Sr ratio reflect pre-existing target rocks isochron dates for impact event enrichments siderophiles / platinum group / Cr non-siderophiles are basaltic achondrites In sedimentary targets, what is generally described as a polymict , allochthonous breccia deposit occupies the equivalent stratigraphic position t o an impact melt sheet in crystalline targets Bulk of the melt glasses correspond t o the underlying crystalline target rocks It is the occurrence of shocked clasts within these lithologies that designate them as being of impact origin
PSEUDOTACHYLITES Frequently as dikes below the crater floor
DECARBONATION Limestone and dolomite target rocks Expulsion cavities
DISTRIBUTION OF SHOCK METAMORPHISM PDFs in allochthonous breccias, parautochthonous rocks of the crater floor PDFs in quart z as a parameter in creating a shock index for the allochthonous breccia lens at West Hawk Most of the material in the breccia lenses at simple impact structures however does not display obvious shock metamorphic features In the paraut ochthonous rocks on the floor of the impact structure The PDFs ranges from 23 GPa to 5.7 GPs over the 85 m The higher shocked material in the paraut ochthonous rocks in the floor are allochthonous Paraut ochthonous rocks of the crater floor at tenuate from 25 GPa at the center t o 5 Gpa at radial distances of less than half the rim diameter Clearwater : 40 GPa at the top of the struct ural uplift to 10 GPa at 5 km depth
SHOCK METAMORPHISM IN THE STRATIGRAPHIC RECORD
Ejecta can be linked to a known impact site Spherules linked to impact site Tektites & micro-tektites Well-removed ejecta is linked t o a known impact site Ğ Acraman Ejecta from the Chicxulub structure occurs worldwide at the K-T boundary Spherules Brist ol 40 Ar Ğ 39 Ar age of 214 Ma prob. From Manicouagan or Rochechouart T ektite and micro-tektite Chesapeake for the N.American tektites Australasian strewn field over 50 X 106 km 2 unknown source
GEOLOGY OF IMPACT ST RUCT URES:SHOCK METAMORPHISM The geological effects of impact at simple structures are visible t o a depth of ~ 1/3 the final rim diameter The geologic evidence for the largest terrestrial simple impact structures (~ 4 km in diameter) can be removed by ~ 1.5 km of erosion Completely removed by erosion Craterform the burden of proof lies in the shock metamorphic effects By iron or st ony-irons relatively undiminished velocity As a result of the passage of a transient high pressure shock wave Quartz mim. 5 Ğ 10 GPa in seconds
SPHERULE LAYERS : Records of ancient impacts SUMMARY What are spherules & spherule layers How do impact spherules and spherule layers form
How can studies of spherule layers enhance our understanding of terrestrial impacts
Abstract An impact melts and vaporizes silicate materials which can condense int o highly spheroidal, sand-size particles that get deposited hundreds t o thousands of km from the point of impact . These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years t o 3.47 by. Unalt ered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). These layers form very rapidly and can be very extensive, even global in extent, so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context these layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers.
In trodu cti on Impact craters are the most common type of landform in the solar system, so it seems inescapable that earth has been the recipient of a vast number of impact ors. The number of large impacts that are well documented is surprisingly small. Those that are known t o have cause environmental damage is a very small number. This is because the science is new and the effects and the traces they leave behind are still being worked out. The actual list of impacts will never approach the true number of impacts that must have taken place. This is due to the active surface, weathering processes and internal tect onic processes. However, we can learn about impacts whose craters have been obscured or even obliterated by studying the layers of ejecta that inevitably form and can be preserved in sedimentary successions far from the point of impact . Some have been inferred on the basis of ejecta alone, ie the Australasian strewn field. Indeed, the endCretaceous impact was initially identified from ejecta i.e. the Cretaceous-T ertiary boundary layer. The ejecta was not connected to its source, the Chicxulub struct ure until years later.
Impact ejecta take many forms. One key distinction is between proximal and distal ejecta. Any ejecta deposited more than 5 crater radii from the rim is considered di stal. Many layers of distal ejecta are not readily recognized as such in the field. Most contain particles diagnostic of hypervelocity impacts such as crystals with planar deformation features and / or high pressure polymorphs such as coisite and diamond, but these are not readily identified without examining suitably prepared samples under a microscope or via X-ray diffraction analysis. Detecting the minut e amounts of truly extrat errestrial material present in most impact ejecta requires even more elaborate geochemical procedures. However, one type of distal impact ejecta payer can be recognized with just a hand lens. These are distal layers containing impact spherules which are formally molten particles that are highly spheroidal and can occur in great abundance. Where they are well preserved impact spherules can be reliably differentiated from other types of spherical particles such as ovoids and accretionary lapilli in hand sample by the distinctive shapes and internal feat ures. The purposes of this review are t o outline what is currently known about them, then t o speculate on what we might learn about the extrat errestrial objects that hit earth and the nature of the surface they hit by studying spherule layers most extensively. WE hope t o stimulate more researchers t o hunt for spherule layers and accelerate the pace at which they are being discovered.
W h at are sph erule s and sph erule layers Sizes up t o 5 mm Different from volcanic glass : low water content , iron in the Ferrous state, traces of Pt group elements (anomalously high compared to earth rocks) Chemical composition indicates derivation from t errestrial rocks rather than from meteorites. Minerals present are pyroxene, olivine, Ni-rich spinel, K-spar. Usually layer is very thin but extensive and forms an excellent time marker. How do impact sph erule s an d sph erule layers form Ballistically ejected impact-melt droplets and droplets that condensed en route from rock vaporized by the impact. Derived largely from target rocks. However, microtektites have a higher content of extraterrestrial material because they have higher Iridium and other PGEs. Almost all of the Precambrian spherule layers are preserved in marine successions and most were deposited below wave base in relatively deep water. The best known potential Phanerozoic analog for the thicker and coarser P recambrian spherule layers is the K/T boundary layer in the Gulf of Mexico area. Phanerozoic layers approach a thickness on the order of 10 cm.
How can stu di eof s sph erule layers e n h an ec ou r u n derstan di ng of terre stri al impacts Studies of impact structures have taught us much about the record of terrestrial impacts though earthÕs hist ory, but spherule layers can provide additional information some of which is not easily won from the cratering record. The strata enclosing the spherule layers may contain a record of life on earth in the form of skeletal remains or organic biomarkers. Because most of the mass in the spherules comes from terrestrial rocks, this suggests target materials were on average more mafic early in earth hist ory. This could be a natural consequence of the areal expansion off continental crust at the expense of oceanic crust though geologic time. Secular changes in the atmosphere and hydrosphere could also produce observable differences in spherule layers as a function of geologic age. The small number of spherule layers found so far have already extended the terrestrial impact record much further back in time than the one provided by craters. The oldest crater is 2 b.y. old whereas the oldest spherule layer is 3.47 b.y old. Because the area of earthÕs oceanic crust exceeds that of continental crust t oday, large oceanic impacts have outnumbered large continental impacts throughout earth hist ory. Many, if not most , of the Precambrian spherule layers appear t o have been generated by oceanic impacts. Spherules may also give us information about the composition of meteorites throughout earth hist ory because they appearto contain traces of the impactors. Importan t qu e sti on sabout sph erule s / sph erule layers for future re se arch Composition of terrestrial rocks and meteorites throughout earth hist ory A method to determine the distance of spherules from impact
Distribution of spherule layers
CHAPTER
5
Mechanics of Impacts
KINET IC ENERGY con ve rte d i n to digging a crater explosion heat t o melt & vaporize rocks plus meteorit e, + shochwaves through the rocks deformation (shat ter cones) P DFs in quart z / feldspar (shocked crystals) accumulation of impact breccias from broken-up fragments possible mixed with melt rocks (lava-like from meteor plus t arget rocks)
IF FRAGMENTED Each fragmentÕs velocity / mass will start accumulating its kinetic energy after the split (it will not be as much when it lands as the original meteor) Smaller craters will be produced with less of an effect
7-page Abstract
HYPERVELOCITY (greater than speed of sound) COLLISION BETWEEN TWO SOLIDS Produce melting, vaporization (don’t happen in sub-sonic collisions) On earth, lowest impact velocity with an object from space = gravitational escape velocity of 11 km / sec The fastest is more than 70 km / sec
Grav. Escape + escape vel. From the sun at the earth’s orbit + earth’s orbit 11
30
30
= 70 +
average impact velocity = 17 km / sec at these speeds impacts produce shock waves in solid materials both meteor and target are compresses to hjgh density, then rapidly depressurizes exploding violently giving rise to a crater (similar to cratering by high explosives)
since craters are caused by explosion, they are always circular – only very low angle impacts cause elliptical craters (Argentina, Carolina Bays)
Impact process has 3 stages overlapping : initial contact & compression 2. Excavation 3. Modification & collapse CONTACT & COMP RESSION After t ouch down, rear of object moves a significant distance during this short but finit e time Impact or is compressed, density rises & P within rises dramatically- exceeds T P a (T =tera or 10 t o the 12th) Suc h values are found in the interior of planets or generated in nuclear explosions A supersonic shock wave initiates It expands by decelerates & compresses impact or ALSO accelerates & compresses the target
Stress far exceeds strength of solid materials both meteor & t arget are irreversibly damaged High pressure minerals form (coesite) Damage produced by shockwaves raises the temperature of rock, enough to melt the meteor Or (large impacts) vaporize and melt volumes of the target As well as being heated, the target near the impact is accelerated by the shockwave
STANDARD, ONDEP ENDENT OF SIZE, VELOCITY, G, ET C DEP T HOF MAX. EXCAVAT ION = 1/3 T OT ADEPT H 1/3 of volume of transient crater is formed by the ejection of material + 2/3 by displacement of material downwards, out & up t o form the elevated rim in large impacts : rock & meteor may melt, vaporize some of melt rock may be ejected but most remains inside transient crater in contrast, the hot dense vaporized rock expands out of the cavity as this hot vapor cloud expands it rises & cools like a mushroom cloud of nuclear blasts some of the ejected may acquire escape velocity
MODIFICAT ION & COLLAPSE T ransient cavity is not stable Only stable in room experiments It collapses under gravity In craters less than 4 km : simple crater, bowl-shaped Melt breccia, ejecta on the floor In bigger craters , more than 4 km, collapse is driven by gravity Complex craters have uplifted centres, broad flat shallow floors, terraced walls The meteor is under hot elastic rebound, but trying to get t o gravitational equilibrium
MULT I-RING BASINS The very largest also have many more rings Observed at the moon, Callist o There must have been a low-viscosity or low-strength layer below the surface None on Mercury, moon has 9, Venus has 4, Chicxulub is a multi-ring basin, so is Sud bury But the 2,000 km Hellas Basin has not , nor the 1,200 km Argyle Basin They form by a type of collapse qualitatively different from the collapse that yields complex central peak or peak ring craters The transition t o multi-ring scarps does not scale as 1 / g, seems t o depend on the rheological conditions near the surface : a weak sub-surface layer than can flow on the timescale of crater collapse
SUMMARY THE CRATERING PROCESS QUANTIFIED Coesite, stishovite,impactite, suevite, tagamite, etc : transfer of energy Total kinetic energy = 1/2 Mass x velocity 2 Energy of meteorite = volume of rock displaced x rock density x gravity x height of excavation Portion of energy is used to produce the shock wave & some lost as latent heat Calculation of volume of melt rock Relationship between size of meteor, composition of meteor and size of crater STAGES IN THE DEVELOPMENT OF AN IMPACT CRATER 1.
Excavation of a transient cavity
2.
Initially motion is downwards & outwards, then outwards & upward
3.
Collapse of walls
THE C RATERING PRO C ES SQ UANTIFIED In figures, the parameters are: meteorit e radius= km,
density= 4 g/cm3
ch .5 speed= 30 km/sec
Impact-induced mineral assemblages and partially melted breccia are sure signs of these interstellar bombs while impact craters and shat ter cones represent convincing landforms. To gain a basic understanding of the sheer magnit ude and striking spectacle that is a meteorite impact , it may be more effective to focus on simple e n ergy relationships. Coesite discovered in the early 1950Õs in the Meteor Crater, Ariz.: pressure of 700 kb, temp. of 700C-1700ÕC 1987: stishovite found in the same crater; it is an even higher P & T quartz polymorph Tiny diamonds also found with iron : formed during impact, NOT in space Impactite: any rock that forms as a result of melting during impact. Names like suevite and tagamite along with many others distinguish between the chemical composition and degree of melting within these rocks. Impactites are commonly quit e brecciated and phenocrysts have a shat tered appearance. P seudotachylite is a general t erm referring to impact related breccia commonly showing some melting and / or flow text ures. In 1959, nuclear explosions produced a man-made shat ter cone. T ravel through the earthÕs atmosphere is approximately the same
T ravel through the earthÕs atmosphere is approximately the same as travelling through 4 feet of solid rock. When a, Fe-rich meteorit e comes in contact with the earth a fantastic shock wave is produced in both the meteorite and surface rocks. This is referred t o as the compressions stage. During this process the target material is accelerated downward by the shock. Because the tremendous pressures produced within the t arget by the shock wave are adjacent to the outer limit of impact, which is still only subjected t o the earthÕs atmospheric pressure, material is catastrophically forced out the sides of the impact area. This squishing of material out the sides of the impact is called jet ting. The material ejected forms a hydrodynamic jet that has a velocity several times that of the meteorite itself, and is composed of an incandescent liquid or superheated vapor spray. A complicated system of shock waves and rarefaction waves envelop the target and projectile. One of the main results of this process is the transfer of kinetic energy from the meteorite to the target in the form of internal and kinetic energy. The rarefaction waves catch up with the initial shock wave several projectile distances from the impact , greatly reducing the shock intensity. After only a fraction of a micro-second, the compression stage ends with the complete transfer of energy in the form of a shock wave and latent heat.
The crater excavation stage overlaps somewhat with the compression stage. T his stage is likened to an at omic explosion, and is characterized by rapid crater expansion. Material leave the crater in 2 ways, ballistically or plastically. The most highly shocked rock nearer the surface is unloaded so rapidly it has a net upward force that propels the rock from the opening crater. Slightly deeper rock is pushed laterally from the crat er. During this stage, depending on exact P/T conditions the target rock may be melted, partially melted, brecciated, or vaporized. Commonly a large amount of impact melt is formed. Several authors t ried t o calculate the decay rate of the shock wave, one estimate indicates a pressure decrease was pr oportional to the 1/radius2.6. Another estimate indicates a decrease rate of 1/radius2. Crater growth finally stops when the net upward propelling force at the crater rim is not large enough to eject any more material.
The final stage of crat er growth, the modification stage, lasts only 20 s or 30 s. Small simple craters donÕt undergo much modification, but they fill themselves, at least partially with impact breccia or melt from the impact. Large craters, however, often undergo tremendous modification. This produces raise centers and double rings. Gravitational collapse seems t o be the main driving force. At least 3 main theories for central uplift of craters exist, but it seems obvious that the rebound from shock is the most likely of these. This process could be likened t o the initial upward movement of water droplets when a st one is t ossed in a pool. T he development of a second ring around a complex crater is poorly underst ood. A second ring appearst o be related to faulting found around impact sites. The cratering process represents the greatest release of energy per unit time known to man. The mechanisms are complex and the results are spectacular. We donÕt have the ability t o shoot projectiles at hyper-velocities equivalent to those of meteorites and the extrapolation from moderat e velocities t o hyper-velocities is not adequate.
If we call the hemispherical radius of the crater R, we let the height needed to move the rock (h) be equal to R, it is easily seen that Energy of me te ori te = K * R crater 4, where K is a constant e qual to 2/3 pi* d*g (table) Of course, not all meteorite energy is used t o make crater, some is used t o produce a shock wave and most is lost as latent heat . About 80 % - 95 % of the meteoriteÕs energy is expended as a shock wave and heat, leaving only a small % t o excavate the crater. But calculating the shock wave & heat is not easy. Calculating the shock propagation speed and particle velocity depends heavily upon the property of the target rock. The amount of heat produced and the amount of melt is also very complicated to calculate and varies only roughly as a function of impact energy (or crater diameter).
Using the relationship P i n i ti al= d targe t * v to 2 me te ori te we can calculate the initial pressure. Using a decay rate of about 1 / r t o3, where r is the radial distance from the point of impact (not a radius) we can roughly calculate the pressure of a shock wave at a distance (d) from the crater. To accomplish this, first we set P i n i ti al= K * 1/r to3 i n i ti al where K is a proportionality constant . When P is at its greatest we can assume r is approx. equal t o the radius of the meteorite itself (at impact the initial pressure would be roughly constant at the perimeter of a spherical shape about the size of the meteorite) Now, for various R meteorite and P initial values (dependent upon initial velocity and target density) a K value can be calculated. Using this K value, we can plug a P value int o the above equation and find the distance from the impact site where the shock wave would reach this pressure. Conversely, we could plug in a distance, r, to the above equation and find what the shock pressure P would be at this distance.
Implications of Results Even a small meteorite would release the energy equivalent of a nuclear arsenal (many megat ons of TNT ) Fig 5 provides the best insight int o the magnitude of cratering. Conclusions, application t o Sud bury Did the magma that brought with it the nickel-bearing sulfide ore form from the fractionation of an impact melt , or from a mantle derived melt ? This is the question All other known large nickel deposits have a deep mantle-derived source, and impact melting could not have reached such depths. Sudbur y Igneous Complex (SIC) need not be formed from an impact melt and an endogenous magmatic process is all that is required. Concentrations of PGEs and Re-Os isotopic compositions are further evidence t o disqualify an impact melt. Lately, the impact melt theory has received much support. Isot opic evidence and melt volume support this theory.
The parameters estimated for the Sud bury impact by various authors : 1) meteorit e = 14 km
2) crater = 110 km 3) density of meteorite= 3000 kg/m3 4) melt volume = 12,500 km3
Assuming a radius of 5 km for a spherical meteorite, a density of 3000 kg/m3. my calculated crater diameter is 57 km and melt volume is 15,500 km3 I assumed 50 % of energy went int o cratering (it may be closer to 15 % or higher than 65 %) If we assume that a percent of the meteoriteÕs composition is incorporated in the melt (say 20%), then a spherical st ony meteorit e with a 0.05 % Ni , a radius of 5 km and a density of 3000 kg/m3 could supply approx. 1.6 billion kg of Ni. The average grade of ore at Sud bury is about 3 %, so the meteorite could account for 52 billion kg of Ni ore. During the best Ni production year ever, Sud bury produced 209,000 kg of Ni, but amounts around 120,000 kg /year are about average. The Ni from meteorite alone could supply 13 solid years of average Ni production. If you assume the entire meteorit e was incorporated into the melt a figure like 60 to 70 years of production could be accounted for. This appearsto be adequate to explain the formation of the greater portion of Sud bury ore. An impact melt then seems like the only logical way this deposit could have formed.
SUMMARY SHOCK WAVES
THE MEANING OF THE CRATER DIAMETER Simple craters
Complex craters The many diameters of an impact crater Geological Investigation Geophysical Investigation Numerical modeling
IMPAC T C RATER Hypervelocity impact EITHER OR
ch .5 MEC HANIC S raised rims Small, simple with bowl-shaped depressions Large, complex, multi-ring impact basin
Intense Early Bombardment ended about 3.8 b.y. Lowest impact velocity with an object from space is equal t o the gravitational escape velocity of about 11 km / sec Fastest impact = 70 km / sec calculated by summing up The escape velocity + escape velocity from the sun + motion of earth around the Sun from earth at the EarthÕs orbit 11 30 30 = 70 Impacts at such speeds produce sh ock wave s in solid materials Both objects are rapidly compressed t o high density, then depressurized, exploding violently. Since craters are caused by explosions they are always circular (except lowangle collisions)
Stages : 1. Initial contact + compression 2. excavation 3. Modification & collapse -
a supersonic shockwave initiates from point of contact. As it expands, it decelerates & compresses the impact or & it accelerates and compresses the target. Stress levels within the shockwave far exceed Stage 1 the strength of solid materials; consequently, both the impact or and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher density phases by shock waves, for example quartz can be transformed int o the higher pressure forms coesite and stichovite. Many other shock-related changes take place and are diagnostic of impacts. As the shockwave decays, the shocked region decompresses t owards more usual pressures and densities. The damage produced by the shockwave raises the temperature of the material and this is enough t o melt the impact or and in large impacts t o vaporize most of it and to melt large volumes of the t arget. As well as being heated, the target near the impact is accelerated by the shockwave and it remains moving away from the impact behind the decaying shockwave.
-
Contact, compression, decompression and the passage of shock waves all occur within a few tenths of a second for a large impact. The subsequent excavation of the crater occurs more slowly. During excavation, the crater grows as the accelerated target moves away Stage 2 from the impact point. The motion is initially downwards and outwards and with time this evolves to become outwards and upwards. The flow initially produces a hemispherical cavity. The cavity continues t o grow eventually producing a paraboloid (bowlshaped) crater in which the center has been pushed down, a significant volume of material has been ejected and a t opographically elevated crater rim has been pushed up. When the cavity has reached the maximum size it is called the tran si e n tcavity. The depth of the transient cavity is typically a quarter t o a third of its diameter. The depth of maximum excavation is only about a third of the total depth. As a result , about 1/3 of the volume of the transient cavity is formed by the ejection of material and the remaining 2/3 is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. Most of melted rock remains within the transient crater. Vaporized material may expand outside the crater. In other planets or moons some ejected material may travel faster than the escape velocity.
Stage 3
The transient cavity is not stable & it collapses under gravity. Small lens of collapse breccia, ejecta and melt rock would cover the original excavation cavity. In complex craters we have the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound. Rather the collapse is a process in which a material with elastic strength at tempts to return to its original geometry. Broad shallow crater floor & terraced walls. One or more exterior or interior rings is possible
Shock metamorphic effects o a layer of shat tered or brecciated rock under the floor of the crater (may be buried) o shat ter cones. Most commonly in fine-grained rocks - high-temperature rock types. Melted rocks resemble volcanic rocks but incorporate unmelted fragments and have mixed composition. They may also have a large amount of trace elements that are associated with meteorites, such as Ni, Pt , Ir, & Co.
SHO CK W AVE A shock wave is a type of propagating disturbance. Across a shock there is always an extremely rapid rise in pressure, temperature & density of the flow. It travels through most media at a higher speed than an ordinary wave. The energy of a shock dissipates relatively quickly with distance. The accompanying expansion wave approaches and eventually merges with the shock wave, partially cancelling it . Thus the sonic boom associated with the passage of a supersonic aircraft is the sound wave resulting from the degradation and merging of the shock wave and the expansion wave produced by the aircraft.
C RATER DIAMETER T ransient crater is max. excavation. Has a depth to diameter ratio of about 1 : 3 For a 100 km transient crater, the max. extent of the opening cavity is achieved in roughly 80 Ğ 100 sec. Simple craters : has a final crater form only lit tle-modified from the transient crater. Complex craters : significantly modified from the transient crater. When the maximum depth of the transient crater is achieved, uplift of the crater floor begins before the opening cavity reaches its maximum radial extent. As a result, complex craters have flat floors whose depths are not strongly dependent on crater diameter. Above the floor rise central peaks that consist of bedrock that has been fractured into blocks, which are chaotically mixed in places and elevated relative to its original stratigraphic position. The uplift occurs rapidly enough that the breccia and melt that slide down the crater walls accumulate in an annulus around the central rise, rather than forming a central breccia lens as in simple craters., although some impact breccia can still be found on central peak complexes. The uplift of material extends below the crater floor and deep structural uplift is apparent in eroded complex craters on earth. Rim terraces develop as wide blocks slump from the rim enlarging the transient crater, typically by a fact or of 1.5 Ğ2 t o form the final crat er.
For even larger craters rather than a central peak, an internal ring of mountains often referred to as a peak ring or inner ring can be formed. P eak rings are typically concentric with the crater center and rim and occasionally surround a central peak. They are commonly discontinuous. Some of the largest impact structures are surrounded by one or more concentric scarps or fractures. These structures are called multiple or multi-ring craters. The rings that define multi-ring craters are distinct from the peak rings. Morphologically, peak rings generally appearmountainous and have symmetric profiles, whereas outer rings more strongly resemble scarps, having asymmetric t opographic profiles with a steep face on one side and a gentle slope on the other, or in some cases graben. Moreover, these two types of ring struct ures form under different conditions: peak rings form within the rim of the final, collapsed crater and consist of significantly disrupted mat erial, whereas the rings that are characteristic of multi-ring craters form outside the final crater and only locally disrupt pre-existing features. At Silverpit a 6 km diameter crater is multi-ringed and is at tributed t o an impact into a layer of brit tle chalk overlying weak shales. External low-relief, ring features that dip inwards at shallow angles are also observed at the Libyan BP and Oasis structures which are 2-3.2 km and 5.1 -18 km in diameter. Such outer concentric features maybe the actual crater rim, ring faults outside the actual rim, or expressions of the outer limit of the original ejecta blankets. These features could also represent faults resulting from inward slumping of rim t errace blocks during the modification stage.
Th e man y di ame ters of an Impact crater Explosion experts distinguished the true craterdefined by the boundary between fractures and unfractured bedrock, from the apparent crater which is the surface manifestation of the crater. Crater diam eteris defined as the apparent crater diam eter at the level of the pre-explosion ground surface, not at the rim and so is smaller than the rim-t o-rim diameter of the same crater. The crater depth is the depth of the apparent craterÕs center below the pre-explosion surface. Rim heightis also measured with respect t o the pre-explosion surface. Lab studies use sand or pumice. No struct ural collapse was observed. However, by increasing the acceleration of gravity the process of crater collapse could be observed during actual crater excavation using both explosive and impact energy sources. Even with large gravitational accelerations that leave a nearly flat , collapsed final crater, the transient crater undergoes a transient deep excavation phase, as was suggested by earlier sand target experiments. This observation is now generally recognized under the name proportional growth and agrees well with computer modeling results. While no new measures of crater diameter came out of this work, the experiments did show very clearly the stratigraphic uplift observed in terrestrial complex craters and underlined the large scale smoothness of the collapse flow, which rest ores rock layers deeply underlying the crater t o nearly their pre-impact locations, despite initial displacement by the excavation flow.
Ge ologicInve stigation The diameters have to be adjusted to account for the amount of erosion that is inferred to have taken place since the impact . Field studies have shown that the original impact melt volume scales with transient crater diameter. Therefore, it may be possible t o estimate the transient cavity diameter based on the amount of melt preserved. However, the terrestrial impact record is still poorly documented and estimating the original melt volume is difficult for the majority of craters that have been eroded to some degree
Ge oph ysica l In ve sti gati on Impact craters commonly exhibit strong geophysical signatures. Most craters have a gravity low whose diameter increases with increasing crater size. Larger craters may also have a central gravity high (Manicouagan) that is most likely caused by structural uplift of denser rocks. Magnetic signatures at impact craters can originate from deposits of suevite impact breccias (Ries ) impact melt rocks (Sud bury) and /or impact-induced thermal overprint of stratigraphically uplifted material. Magnet otelluric data from Chicxulub and Ries indicate that there are zones of high conductivity beneath some impact structures that are probably related to the presence of fluids within the porous impact breccias and fractures crater floor. Seismic reflection data are particularly good at locating subvertical faults within the target rocks. Although estimates of crater diameter that are obtained from geophysical data alone may well be accurate, they should always be interpreted with caution.
Numeri cal Modeli n g Estimates of rim diameter from numerical modeling should be treated with caution. No simulation t o date has accurately reproduced terrace slumping of a crater wall, because the resolution required to simulate strain localization on this scale (meters t o tens of meters) is prohibitively high. Results of numerical modeling simulations of impact events suggest two further metrics of a craterÕs size that may relate to observable parameters: the diameter of the heavilydisturbed zone and the diameter of the damaged zone. The heavily-disturbed region is the portion of the t arget that deforms during collapse; it has the form of a shallow bowl with a broad lip and is characterized by very high strain. T his zone may correspond t o the source of the broad gravity, magnetic and seismic-velocity anomalies associated with terrestrial impact struct ures. The damaged zone corresponds t o the spatial extent of fracturing predicted by the numerical simulation. This region may correspond t o the truecrater defined from nuclear explosion experiments. The limit of this region is hard to define because of strain localization. Delimiting this zone from observation may be impossible due t o the difficulty of distinguishing impact-related fracturing from nonimpact fracturing outside of the final crater rim.
REC OMMENDED TERMINO LO GY
Transient crater diameter Defined as the diameter of the rim of the idealized transient crater measured at the preimpact surface; that is the maximum radial extent achieved by the opening cavity before rim collapse begins. The diameter of the transient crater is equivalent to the diameter of the excavated zone of target material.
Rim or final crater diameter Defined as the diameter of the topographic rim that rises above the surface for simple craters or above the outermost slump block not concealed by ejecta for complex craters. The physical meaning of the rim-to-rim diameter is different in simple and complex craters as it refers t o the outermost steep scarp that is recognizable, which is not necessarily the same position with respect t o the transient crater diameter.
Apparent crater diameter Defined as the diameter of the outermost ring of continuous concentric normal faults measured with respect t o the pre-impact surface. For the majority of terrestrial impact structures this will be the only measurable diameter. It is not clear in all cases how this apparent diameter is related to the rim-t o-rim diameter. Some measurements of apparent diameter have been made with respect t o the present surface rather than accounting for erosion.
Central Peak diameter The diameter of a central peak measured at the contact with the surrounding crat er floor. For eroded craters the central uplift diameter is less easily defined because distinguishing uplifted material depends on the availability of stratigraphic markers and their positions relative to the amount of vertical uplift and the amount of erosion.
Peak ring diameter Defined as the diameter of the peak of the often discontinuous internal mountainous ring that rises above the crater floor. To unequivocally call a ring-like feature in an eroded terrestrial structure a peak-ring, it must be proved that this ring originally protruded through the crater-fill deposits in the fresh crater. Where this is not possible, it is suggested that the term inner ring be used. External ring diam eter Defined as the diameters of the crests of semi-continuous t o continuous concentric fault scarp external t o the final crater rim. In some cases external rings can stand t opographically higher than the struct ural rim. However, for the purposes of crater scaling, the origins of features are more relevant than simply their elevations so t opography alone should not influence the identification of the final rim. Outer lim it of deform ation Defined as the maximum distance at which impact-related deformation can be identified. This metric clearly depends heavily on the types of data that can be acquired; only large features can be identified from orbit whereas even faults with little offset can be identified if they are preserved in the terrestrial geologic record.
Terrestri al Example s : Ch i cxulub The structure is well preserved because the impact occurred on a stable carbonat e platform and the crater was subsequently quickly buried by 1 km of T ertiary sediments. Thus, the struct ure experienced little erosion, however, its burial makes access difficult . The structure was identified by circular geophysical anomalies during oil surveys in the 1950Õs. Its identification was confirmed through a combination of geophysics, petrologic and identification of shocked quartz. Studies on its size have been the source of heated discussions until recently. The size was initially inferred t o be 180-210 km. This corresponds t o a 30 mal negative Bouguer gravity anomaly and a small (5 Ğ20 nT) short-wavelength magnetic anomaly. Which lies outside a 90 km diameter, high-amplitude (100 Ğ 1000 nT) magnetic anomaly. 3 to 4 rings in the gravity data have been outlined. Chicxulub is a 295 km diameter multi-ring crater formed from a 170 km diameter transient crater (which would place the transient crater rim between the second and third rings). The internal crater had a diameter of 170 km from the combined analysis of gravity, magnetic and seismic data. Kring used t o the physical properties of the K-T boundary impact ejecta and Chicxulub impact melt sheet to test the geophysical estimates of the crat erÕs size, concluding that 180 km is the best estimate for the diameter of the final crater rim. Espindola inferred the presence of a single central peak and estimated the diameter to be 200 km based on the analysis of both gravity and magnetic dat a. Hilderbrand reanalysed the gravity data set with five new closely spaced onshore gravity profiles t o the SW of the crater center.
The examined the horizontal derivative of the Bouguer gravity anomaly (which emphasizes the effects of lateral density changes and suppresses regional gradients and broad features) over the struct ure revealing several circular features the most prominent of which is a steep gradient at a diameter of 170 km that correlated with a ring of cenotes ( sinkholes in the carbonate bedrock). No circular gradient features were evident beyond a radius of 90 km so H. concluded that the crater was 180 km. The initial diameter of Pope 240 km was later revised to 260 km based on surface t opography across the crater. On a profile to the SE of the crater they noted a prominent depression that coincided with the ring of cenotes (166 km). The combination of geophysical data has produced some consensus regarding the size of the crater. There is now general agreement that Chicxulub is a multi-ring structure and that the rim diameter is 180 Ğ 200 km and the transient crater diameter is 80 Ğ 100 km. According t o our definition above the rim diameter is 150 km. Disagreement over the size of Chicxulub was often the result of different interpretations of the geophysical and t opographical data as well as different definitions of the diameter of a complex crater. Such discrepancies have been responsible in estimating the transient crater size and thus the energy which is essential in understanding the environmental perturbations produced by this event to which the Cretaceous tertiary mass extinction has been at tributed.
S u dbury Of the 3 largest recognized impact structures on earth, the 1.85 by Sud bury structure is particularly important because it is the best exposed and although it has undergone erosion it is mostly intact . It has gone a complicat ed tect onometamorphic evolution since its formation. It possesses a complete sequence from impact-damaged basement rocks though footwall breccias and a 2.5 km thick impact melt sheet (the SIC), to fallback / flowback breccias (the 1.5 km thick Onaping formation) and overlying post-impact sedimentary cover (the Onwatin & Chelmsford formations). The post-impact folding and thrusting deformed the SIC into an elliptical shape. Four concentric pseudotachylite-breccia-rich zones have been identified. There is a wide range of proposed rim diameters from 55 km t o 280 km. There are ambiguities surrounding some of the original dimensions of the SIC. Landsat Multispectral Scanner data and regional geophysical data document the presence of 4 concentric rings associated with the structure. These rings are at 90 km, 130 km, 180 km and 260 km. Rings 1,2 & 3 correspond to zones of enhanced endogenic pseudotachylite development and have been hypothesized t o be related t o concentric fault systems that experienced very large displacements. The region between rings 1 & 2 consists of an annular trough 20 km wide. Rocks within ring 1 which have experienced shock pressures between 5 and 10 GPa are int erpreted to constitute the central uplift . The central uplift diameter was 90 km. The central uplift and annular trough are believed to roughly coincide with the transient crater, although some collapse may have occurred along the inward-facing walls of ring 2 during the modification stage. It is suggested that the impact melt sheet would have originally occupied much of this area. Rings 3 & 4 outside the collapsed transient cavity, but it is not clear which, if either, coincides with the original final crater rim. Given that ring 4 is the most distant recognizable feature found t o date associated with the structure it represents the outer limit of known impact-related deformation. However, due t o the lack of ground-truth information from the field the significant amount of erosion and post-impact tectonic deformation, it is not clear whether this outer ring structure corresponds t o the rim diameter, the apparent diameter or an external ring.
Vredefort Sum mary There are many ways t o measure an impact struct ure. The rim diameter is extremely difficult t o determine if the crater has been eroded and / or is not well exposed, if the target has distinct rheologically variable layers or if it has multiple concentric rings. If the t opographic rim has been eroded away one has t o look to other structural features present in the target rocks. Outer, continuous, ring structures can often been identified with detailed mapping. However, this measurement should not be interpreted as rim diameter unless strong evidence exists that the st ructural feature mapped corresponds t o the fault scarp of the outermost slump block that was not concealed by ejecta. High resolution seismic reflection can reveal crater-related fault systems that lie beneath the ejecta blanket. Although gravitational collapse is the driving force behind both final crater rim and external ring formation, the processes that create these structures differ. REFERENCES : Impact crater by Wikipedia Shock wave by Wikipedia T urtle, et al, 2005, What does crat er diameter mean ?
GEOPHYS IC ALS IGNATURE OF TERRES TRIAL IMPAC T C RATERS
ch .5
About 20 % of craters are buried beneath post-impact sediments. The most common signature is a circular gravi ty low. For simple, bowl-shaped craters, gravity models indicate that the anomaly is largely due t o the presence of an interior allochthonous breccia lens. In complex craters modeling indicates that the main contribution to the gravity anomaly is from fractured parasutochthonous target rocks in the floor of the crater. The size of the anomaly generally increases with increasing crater diameter reaching a max. of 20-30 mGal at diameters D of 20-30 km. Further increases in D have negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity low can be modified by target rock and erosional effects, and there is a tendency for larger struct ures (D more than 30 km) to exhibit a relative gravity high restricted t o the crater center and extending out to less than 0.5 D. The magneti c signature of craters is more varied. The dominant effect is a magnetic low due t o a reduction in susceptibility. Large structures ( D more than 40 km) tend t o exhibit central high-amplitude anomalies, with dimensions of less than 0.5 D due to the remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat and chemical alteration. The few studies over craters involving ele ctri cal methods indicate re si sti vi tylows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques particularly refle cti on surveys, have provided det ails of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due t o brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the struct ure. From the various geophysical techniques a set of general criteria can be established that correspond t o the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due t o impact . Confirmation of an impact origin, however, is based on geological evidence.
Examples Maximum negative residual gravity anomalies for 58 terrestrial impact structures Variation in the maximum negative gravity anomaly with crater diameter Residual magnetic field intensity over Deep Bay, Sask. Cont our int erval is 20 nT, flight height 300 m. Total magnetic field intensity over West Hawk lake, Manit oba. Cont our interval is 100 nT , flight height 300 m Residual magnetic field intensity over Ries, Germany Magnetic Anomaly character for 37 t errestrial impact structures Structure cont ours on the base of the Fish Scales formation at the Eagle But te structure, Alberta Rochechouart Impact structure, Bouguer residual anomaly Ries impact structure Bouguer anomalies Steinheim impact struct ure Bouguer residual anomaly Ries impact structure aeromagnetic survey, t otal field anomalies References: Pilkingt on-Grieve:Review of geophysics,May1992 Ernst on Claudin Impact Structures (www. impact-structures.com)
RECOMMENDED TERMINOLOGY
Transient crater diameter Rim or final crater diameter Apparent crater diameter Central peak diameter Peak ring diameter
Examples : Chicxulub, Sudbury, Vredefort
SUMMARY GEOPHYSICAL SIGNATURE OF TERRESTRIAL IMPACT CRATERS Gravity, magnetic, electrical (resistivity, reflection)
Examples LAB. EXERCISES
To calculate size of crater from size of meteor To calculate size of meteor from size of crater To calculate projectile size from transient crater diameter
Calculate energy released during impact
CHAPTER
6
Classification of Terrestrial Meteorites
OVERVIEW & CLASSIFICATION Simplest : stones, irons & stony-irons Stones are chondrites & achondrites METEORITES : FRAGMENTS OF ASTEROIDS
Meteoroids & meteorites : Lessons in Survival Brecciated meteorites and Multiple Falls Meteorite Surface Features
A
METE O RITE S : FRAG M ENTS OF AS TE RO IDS
IDT s :interstellar dust particles Asteroids in Hist ory : minor planets, asteroid planet bodies and their ÒchildrenÓare meteorites. Planets are in an orderly geometric progression. PlanetsÕs mean distance from the sun and its period of revolution: KeplerÕs Harmonic Law. Also Law of Areas & the Ellipse Law. First asteroid : Dec.31, 1800 Ceres: Roman goddess of agriculture = Demetra 1 Ceres, 2 P allas, 3 Juno, 4 Vesta, Astraea É. . 433 Eros Astronomical phot ography, dry-plate phot ography, 30,000 known Charge-coupled devices (CCDs) aut omated electronic telescopes possible t o image asteroids t o the 16th magnitude Smithsonian Astrophysical Observat ory in Cambridge, Mass. Small Bodies Names Commit tee Main belt : 2 AU to 4 AU COMPARING AST EROIDS WIT H MET EORITES Meteorite collections must have pieces from at least 135 separate asteroids. A conundrum arises: only about 16 % of the S-type asteroids have chondritic composition. The disparity between the carbonaceous chondrites, rare on earth but plentiful in the asteroid belt, and the ordinary chondrites, common on earth but relatively rare in the asteroid belt, seems t o be telling us that ordinary chondrites probably came from one or at most a few asteroid planet bodies. 4 Vesta, 1 Ceres, Asteroid Close Encounters, 253 Mathilde, 433 Eros, Hayabusa (this is the first time a spacecraft has landed and taken off froma solar system body other than the moon), The Dawn Mission t o 4 Vesta and 1 Ceres, Manned Missions
Brecciated
M ete orite s and M ulti pl e Fall s
Th e stony me te ori te sin parti cular often bre ak in to man y fragme n tswh ile stil l in space Th e melti ng i s u su al ly i n comple te and th e re sult i s a mix of me l te drock an d u nmelted pie ce s(clasts) Pe ekskil l, N.Y. main mass 12.6 kg bre cciate dch on dri te Dark ve i n sof glass an d re crystal lize d min erals perme ate dthe fragments h oldi ng th em toge th er mon omi ct bre ccia Bre cciasfragme n tsdifferi n g i n texture and com positi onfrom th e h ost rock polymi ct bre ccias Re goli th bre ccia
Mete orite Surface Fea tu re s Fusi on crust : n ot s e e n i n an y te rr estri al rock te mpe ratu res more th an 1,800Õ C covere d by a da rk brown to black crust less th an 1 m m fire ba l l stage : th e te mpe ratu re is at i t s h i gh est th e c rust can on ly form i n th e last fe w se con ds of th e fire ba l l stage th e above mi n eral s can n ot re crysta l lize for iron s th e c rust is cre am co lore d or b e i ge con t racti on cr ack s iron me te ori tes su bstan ti a l ly th i n n er th e m ost fr agi le an gulari ty of ston e s : n e a r 90; an g les pre do mi n ati n g re gmaglyph s Ò th umb pri n tsÓ iron s: shrapn e l fl ow stru ctu res de li cate fl ow fe atures th e C abi n C re ek, Ark an sas iron : th e largest iron e ve r se e n to f al l, lou de r th an th u n de r h i ssi n g sou n d con e s s h i eld sh ape s W il lame tte : ori gi n al ly fel l on C an ada t h e n , co lossal fl ood
FROM HAND LENS TO MICROSCOPE
--- Petrographic microscope --- Interference colors, crossed polars --- Reflected light & transmitted light --- examining meteorites in thin section --- chondrules --- matrix --- CAIs : calcium aluminum inclusions --- classifying your chondrite --- textures: their look and meaning --- the look of shock: textures & stages --- weathering: the enemy of meteorites --- photographing thin sections
B.
FROM HAN D LE NS T O MIC ROSCOPE
P etrographic microscope for 150 years polarizing filters, they can be rotated t o completely block out light passing through them, a position called crossed polars abbreviated XP see interference colors The thin section may be polished and uncovered if it is t o be examined for opaque minerals or for chemistry with an electron microprobe or scanning electron microscope Most silicate minerals in rocks and meteorites become transparent when ground down to the standard thickness of 0.03 mm Minerals that produce i nterf ere nce colors are said t o be birefringent , such as olivine, pyroxene and plagioclase. Some minerals and materials are isotropic, such as glass, garnet and diamond. Light entering these materials does not divide int o separate paths. When viewed between crossed polarizing filters the field of view remains black.
Make your own simple polarizing microscope Place a thin section between two polarizing filters Use only linear filters not circular Look at the thin section with a 5 Ğ 15 X hand lens while you shine light through filt ers from below Before shining light make sure the filters are crossed, by rotating one filter until all light is extinguished Or microscope with magnification of 10X t o 50X Impressive colorful minerals textures and pat t erns of most meteorites in thin sections!! Using Reflected light and T ransmit ted Light Isotropic (quart z, calcite) and anisotropic (olivine, pyroxene, feldspar) minerals T able 11.2 Minerals commonly found in chondrites & identifying features T able 11.3 T extural classification of chondrules a. Classifying chondrites using distinguishing petrographic features b. Selected diagnostic features of petrographic types Weathering : The enemy of meteorites
The strewn field
Jilin, China
The Big Ones
Hoba, Namibia
The Stre wn Field Th e more massi ve pi e ce s are ca rri e d furth er by th e i r gre at er mome n tum 30 de gre es fr om ve rti ca l le ast massi ve on e s 20 de gre es off th e ve rti ca l broke at 35 k m It i s m ore di ff i cu lt to l ocate a s tre wn fi eld wh e n th e me te ori tes h ave be e n on th e g rou n d for a th ou san d ye ars or more
Th e Bi g O n es Hoba i ron : 60 ton s n i ckel-iron ata xi te as large as iron can be An yth i n g large r wou ld fr ag me n t S ikh ote Ali n : 200 ton s, re cove re d 22.5 t on s : mass loss of 89 %!! Largest ston e : Kiri n , C h in a 15 to ns largest 1.77 k g C ampo Ir on , Arge n ti n a : date s fr om 1576 AD th e y h ave be e n on th e grou n d for 400 years W e ath eri n g of Met e ori tes Most me te ori tes are fi n ds n ot fal l s Oxi dati on , h ydrati on , solu ti on Ru st alon g th e fr ag me n t bou n da ri es
THE FAMILY OF METEORITES
1. THE CHONDRITES --- metal: definite surprise
--- only 280 minerals in meteorites --- most abundant: olivine, pyroxene, iron-nickel alloys --- most surprising primary mineral : iron-nickel --- the most primitive of all the meteorites
--- Petrographic types of Ordinary chondrites
Carbonaceous chondrites
---The most impressive : Tagish lake --- Allende --- Murchison
A chondrule gallery --- 0.1 - 3.8 mm --- growth within solar nebula, 4.56 b.y. ago --- thin sections under polarized transmitted light
2. Primitive Differentiated Meteorites : Asteroidal Achondrites --- 85 % of all known meteorites --- thermal metamorphism --- water alteration --- differentiation --- Achondrites --- Asteroidal achondrites
3. Differentiated Meteorites : Planetary & Lunar Achondrites --- Martian --- Lunar
4. Differentiated Meteorites : The Irons --- cores of asteroids --- alloys of iron & nickel
--- octahedrites: the most common --- IAB, IC, IIAB, IIIAB, IVA, IVB --- Ungrouped Irons
5. Differentiated Meteorites : Stony - Irons --- 2 main groups --- pallasites --- mesosiderites
O VERVIEW AND C LASSI FICATIO N OF METEO R ITES Meteorites: unique source of information about materials present & conditions prevailing in the solar system during the earliest phases of its hist ory. Not all are the same Compositional & physical differences Must be fundamental properties attributable to the starting materials Later events may destroy primary properties Simplest: st ones, irons & st ony-irons Among the st ones, enormous diversity in chemical & physical properties. Some (chondrites) are remarkably similar to the of the solar prot osphere for all but the most volatile elements, while others (achondrites) differ much from the sun, are enriched in lithophiles, depleted in siderophiles & chalcophiles. The sun is a fairly normal star, so its composition has Òcosmic abundancesÓ.
The chondrites divided in 9 classes: - CI, CM, CO & CV: have relatively high carbon, so Òcarbonaceous chondritesÓ - H, L & LL :the most abundant, so Òordinary chondritesÓ - EH, EL Òenstatite chondrites Isot opic differences can also be used to classify them. Chondrites are aggregates of Òcosmic sedimentsÓ, while achondrites are either igneous rocks or derived from igneous. Major components of chondrites are the chondrules, matrix, metal and sulfide & in certain classes, inclusions rich in Ca, Al & similar refract ory elements. Chondrules are especially important. Silicate mineral assemblages typically 0.1 t o 1mm in size which had an independent existence prior to the formation of the meteorite. Wide variety of internal structures but generally consist of silicate grains enclosed in either glass or in crystals which formed from a glass. Have been partially or t otally melted & owe their droplet form to their liquid origin.
Achondrites & st ony-irons are relatively few, but very diverse in properties & hist ories. All have experienced major planetary processing, typically including melting, which has virtually obscured the nebular phases of their history but provide indication of the scale & type of igneous processes that occurred in the early soler system. P allasites consist of networks of Fe-Ni alloy containing angular or rounded nodules of olivine ( 4-5mm size). Igneous origin, prob. formed in the interface between a large molten metal body & a large magma chamber in which olivine could form and sink to the bot tom. Three groups of achondrites (eucrites, diogenites, howardites) & the pyroxeneplagioclase st ony irons (mesosiderites) are closely related. The eucrites & diogenites are magmatic rocks while the others are breccias of these 2 classes. The parent material was chondritic, the variations reflect differences in cooling hist ory complicated considerably by metamorphism and shock.
The SNC meteorites are achondrites. The trapped noble gases & relatively young formation ages (around 1.25 Gyr) suggest they come from Mars. The 4 anorthositic breccia achondrites are of lunar origin. There ar emany unusual achondrites & st ony-irons some of which may be related to established classes. A unique achondrite is Angra dos Reis that was formed by magmatic processes 4.55 Gyr ago, very close to the time of formation of the solar system at 4.56 Gyr. Irons are essentially Fe-Ni alloys with minor amounts of C, S, & P. The alloy segregates int o 2 phases, a low-Ni body-centered-cubic phase known as the alpha phase or kamacite and a high-Ni face-centered-cubic phase known as the gamma phase or taenit e. Structures may range from pure kamacite with shock twins as their only structural feature (hexahedrites) t o some which are structureless t o the naked eye but consist of taenite with microscopic grains of kamacite (ataxites). In between these extremes are the majority of irons, which consist of plates of kamacite in an octahedral arrangement with taenite filling the interstices. These are termed octahedrites & their characteristic structure is called the ÒWidmanstat ten structureÓ.
The structure depends on the bulk Ni content and the cooling rate. However, the structures & Ni contents do not produce well defined groups. This is not true for the volatile trace elements Ga & Ge which show a 10000 t o 100000 Ğfold range in abundance, but produce many tight clusters. It seems these elements are unique because their volatility made them especially sensitive t o small differences in the P & T of the region of the solar nebula where the irons formed. Also, their distributions were insensitive t o subsequent igneous processes. Classified according t o their Ga, Ge & Ni content qualified occasionally by structural & mineralogical fact ors. The most populous classes are known as IAS, IIAB, IIIAB, IVA & IVB plus many smaller classes. However, genetic relationships maybe independent of classification mechanisms. For example, the 2 types of st ony-irons are half metal and half silicate, but the pallasites are much more closely related to irons than t o mesosiderites.
C HO NDRITES More than 100 minerals have been identified in meteorites, many of them unknown elsewhere. However, most of them are found in trace amounts. The important minerals for the classification of ordi n ary ch on drite s are olivine, Ca-poor pyroxene, feldspar, sulfide and metal. Olivine and pyroxene are solid solutions of Fe-, Mg- and Ca- silicates. Olivine is a solid solution of the 2 main components fayalite (symbol Fa) and forsterite (symbol Fo). Pyroxenes are solid solutions of ferrosilite (Fs), enstatite (En) and wollast onite (Wo). Feldspar ( Ab, An, Or). The sulfide is almost always pure FeS, known as troilite after an 18th cent. monk who observed the mineral with the naked eye in a newly fallen meteorite. T hemetal occurs as two phases (kamacite and taenite) as in irons. They exist as small widely dispersed grains surrounded by silicates.
The carbonaceous chondrites likewise have varied mineralogy. The CI and CM consist mainly of a variety of clay-like hydrous sheet silicates often identifiable with terrestrial phyllosilicates such as serpentine. They also contain hydrous Mg- and Ca- sulphates (sometimes in veins that suggest deposition from aqueous solutions), magnetite and considerable quantities of C, as carbonat e or complex organic compounds. Unlike the CMs, the CIs do not contain chondrules. The CO and CV are similar t o the ordinary chondrites in their major mineralogy, but contain olivine with a mean composition that is richer in Fe. The CO are noted for the smallness of their chondrules. The CV are known for the presence of mm-sized white or pinkish inclusions of Ca- and Al- rich minerals. (Ca-Al-rich inclusions or CAI).The inclusions are spinel, pyroxene, anorthite plus occasional sulfide-metals referred to as Fremdlinge whose metal is rich in rare refract ory elements like Re, Os and Ir.
Classification of chondrites The degree of oxidation of Fe is important. The reduced form is the pure metal (Fe), the oxidized form is in silicates and the sulfurized as sulfides. A plot of Fe in metallic form against Fe in oxide and sulfide form (both have Sinormalized). On this basis the enstatite chondrites are highly reduced, the carbonaceous chondrites are highly oxidized and the ordinary chondrites are intermediat e. These differences in Fe oxidation within the ordinary chondrites are also refelected in their mineralogy. A plot of the fayalite content of olivine against the ferrosilite content in pyroxene clearly resolves the H, L and LL classes. Other fact ors are involved. The Co content of the kamacite is also in large part determined by the degree of Fe oxidation.
The relative abundances of the 3 isot opes of O have provided an independent means of classification. A convenient method to examine the data is t o plot differences in the 17O/16O ratio between the sample and a standard, such as mean ocean water against the corresponding differences in 18O/16O.T wo processes seem t o have determined the isot opic proportions of O on a given meteorite or pert of one: (1) mixing of 2 or more components with different isot opic proportions and (2) mass fractionation mass fractionation is the adjustment of isot opic proportion by physical and chemical processes such as volatilization, crystallization, chemical reactions. The Allende CV chondrit e had an exotic 16O Ğrich material probably of extrasolar origin The chondrites : secondary properties
Several processes have modified original: metamorphism, shock, aqueous alteration and brecciation. Few chondrites have escaped significant levels of metamorphism ( up to 1200 ÔC) on their parent bodies. Brecciation and shock are common features in all classes. The terms polymict , monomict & genomict describe breccias. Shock played a major part in disrupting the parent objects and perhaps in sending the fragments to earth. It should be stressed that chondrites which are complex mixtures of minerals with highly diverse physical properties behave very differently from pure minerals during the passage of shock waves.
Significance of the chondrit e class The 9 classes offer unique insights int o conditions in the early solar system. First, they are compositionally very similar to the sun and quite unlike other known samples of solid material in the solar system. This suggests a relatively simple origin from a common pool of starting mat erial. Second, they have formation ages similar t o those inferred for the Earth, Moon and Sun, and presumably that of the solar system as a whole namely about 4.55 Gyr. Radiometric systems which can readily be disturbed, such as those dependent on the retention of gases, sometimes yield lower ages, but these ages are those of subsequent events such as shock heating. Third, all chondrites contain highly nonequilibrium assemblages which have been altered very lit tle, if at all, since the components came t ogether. The existence of 9 distinct classes of chondrite is significant because it leads t o the conclusion that those classes were established in the solar nebula. This means that the properties that distinguish one class from another Ğthe variations in Mg/Si, Fe/ Si, oxidation state and O-isot ope ratios- in some way reflect nebular processes. For example, there was apparently a nebular process for separating metal and silicate to produce the Fe/Si variations. Another process caused mixing of reservoirs with different O-isot ope properties. Chondrit e classes represent different regions of the nebula.
The Chon drit e s Ti n y silve r colore d flake s of me tal: a de fin ite surpri se El eme n tal iron is al most n e ve r fou n d i n te rr estri al rock s C h on drules : mil lime ter size sph e ri ca l i n clusi ons made of ve ry h ard yel lowi sh crysta l l i ze d mi n eral s 4,000 mi n eral s on e arth , bu t on ly 280 m i n eral s fou n d i n m e te ori tes on ly 8 eleme n ts make u p th e sili cate s on ly 3 m i n eral s are th e m ost abu n dan t in me te ori tes : oli vin e , py roxen es an d i ron -n i ckel al loys ca l le d me tal oli vin e s are soli d solu ti on of iron -ri ch to m agn esi um-ri ch py roxen es are al so soli d solu ti on s wi t h ca lci u m i n addi ti on to i ron an d magn esi um oli vin e s h ave more me tal th an py rox e ne s th e most surprisi n g primary mi n eral fou n d i n ordi n a ry ch on d ri tes is eleme n tal iron -n i ck el (me tal) bri gh t, sta r-like me tal li c grai ns, att racte d by m agn e t, th e p rese n ce of iron -n i ck el meta l i n a te rr estri al ro ck is a sure si gn of a ston y me te ori te
n i ckel con te n t fr om 5 % to 25 % as mu ch as 25 % of iron can be i n eleme n tal state troi li te is bron ze-like i n co l or an d stan d s ou t co mpa re d to th e silve ry co lor of iron -n i ck el me tal troi li te is like py rr h oti te bu t i t is n ot magn e ti c iron oxi de is prese n t as magn e ti te smal l amou n ts of p lagi oclase esp. i n basa lti c ach on d ri tes C h on dri tes are con si de re d th e m ost primi ti ve of al l th e me te ori tes Th e y h ave co mpo si ti on s close to th e sun (73.5 % h ydroge n , 25% h e li um + 1.5 % f or th e remai n i n g of n on vo lati le eleme n ts Th e eleme n tal co mpo si ti on of th e C I ca rbon ace ou s ch on d rit es is usu al ly co mpa re d to abu n dan ce s to th e solar ph oto sph e re- se e graph Al l ch on d ri tes wi th th e e xce pti on of th e CI ca rbon ac eou s ch on d ri tes con tai n ch on d rules e ve ry or di n a ry ch on dri te h as some magn e ti c att racti on du e to th e prese n ce of some eleme n tal iron
PET RO GRAPHIC TYP ES OF O RDINARY C HO NDRITE S Se conda ry proce ssing due to he at give n by radioacti ve de cay of aluminum 26. Type 3 m e te orites a re rel ative ly unaff ecte d by therm al me tamorphi sm, so the y are scie ntifical ly the m ost va luable of the chond rites. The y are une quili brate d. Type s 4 to 7 a re con side re d e quili brat e d which m e ans these type s have co mple te d the ir chem i cal re action s with adja ce nt mineral s and g lass and have ce a se d to r e act Any further he ating be yond that point (type 7) and the chond rite may suff er melting
C ARBO NAC EO US C HON DR ITE S Th e most impre ssi ve ston y me te ori tes Tagi sh lake 97 % bu rn e d u p i n th e at mosph e re , remi n d us of coa l bri qu e ts, ve ry n e arl y th at of th e su n i n co mpo si ti on , lack of me tal, wate r-beari n g mi n eral s, wi th s i gns of aqu e ou s alte rati on Murch i son , Al le n de A C HO NDRU LE GALLE RY C h on drules gre w wi th i n th e solar n e bula 4.56 b.y. ago, 0.1 Ğ 3.8 m m i n di a me ter, n ot ve ry pre tty Bu t th i n se cti on s place d u n de r cross-po larize d tran smi tte d li gh t e ve ryth i n g ch an ges. Loo k u n de r pe t rog raph i c mi crosco p e pp 107 -111
Pr i mitive Dif fer en tia te d Me te ori te s :As teroi dal Acho ndri tes 85 % of all kn own me te ori te sare ch ondri te s.Th e remai n i ng are ach on dri te s iron-n i ckel me tal is scattere d li beral ly throu ghou t th e ir i n teri ors a classifi cati onsystem base du pon th e amount of iron (oxi dized & eleme n tal) save on e rgou p Ğth e C I man y of th e sech ondri te sh ave be en su bje ct toth ermal metamorph i sm i n wh i ch the ch on drule fi elds were h eate d to a temperature of 950Õ C or more, n ot e n ou gh to melt th e ch on rdule s bu t suffi cie nt to slowly alter them from th e i rprimordi al state 400Õ C dow n to 150Õ Cal terati ondu e to th e pre se n ceof wate r produ ci ng h ydrated mi n erals su ch as magn eti te and clay-like ph yl losili cate s 15% of th e remain i n gme te ori te sare ach on dri te s, iron s an d ston y-iron s ach on dri te sformed by melti ng de e pwith in th eir asteroi d pare nt bodi e s.Th ey were on ce ch onrdi ti cbut that primary stru cture was de stroye dduri n g plan e tbu ildin g
Differe n ti ati on C ompre sse d gravi tati on ale n ergy into h e at plus h e atof de cay of radi oi sotope s melt differe n ti ate Iron , n i ckel an d some of th e noble me tals like gold, plati num an d iridi um Ach on dri te s Th e large stclass of differe n ti ated me te ori te sand i n clu de me te ori te sfrom th e asteroi d belt, the moon and plan et Mars Produ ct of com ple te melti n g Primi ti ve ach on dri te s Aste roi da l Ach on dr i tes 5 % of all kn own me te ori te s e u cri te s: calci u mri ch n ot h ydrous mi n erals virtu al ly all e u cri te ssh owi m pactbre cciati on di oge n i te s h owardi te s ure ili te s an gri te s au bri te s brach i n i te s
Di ff ere ntiated Mete orit e s : Plan etary and Lunar Ac hon drite s Marti an an d L u n ar W ere e i th er ful ly melted and differe n ti ated i n to mi n eral zon e swith in e achpare nt body or th e ysh owe dparti al melti ng in an in te rru pte dattempt to differe n ti ate. Th e h e ati n g occu rre d with i n th efirst few mil lion years aafter the ch on dri tic pare nt bodi e s acc re te d.Both ch on dri te sand ach ondrite s h ave virtu al ly the same age S ome ach on drite grou ps h owe ver are com ple te lydi ffere n t : th ey turn out to come n ot from th e asteroi d be l t, but from th e moon an d Mars Marti an me te ori te sS NC are thou ght to be cu m ulate i gn e ou srocks th e forme d on th e floor of an ign e ou sch amber Lu n ar ach on dri te s: an orth osi tic re goli th bre ccias,Mare basalts, Mi n gled bre ccias
Di ff ere ntiated Mete orit e s : The Iron s Melte d an d di fere n ti ate .dAn example i s 4 Ve sta If ach on dri te scome from th e cru stsand man tles of oth e rworlds, th e n iron me te ori te s mu st re pre se n tth e de ep foundation of th e seworlds Al loys of iron-n i ckel me te ori te s Hexagon al Ğcubi c crystals Th i s dif fere n cein composi ti on le ads to vi si ble stru ctural differe n ce sby wh i chth e y can be classifi e d Poli sh e dand e tch e d O ctah e d ri te s,th e most com mon type of iron me te ori te s IAB grou p : C an yonDiablo, Ode ssa,Texas, C ampo de l C ielo IC grou p : IIAB : S ikhote Ali n IIIAB : C ape York, W il lame tte IVA : Gi be on IVB : Hoba Un grou pe dIron s : Hexah e dri te O ctah e dri te : Ataxite ( wi th out stru cture) : C hin ga S i l i cate dIron s
Di ff ere ntiated Mete orit e s : Sto ny Ğ I rons Primi ti ve ch on dri te sare th e vast majority of m e te ori te s Be cau se of th ermal me tamorphi sm in the e arly solar system, i n tern al h e ati ng on some of th e more massivepare n t asteroi ds produ ce da broad ran geof me te orite type sfrom th e ori gin al pre cursor ch on dri te s 2 main grou ps:pal lasi te sformed at th e bou nary d be twe e nme tal core s an d sili cate man tle s of melted and differe n ti ated pare nt bodi e s.Me sosi deri te sh owe ver forme d by impact me l ti n g, vi olent col li sion s i n the e arly solar system
Palla si te s : from the 1,600 lb meteorit e found in 1749 in Krasnojarsk, Siberia Whne the olivine is of gem quality, it is called peridot Olivine t o metal ratio is roughly 2 to 1
Mesosiderite s Polymict breccias they are simply accidental mixtures Olivine is all but absent in mesosiderites
SUMMARY SOURCE REGIONS METEORITE ORBITS --- Astrometric photography & photometric photography DYNAMIC MECHANISMS FOR DELIVERING OF METEORITES TO EARTH --- most are 10 m to 100 m in diameter --- 2 major asteroidal source regions --- most ordinary chondrites from the 3:1 gap --- innermost belt as source of achondrites --- not many from beyond 2.6 AU
S O URC E REGIO NS Asteroids and meteorites Meteorites are fragments of asteroids. Some may be cometary in origin. The heliocentric orbits of meteorites appearedt o be parabolic. Because Òthere has never an observed fall from a shower meteorÓan association between the cometary sources of shower meteors and meteorites is unlikely. However, there was a coincidence of the Mazapil iron fell during one of the greatest meteor showers ever observed (1885). The Apollo mission taught us what lunar meteorit es should look like.Cometary spectra provide strong support for the belief that comets are volatile-rich bodies consistent with their orbit s and their presumed origin in the outer solar system. In contrast , the most common meteorites, the ordinary chondrites, are depleted in volatiles to about the same extent as the highly depleted planet earth. Comet nuclei resemble D- and P-type asteroids, very unlike the ordinary chondrites. Spectrophot ometric data provide a way t o compare the mineralogical composition of asteroids and meteorites. However, there are important disagreement between the contents of olivine, pyroxene and metal of the most abundant meteorit e class, the ordinary chondrites and those inferred for the S asteroids, otherwise the most promising asteroidal source for these meteorites.
METEO RITE O RBITS A body in an earth-crossing orbit will usually be above or below the plane of the orbit of the earth at the time it is at the earthÕs distance from the sun and therefore will not actually intersect the orbit of the earth. The orbits of 3 meteorites, all ordinary chondrites, have been determined by astrometric phot ography & phot ometric phot ography by bright meteor fireball networks. All of these orbits have perihelia fairly near earthÕs orbit, penetrate int o the asteroid belt, but remain well inside the orbit of Jupiter. By use of phenomena associated with the observed atmospheric traject ory of these meteorites, it has proven possible t o identify about 30 phot ographed, but unrecovered, very bright meteors that appeart o have physical properties similar from those phot ographed ordinary chondrites. In contrast , the majority of the fireballs do not appear to share the properties of ordinary chondrites, and must be different , probably carbonaceous chondrites, as well as cometary fragments. Because the recovered ordinary chondritic fireballs are among the very brightest phot ographed meteors. It may be expected that they should be accompanied by a much larger number of less massive ordinary chondrites. An important feature of the 3 observed orbits is that of the perihelion distribution: these are strongly concentrated between 0.9 and 1.0 AU. Ordinary chondrites are observed t o fall about twice as often in the daylight morning hours. In contrast , on a 24-hour basis, probably biased against midnight to 6am falls, the basaltic achondrites exhibit no significant pm excess. Although their source is also the asteroids, it is different t o those of the ordinary chondrites.
DYNAMIC MEC HANISMS FO R DELIVER ING O F METEO RITES TO EARTH It is calculated that the immediate parent bodies of most meteorites are quit e small asteroidal bodies (10m Ğ 100 m diameter) and that most of the meteorite-size fragments are produced by collision of these immediate parent bodies with even smaller main-belt bodies while in earth-crossing orbits. All calculations argue against a cometary origin of ordinary chondrites. Neither active comets nor those Apollo objects most likely to be volatile-depleted comets exhibit the concentration of perihelia near 1AU required t o satisfy the orbital requirements of ordinary chondrites. It has been assumed that asteroidal collisions and ejection int o earth-crossing orbits is a continuous process. In conclusion - there ar e2 major asteroidal source regions for meteorites and ApolloAmor objects. The first is adjacent to the 3:1 Kirkwood gap that extends from 2.48 to 2.52 AU. It includes main-belt asteroids between about 2.45 and 2.48 and 2.52 to 2.55 AU, with smaller contributions from asteroids with somewhat greater and smaller semimajor axes, The second is in the innermost belt, in the vicinity of the Flora asteroidal group. The V6 resonance at 2.04 AU plays an important role in accelerating fragments of these asteroids int o earthcrossing orbits. - Most ordinary chondrites should be derived from the vicinity of the 3:1 gap.It is also expected that a comparable number of carbonaceous meteorites originate in this region. - The innermost belt is a good candidate source region for several kinds of achondrites, including the basaltic achondrites. Both the calculated mass yield and the time-of-fall statistics of these meteorites can be underst ood if this is the case. - It is unlikely that any siginificant number of meteorites originate from orbits beyond about 2.6 AU, although further study of the 5:2 resonance at 2.82 AU is warranted.
SPECTROPHOTOMETRIC & RADAR EVIDENCE LINKING ASTEROIDS & METEORITES Remote sensing techniques The compositions of the S asteroids --- paradox: no asteroids as sources of ordinary chondrites! --- most abundant are S and C Description of asteroid classes --- low-albedo classes --- moderate albedo classes --- high albedo class Asteroidal sources of differentiated meteorites Zoning of the Asteroid Belt Igneous activity in the early solar system
SPEC TROPHO TOMETRIC AND RADAR EVIDENC E LINKING AS TEROIDS AND METEO R ITES Remote se n si n g te ch nqu i e s. Ideally, the technique is t o identify mineralogical composition of asteroids so as the match with the different meteorite types. The chief technique is visible and near-infrared reflection spectroscopy. Also, useful results have been obtained from groundbased radar-reflection measurements. Vesta for instance was identified to be basaltic achondritic type, but it may not be a parent asteroid of those meteorites found on earth. In essence, sunlight reflected from an asteroid is transmit ted through mineral crystals in the surface layer and acquires a spectral signature caused by the composition and particular lat tice struct ure of the minerals. These are characterized by having ÒslopesÓ(colors) and several broad absorption feat ures, whose positions, widths and shapes may be diagnostic of even fairly minor differences in mineralogy. These data are combined with an understanding of ÒplausibleÓassemblages based on cosmic abundances. These results are then compared t o measurements made on known meteorites of all types. The best matches t o asteroid spectra are found when the meteorite samples are pulverized t o simulate an asteroidal regolith.
Th e composi ti onof th e S asteroi ds and th e spectroph otome tric Paradox In the 1970Õs there were 2 paradoxes: - There was no mechanism t o transfer material from the main asteroid belt into earth-crossing orbit.- Even if the above was solved, the problem remained that there were virtually no asteroids in the main belt that had the reflectance spectra expected for ordinary chondrit es. The most abundant asteroids in the inner belt are classified as S and C. The Cs are most plausibly associated with meteorites rich in opaque minerals with low albedos, i.e. the carbonaceous, and the S asteroids are therefore the most promising ordinary chondrite source. The mineralogy as obtained by spectra of Ss is the same as found in ordinary chondrites: olivine, pyroxene and metallic Fe. However, the proportions and compositions of these mineral constituents differ from those found in ordinary chondrites. The major problem of sorting out the implied composition of S types in terms of wellknown types related to determining the relative proportions of the 3 major components. By testing the spectra of known meteorites the S types fail to match ordinary chondrites. S types have a larger signature from metal than is shown by lab measurements of ordinary chondrites. Another important difference is that most S types are very olivine rich compared with the olivine/pyroxene ratios in even the olivine-rich LL types.
Several solutions t o this paradox: - Ordinary chondrites are fragments of S asteroids, but the present sampling is incomplete There are about 36 S-type asteroids larger than 100 km diameter. Only a few have comparable pyroxene/olivine ratios however. - Ordinary chondrites come from S asteroids, but the quantitative mineralogical interpretation of the S type spectra is in error, because of surficial effects that mask the actual mineral composition of the asteroids. - Ordinary chondrites come from S asteroids and represent a portion of a differentiated asteroid that somehow escaped differentiation - Ordinary chondrites do not come from S asteroids, but for example, come from some other asteroid class that has not been sampled well, possibly because its members are concentrated in the smaller-size range
Description of Asteroid Classes ------------------------------------------------------------------------------------------------ Low-Albe do classe s C
a very common asteroid type in the outer part of the main belt
D
rare in the main belt
P
a fairly common type near the outer edge of the main belt
T
rare of unknown composition
Moderate albe do classe s A
rare
M
a fairly common in the main belt. Presumably metallic
Q
a spectrum unique to 1862 Apollo
R
a spectrum unique t o 349 Dembowska
S a very common type in the inner part of the main belt and also among earthapproaching asteroids; their spectra are moderately t o very reddish shortward of 0.7 mewm. Varying amounts of pyroxene and olivine indicated V
a spectrum unique t o 4 Vesta
High Albe do Class E an uncommon type; surface composition is possibly similar to enstatite achondrites ---------------------------------------------------------------------------------------------------- - - -
Asteroidal Source s of Differe ntiated Me te orite s There is no reason to believe why an entire asteroid should have the same composition as a particular sample of an igneous differentiated sequence, particularly if any original igneous layering was disrupted by a fragmentation hist ory sufficiently intense to expose metallic asteroidal cores. Zoningof the Asteroid Belt S types in the inner belt , Cs in the out er, Ps in the Hilda regions and Ds among the T rojans
ORIGIN OF AST EROIDS They are recently derived fragments of asteroids that exist t oday, or have existed until recently Most meteorites were hammered out not long ago from asteroidal outcrops. Have been processed by events like irradiation, impacts & fragmentation since the solar system became normal, perhaps 4 by ago. It is supposed that density of mat ter in the solar nebula varied smoothly with distance from the sun. The original number of asteroids was1000t o 10000 times greater than t oday. SECONDARY PROCESSING (chondrites are primary) Isot opically inhomogeneous nebula ------t o P rimitive parent bodies--------to evolved parent bodies-------t o source objects
IGNEOUS ACT IVITY IN T HE EARLY SOLAR SYST EM A large number of meteorites record melting events in at least 70 small parent bodies essentially at the time of formation of the solar system and remot e sensing demonstrates that a large number of asteroids are differentiated, ie were partly or wholly melted. To explain such heating could be due decay of radioactive 26Al, electromagnetic induction. Although only one large asteroid with babsaltic rocks on its surface is known, there are many others inferred to be differentiated, including As (olivine-rich), Ss (metal-olivinepyroxene) and Ms (metal-rich). The lack of currently detectable basaltic asteroids is undoubtedly due to the collision hist ory of asteroids since silicates are brit tle and can be stripped off the t ougher metallic cores. The more strongly heated bodies are in the inner part of the main belt , due to closer t o the sun. Among all the sort-lived radioactive isot opes 26Al has the greatest potential for asteroidal heating and there is evidence for the former presence of it in chondritic meteorites.
T HE MET EORIT IC RECORD Iron s A very wide extent of melting is implied for small bodies in the early solar system, particularly from iron rather than silicate meteorit es. There are over 60 groups of irons, the majority of which formed by fractional crystallization of molten metal most probably in the cores of small asteroids. The irons resulted from solidification of a liquid. (except the IAB group). P artition coefficients depend on the concentrations of minor elements such as S and P. Irons with intermediate Ni contents cooled through the 2-phase region of the Fe-Ni phase diagram, so as to produce exsolution lamellae of the low-temp. kamacite in an octahedral pat tern. S ili cate-ri ch Differe n ti ated me te orite s
CONCLUSIONS Even though the chondrites are numerically more abundant among the meteorites, the number of parent bodies, about 70, represented by the igneous meteorites is much larger than the number of parent bodies represented by the primitive meteorites. This conclusion is consistent with the observation that the parent bodies of the igneous meteorites, the differentiated asteroids, are relatively close t o us in the asteroid belt. The howarditeeucrite-diogenite parent body, though not conclusively identified with a known asteroid, is the one we know most about . The data for HED basalts and cumulates indicate that their parent body was very extensively but not t otally melted. Establishment of remelting of igneous crust for the origin of any HED magmas would suggest electromagneticinduction heating combined with 26Al decay or 26Al alone, because the lat ter heat source could have migrated to the planetary surface. Complicated igneous hist ories are also observed in samples from the aubrit e and ureilit e parent bodies and in the Angra dos Reis meteorite. Induction heating probably played a major role in differentiating asteroids in view of the location of such asteroids relatively close t o the sun.
T HERMAL METAMORP HISM AQUEOUS ALTERAT ION MET EORITE REGOLITHIC BRECCIAS SHOCK EFFECTS IN METEORITES IRRADIAT ION RECORDS IN MET EORIT ES
Principles of Radiometric dating SUMMARY •Decay constants •Initial ratios : for Pb, ratios taken from Canyon Diablo troilite. For Sr, use Allende CAIs •Isochron diagrams: slope is function of age of samples •Analytical methods & instrumentation
PRINC IPLE S OF RAD IOMET RIC DAT ING Six decay systems are used in meteorit e dating: Rb-Sr, Sm-Nd, K-Ar, T h-Pb, and the two U-Pb systems. As in terrestrial dating, different systems may date different physical processes, even when applied to the same sample material. Assumptions: - the decay constant of the radioactive parent is accurately known - the radiogenic component of the daughter nuclide can be distinguished from the initial, or nonradiogenic component - the isot opic composition of the daughter element was homogeneous at the time of the chemical differentiation - the dated material has been a closed chemical system
De cay constants The constants for alpha emit ters are the most accurately known because the particles are non energetic and create high ion densities in traversing mat ter. The greatest problems are encountered with low-energy beta emit ters. T able 5.1.1 Normalizati onC onventi ons The long half-lives of the parent elements cause Rb-Sr and Sm-Nd ages t o be quite sensitive t o corrections for the non-radiogenic daughter isot opes. Ini ti al Rati os Calculation of ages requires corrections for initial, or nonradiogenic, isot opes. In meteorite dating, the isot opic composition of the initial daughter element may either be measured directly, or inferred. When the initial ratios are inferred, the ages are designated as Ómodel agesÓbecause they use assumed corrections for nonradiogenic daughter nuclides. For meteorite model ages the isot opic composition of intial Pb,Sr, or Nd is assumed t o be that of the primordial element as determined in other meteorite samples. For Pb, the ratios are taken from Canyon Diable troilite, while Sr calculations utilize the initial ratios from basaltic achondrites or Allende CAIs. T able 5.1.2 Primordial Ratios
Isochron Di agrams If a sufficient number of samples are available, it is often possible t o measure the initial ratios directly by isochron methods, which provide age as well as information on the initial isot opic composition of the daughter, providing 4 conditions are met: a) the samples are coeval, b) the initial isot opic compositions of the daught er element were identical for all samples c) all samples have been closed chemical systems for parent and daughter elements since crystallization, and d) there is sufficient variation in parent/ daughter ratios to generate a spread in daughter isot ope ratios. When all these conditions are met , a plot of parent vs daughter nuclide, each normalized t o a non radiogenic daughter-element nuclide, yields a straight line called an i sochron. The slope of the isochron is a function of the age of the samples, while the initial isot opic composition of the daughter element is given by the intercept of the isochron with an axis for which parent/daughter ratio = zero. Fig. 5.1.1 Isochron diagram for the 87Rb- 87Sr system Another isochron method is available for the U-P b system owing t o the coupled decay of the two U isot opes to two isot opes of Pb. Fig. 5.1.2 Isochron in the 207Pb Ğ 206 P b system Although the age is given by the isochron, note that it does not yield information on the initial P b isot ope ratios, which require U and Pb concentrations data.
C oncordia Diagrams When disturbances have occurred in the U-Pb system, Concordia diagrams can still sometimes permit evaluation of the crystallization age and a time of disturbance. Analytical methods and Instrume ntation Mass spectrometry In U-Pb dating, Pb blanks are difficult t o control compared t o other elements owing to the ubiquity of Pb in the environment and the generally low levels of Pb in meteorit e samples
AGE OF THE SOLAR SYSTEM SUMMARY Primordial Pb from Canyon Diablo troilite Allende CV3 chondrite: chondrules at 4.56 b.y., rich in refractory minerals, matrix is younger Angra Dos Reis achondrite: from early planetary igneous process Basaltic achondrites Chondrites Conclusions: Oldest is CAIs from Allende
•Dating secondary processes: gas-retention ages, thermal metamorphism, shock reheating, aqueous alteration •Compaction ages: techniques, radiometric ages, authigenic phases •The early solar system: protostellar collapse, primitive solar nebula, grain growth, effects of the proto-sun •Formation processes and time scales for meteorite parent bodies
AGE OF THE SO LAR SYSTE M We date chemical differentiation rather than physical processes. The generally accepted of the solar system has changed lit tle over the last 30 years since Pet terson (1956) reported a 207Pb/206P b age of 4.57 for 3 st onies paired with the Canyon Diablo troilite Pb, which represented primordial Pb. That age was controlled mainly by the isotopic compositions of the troilite and the Nuevo Laredo eucrite. When that data are recalculated using newer values for primordial Pb and decay constants, the age is reduced t o 4.50 by. Suc h an age agrees well with the present 207Pb/206Pbages for eucrites, while the oldest known ages are still 4.56 by.
Al le n deC V3 ch on drite It has highly refract ory minerals rich in Ca, Ti and Al similar in composition to those predicted t o have condensed early in a gas of solar composition. These inclusions maybe one of the oldest material t o be found in meteorites. K-Ar age of 4.44 by. for a chondrule. Later, model age of 4.6 by. Obtained from several chondrules plus bulk samples and inclusions. Rb-Sr ratios have been disturbed more recently than 3.6 by. In a low-Rb inclusion Gray et al (1973) made the important discovery of the lowest 87Sr/86Sr ratio yet found in any solar system matter, 0.69877+-2. This is an observed value, which corrects t o 0.69876 when corrected for in sit u Rb decay over 4.6 by. Isot opic Pb age of Allende inclusions : model age of 207Pb/ 206 P b of 4.553 by. Fig. 5.2.1 207Pb/204P b- 206Pb/ 204Pb isot ope correlation diagnram for Allende and other chondrite samples Coarse-grained inclusions give a Pb/P b age of 4.559 b.y. T his is the best estimate of the Allende CAI age. The P b-Pb age is a model age. All experiments indicate theat the model ages of the matrix material are distinctly younger than that of the inclusions and chondrules of Allende
An gra Dos Re i sAch on dri te It is a Ca, Al, Ti-rich achondrite that appearson the basis of petrographic criteria t o have crystallized from a magma, thus relating it to an early planetary igneous process. It is also depleted in volatile elements, those that are volatile in vacuum at temperatures of ca. 1000ÕC. These include the alkali elements and Pb. This meteorite is an especially favourable sample for dating because the bulk sample has a high U/P b ratio and especially because the calcium phosphate, whitlockite, is present in sufficient quantities t o allow separation for analyses. The average of the model ages is 4.551 by T able 5.2.2 U/P b ages of Angra The low Rb concentration in Angra, presumably due t o volatilization, causes the Rb/Sr ratio to be exceptionally low Sm-Nd data yield an age of 4.55 +- .04 (high uncertainty) The chemical differentiation age for Angra is ca. 10 my younger than that for Allende, while the initial Sr ratio is higher. It is significant that a magmatic meteorite gives an age so close t o that of Allende, which shows no evidence for such a process.
Basalti c Achondri te s The include the eucrite and howardite classes which like Angra have crystallized from a magma. Both groups are depleted in volatiles causing the whole-rock samples t o contain highly radiogenic P b and nonradiogenic Sr. They are favourable samples for U-P bmethod dating and initial Sr-isot ope ratios, although brecciation and metamorphic hist ory complicate the interpretations in many cases. The initial 87Sr/86Sr ratio for achondrites is measurably higher than those for Angra and Allende CAI. Sm-Nd ages give 4.56 by. U-Pb dating offers the possibility of higher precision dating. The most thoroughly studied eucrit e is Juvinas. Rb-Sr isochron age of 4.50 by, is still one of the two most accurately known eucrite Rb-Sr isochrones. The initial Sr ratio for Juvinas calculated from plagioclase is 0.69898. Many basaltic achondrites appear t o have crystallized between 4.56 and 4.53 by with an average age of 4.54. Some show later disturbances presumably due to impacts. The age of 4.539 is tentatively adopted here as the best age for basaltic achondrites.
C h on dri te s A considerable body of data exist for the Sr, Nd and Pb geochronometeres from chondrites. Most values are consistent with ages of 4.56, but within rather coarse error limits than more than encompass the age range covered by Allende CAIs and the achondrites. Fig. 5.2.2 87Sr/86Sr Ğ 87Rb/86Sr isochron diagram Disturbed systems: terrestrial contamination with Pb is suspected. If corrections for that are made, then chondrites yield a model Pb/Pb age of 4.550 +-0.005 by It is somewhat difficult to place the ordinary chondrites within a highly precise framework due t o uncertainties in the analytical data. Lack of precise knowledge of the 87Rb decay constant and the influence of terrestrial Pb contamination on the model Pb/Pb ages. The model ages mostly fall within the range of 4.53 to 4.56. Three chondrites yield high-precision model Pb/P b ages of ca. 4.552 +-0.003. That is probably the best estimate from U-Pb data. The best estimate for initial 87Sr/86Sr is probably 0.69885. There is considerable evidence for early metamorphic processes that affected some chondrite ages.
C O NC LUS IO NS The oldest dated meteoritic material consists of certain CAIs from Allende, which yield an age of 4.559 +-.004 Since the radiometric ages date chemical differentiation processes we need t o define the process responsible for the ages. St arting in the nebula of solar composition, the isot opic evolution of P b is effectively arrested due t o the low U/P b and Th/P b ratios, while the Rb/Sr and Sm/Nd ratios cause Sr and Nd isot opic compositions t o evolve. The isot opic-P b ages therefore date a process that fractionated volatile Pb from refractory U and T h, allowing isotopic evolution of P b t o commence. This process is reasonably associated with the condensation of mat ter from the nebula which for the Allende CAIs requires cooling through a temperature of about 1600 K. T able/Fig. 5.2.3 Initial 87Sr/86Sr ratios plot ted against Pb/Pb model ages of meteorites Achondrites have younger isot opic-model Pb ages for most BACHs and a higher initial Sr ratios compared to the Allende inclusions. The average Rb/Sr ratio of ordinary chondrites also agrees closely with the solar abundance value. However, O-isot ope data preclude derivation of basaltic achondrites from identically the same sources that produced H, L or LL chondrites. The basaltic achondrite sources must have evolved from 4.56 t o 4.54 either in the nebula of solar composition or in an object with approximately solar Rb/Sr ratios. Since P b data suggest that evolution of Pb isot opes commenced around 4.54, the U/P b ratios in the parent material must also have been near the solar abundance ratio up to that time. The basaltic achondrite material condensed 3 t o 8 my later than the Allende inclusions. Collisions between large-size planetesimals, like during the formation of the Moon, could cause melting or vaporization of parent materials which will account for the low abundance of volatile lithophile elements and the normal or enriched abundances of nonvolatile lithophile elements that characterize the basaltic achondrites and the Moon.
DAT ING OF SECONDARY EVENT S Gas-retention ages and Ar-Ar dating Thermal metamorphism and cooling ages Shock reheating during major impacts The chronology of SNC meteorites The chronology of Aqueous alteration COMPACT ION AGES T echniques and results Radiometric ages Authigenic Phases
T HE EARLY SOLAR SYST EM Chondrites and the early solar system P rot ostellar Collapse, dust grains and solar-system formation -Observations of solar-type star formation - P rot ostellar collapse - P rimitive Solar Nebula - Grain growth in the Interstellar medium and during prot ostellar collapse - Enhanced dust-t o-gas Ratios - Effects of the Prot osun A review of Solar Nebula Models Formation processes and Time scales for meteorit e P arent bodies REFERENCES Kerridge-Mat thews, Meteorites and the Early Solar System, U. of Arizona, 1988, 16 P arts
CHAPTER
7
Analytical Data derived from Meteorites / Comets
SUMMARY: Topics --- The age of the solar system: 4.566 b.y.
--- Choose a clock --- reading a long-period clock: mass spectrometer --- Family portrait: unshocked chondrites: 4.56 - 4.48 Ga, oldest
material in the solar system --- eucrites: very old volcanism (4.56 - 4.53) --- K-Ar age: dates the collision event --- Age of the earth: Patterson, 1955, meteorites & earth have same age
--- Stopped clocks --- Iodine 129 in meteorites: all meteorite parent bodies formed within 20 m.y. --- Solar system bracketed between 4.57 & 4.55 --- Birth date of solar system: Al-26 detected in feldspar of inclusions
that gave a very precise age of 4.566 b.y.+/- 2 m.y. --- The age of the Universe: 3 methods to date the Big Bang estimate of oldest stars in our galaxy,
radioactive dating of chemical elements synthesized in stars in our galaxy measure the expansion by observing galaxies
THE AGE OF THE S O LAR S YS TE M We know the age of the solar system for 45 years. It is precisely 4.566 b.y. The earth formed over the next 100 m.y. and life began t o develop about 4 b.y. ago. However, the sun appearsyoung in comparison t o the Universe which is about 14 b.y. old. A fragment of a molecular cloud in one of the spiral arms gave rise, by contracting, to an infant sun and a swarm of planetesimals. After a hist ory of accretion, some of these planetesimals became planets of the solar system. C h oose a lock c The ages are determined with the help of some clocks, which count time in meteorites. Some at omic nuclei are radioactive, unstable & spontaneously transform into other nuclei. Disintegration is instantaneous, but takes a long time. Each clock ticks with its own rhythm, governed by this half-life. We have 2 kinds of clocks; those that work over a long period & those which st op at the end of a short period. Ga = giga years, billions or Ma (mega years, millions)
Re adi n g alon g-peri od clock A one-gram fragment of chondrite contains an infinitesimal amount of uranium, about 10 billionths of a gram. The tiny mass represents a gigantic number of atoms, twenty five thousand billion. This number was even greater when the meteorite was formed because a fraction of the initial uranium at oms have decayed giving lead at oms. By counting the radiogenic lead at oms and the remaining uranium at oms in this fragment , the time elapsed since the uranium began to decay can be calculated. The counting is done with a mass spectrometer, after extraction and chemical separation of the traces of uranium and lead. Contamination is t o be avoided. The age obtained is close to 4.5 Ga. At that time, the fragment of chondrite contained 67 thousand billion uranium at oms. The ages depend on the chemical hist ory of the material. If it was a closed system, without gaining or losing material due t o external influences, the age would correspond to the age of the formation of the rock. A simple heating would upset the system. For example, if the parent isot ope was firmly in place, within the crystal lat tice, the daughter isot ope is generally less secure because it may migrate away. In this case, the clock would indicate the time elapsed since the last event took place, not the formation of the rock.
Family portrai t In the ÒunshockedÓchondrites, long-period clocks indicate very similar ages between 4.56 and 4.48 Ga. They were started almost at the same time and have ticked away without perturbation right up t o t oday. This age identifies chondrites as among the oldest materials known in the solar system. The oldest terrestrial rocks are about 4 b.y., the oldest lunar rocks 4.4 Ga. In addition, during mineralogical transformations within their parent bodies (metamorphism), the chemical composition of chondrites has only been slightly changed. The formation age of chondrites then corresponds essentially t o the formation age of their parent bodies. The mineralogical transformation took place within the interval of 0.15 Ga, between 4.56 and 4.50 Ga. This period reflects the weak thermal activity which affected planetesimals soon after their accretion. ( accretion is a process in which the size of something gradually increases by steady addition of smaller parts) Dating of differentiated meteorites irons and basaltic chondrites defines the time when their parent bodies cooled after melting produced metal cores surrounded by silicate. The measurements yield a range of ages from 4.56 t o 4.45 Ga similar to that defined by chondrites. The volcanism that occurred on these bodies is very old: the lavas poured out on their surfaces, represented by eucrites, show it began between 4.56 and 4.53 Ga. Thus, thermal activity in the parent bodies of differentiated meteorites was contemporaneous with that which affected chondrit e parent bodies in a less intense manner. Long before clocks used for shocked meteorites generally define varied ages, usually younger than 4.5 Ga. They have been disturbed since they were originally started at 4.56 Ga. Some may have been reset to zero by impacts. This is the case for the potassiumargon (K-Ar) clock: the argon gas generated by the decay of potassium has a tendency to diffuse out of material heated by shock. The K-Ar age will then dat e the collision, which affected the parent body.
Th e age of th e e arth This becomes a subject of scientific debate in the 19th century. Lyell, Darwin (several hundred million years) Lord Kelvin (few tens of millions of years Rutherford in 1929:measured the quantity of helium produced in rocks by the decay of uranium gives a date of a billion years. Hubble (first astronomer to enter debate):age of universe about 2 Ga P at terson in 1955: by measuring the isot opic composition of lead, he shows that the earth and meteorites have the same age, 4.55 Ga. This is also the age of the solar system Today, precisely 4.566Ga. Our planet developed by accretion of planetesimals over the next hundred million years that followed plus differentiation into core, mantle and primitive atmosphere. S toppe d clocks The contraction of the parent cloud of dust and gas formed the solar nebula which became the solar system. The presolar material was no longer fed by radioactive elements (from stellar explosions) except by shock waves. Parent bodies formed shortly after the isolation of the solar nebula were able t o incorporate short period radioactive elements while they were still active. They thus decayed within this material. Comparisons between different classes of meteorites are possible if they initially contained the same amount of parent isot ope. Chemical elements formed within stars that exploded just before.
A Major discovery The first discovery of the decay of a short period radioactive element, iodine 129, in meteorites, happened in 1961. Since then, xenon-129 derived from iodine-129 has been found in most meteorites. This discovery indicated that all the meteorit e parent bodies were formed in a relatively short interval of time, at most 20 million years which is compatible with estimates derived from long period radioactive transformations. Thus, the age of the solar system was bracketed: it is between 4.55 and 4.57 b.y. In addition, this result showed that less than 150 million years elapsed between the last stellar event that generated iodine-129 and the formation of the parent bodies.
Birth date The birth of the solar system was rapid. We are able t o specify the dat e of this event by a combined reading of long period and short period clocks in material formed from the solar nebula. The most primitive carbonaceous chondrites contain refract ory inclusions, which were formed at high temperatures in the solar nebula. In 1976, the presence of Al26 which has a very short half-life (0.7 million years) was detected from the excess of Mg-26, the daughter element , in the feldspar of these inclusions. Al-26 was a very efficient heat source for heating planetesimals. Because of its short period, it must have been produced at most 3 m.y. before it s incorporation in the feldspar. This period of time was reduced t o less than 1 m.y. with the discovery, in 1995, of excess K-41 corresponding t o the radioactive decay of Ca-41 (with a half-life of 105,000 years). Moreover, dating the refract ory mat erial in carbonaceous chondrites with the uraniumlead clock defines a very precise age: 4.566 +- 2 m.y. The combination of these two chronometric data gives us the birth date of the solar system Ğ 4.566 m.y. The level of uncertainty on this number does not exceed 3 m.y.
Th e age of th e Un i verse Hubble established galaxies outside the one that contains the solsr system. Most show a light spectrum shifted t owards the red, which indicates that they are moving away. They move away at speeds proportional t o their distances. Originally incredibly dense and hot , the universe has cooled since by expanding. 3 methods allow us t o estimate the age of the Big Bang: - estimation of the oldest stars in our own galaxy. Answer is 14-17 b.y. - radioactive dating of chemical elements synthesized in stars in our galaxy based on measurements on the isot ope abundances in meteorites. Answer is 13-19 b.y. - measure the expansion by observing galaxies. Values vary between 50 Ğ 100 km/sec per million parsecs (parsec is a unit of dist ance equivalent to 3.26 light years. Answer is 8 Ğ12 b.y.
Time 0 is the birth date of the Solar System and is defined by refractory inclusions, the first objects in the solar system
GALACTIC FOSSILS --- When isotopes talk: abnormal isotopic anomalies were sought
in the 1970’s. Xenon & neon were known --- 1973: Allende had refractory inclusions rich in anomalous isotopic ratios
--- The chemistry of stars: matter ejected into space from dying stars condenses with trapped specific isotopic concentrations --- Isotopic anomalies: most spectacular in presolar grains --- First indications: Allende, then Murchison --- The origin of Elements
--- Stardust: first true grains isolated in 1987 --- How do stars evolve?
--- Red Giants and Supernovae 3 types of presolar grains rich in C, diamond, silicon carbide and graphite. Also, corundum and silicon nitride --- Isotopic analysis: ion probe
--- The interstellar medium --- a clock that has stopped: Mg-26 formed by decay of Al-26. Initially, Al-26 amount was large. Chronometer stopped before the arrival of grains in the solar nebula
GALAC TIC FO S S ILS W h e ni sotope s ta lk .. Until 1970 no isot opic differences between lunar, terrestrial and meteorite samples were detected. It was thought that original material was vaporized and mixed, thus losing every trace of its origin. Mass fractionations, like those caused by evaporation and condensation in the early solar system explained the small variations in certain isot opic ratios in meteorites. However, t o locate the source materials of the nebula itself, it was necessary t o detect abnormal isot opic abundances (isot opic anomalies) the products of processes occurring within stars, the giant fact ories for making the chemical elements. A few examples were known, notably of two noble gases, xenon and neon, but the carrier minerals of these gases remained t o be discovered. Finally in 1973 in the Allende meteorite, scientists identified refract ory inclusions rich in anomalous isot opic ratios; ratios close t o those that certain astrophysical models predict for stars.
e ch emi stry of stars During the Big Bang, in the initial furnace, the lightest chemical elements, hydrogen, helium and one isot ope of lithium, lithium-7 were produced. From the primordial hydrogen and helium the first generation of stars produced elements such as carbon and oxygen. At the end of their lives, stars, in particular the most massive ones, eject gas and dust int o the interstellar medium, and this material later serves t o form new stars. A large number of generations of stars must have succeeded one another before the formation of our sun, 4.566 b.y. ago. The mat ter ejected int o the interstellar medium by dying stars, red giants and supernovae, condenses int o solid grains, which trap the specific local isot opic composition. This is how we can identify stardust in meteorites: its anomalous isot opic signature is the sole criterion, which allows us to recognize it. This st ardust has survived the processes of destruction and re-growth operating during the formation of the solar system.
Isotopi c an m o ali e s Every deviation from the average isot opic composition of the solar system is an isot opic anomaly. The most spectacular anomalies are found in surviving presolar grains. Isot opic anomalies can result from the decay of short-lived isot opes in a grain that formed early in the solar system. More generally, they are created by stars, which are nuclear react ors. The isot ope oxygen-16 t o oxygen-18 ratio of the solar system is close t o 500, but that of presolar grains varies from 2.7 to 40,000. Certain refract ory inclusions in primitive meteorites contain an excess of oxygen-16 that could equally well be an indicat or of a process in a star older than the sun.
First i n di cati on s The first sample of which we can say that it contained presolar traces was extracted from the Al le n demeteorit e. The refract ory inclusions in this carbonaceous chondrite contain large isot opic anomalies in Ca and Ti, elements produced close t o the centre of massive stars, as well as in heavier elements, such as Ba, synthesized by the capture of neutrons in materials ejected by supernovae (r process). Later, hibonite, a very refract ory mineral, was isolated in the Murchison meteorite. It turned out that its bluish crystals contain an excess or deficiency of Ca-48 and Ti-50. The combined presence or absence of these two neutron-rich isot opes are signatures of two different types of supernova. Distinct grains coming from these two types of supernova must have been present in the early solar system and incorporated in these hibonites. However, refract ory inclusions and hibonite are not intact circumstellar condensates: they only incorporated isot opically anomalous stellar components at the moment of their formation in the early solar system.
Th e ori gi nof the eleme n ts Most of the chemical elements are made in stars by nuclear reactions, starting from hydrogen. A star has temperatures and particle concentrations favourable to nucleosynthesis. The first reactions lead t o the fusion of hydrogen int o helium, then helium int o carbon. Low mass stars do not go beyond this stage. In medium stars, get oxygen, neon. In the most massive stars produce elements up t o iron. As the latter has the most stable nucleus nuclear fusion st ops at this point. The heavier elements are produced either by rapid capture of neutrons in a high neutron density environment (r process that takes place in supernovae) or by slow capture at low density (s process, in red giants).
S tardu st Finally, in 1987, the first true pre-solar grains were isolated, after a 20-year hunt t o identify the carrier minerals of the isot opically anomalous rare gases. These minerals represent less than 1/1000of the matrix of carbonaceous chondrites but, by chance, they are extremely t ough. We can isolate them after t otal dissolution of the matrix containing them with very aggressive acids. This stardust condensed in stellar winds from red giants and in the gas ejected by stellar explosions. Generally, all the chemical elements present in these grains have isot opic compositions, which reflect the stellar environments they come from, and which are completely different from those of solar system material. These grains are samples of stars. Their isot opic and chemical compositions teach us about the processes of synthesis of at omic nuclei that take place in different types of star so as t o produce chemical elements with distinct isotopic ratios.
How do stars e volve ? All stars are born from pre-existing dust and gas, and die either in fantastic explosions or by cooling slowly when their internal nuclear fire goes out. Massive stars burn their nuclear fuel much quicker than low mass stars, because of higher internal T & P disappearing within a few million years. The mass of a star controls its lifetime. When the H of a medium mass star is used up, the outer part expands, transforming the star into a red giant. In the case of a massive star, fuel is exhausted after an iron core is produced and contraction starts. Formation of a hard neutron core st ops the collapse and in a rebound an enormous explosion is produced, ejecting the outer layers of the star; it becomes a supernova. During the explosion, a shock wave passes through and heats ejected material, resulting in so-called explosive nucleosynthesis. During red giant or supernova stages, the synthesized products are ejected int o the interstellar medium and grains condense. They constitute the seeds of other stars.
Re d gi an ts nd a su pern ovae Three types of presolar grains rich in C, diamond, silicon carbide and graphit e have been discovered thanks t o the anomalous rare gases they contain. These crystals cannot form in a medium rich in oxygen like our solar system. T wo other types, corundum (Al oxide) and silicon nitride, have been identified by isot opic analysis on the ion probe. With the exception of diamond, all these grains have sufficient size t o be individually analysed with this instrument. Their isot opic compositions reveal that these grains come principally from two kinds of stars: red giants, stars of relatively small mass at an advanced state of evolution, which swell and lose a large amount of material; and massive stars which explode as supernovae at the end of their short lives. The corundum grains and the majority of the silicon carbide grains seem to come from red giants. The range of isot opic ratios of O, Si and Ti indicate that a great number of different stars fed the solar system with dust grains. Most of the graphit e grains, 1 % of the silicon carbide grains and all the silicon nitride grains apparently come from supernovae. The simultaneous presence of all these isot opic anomalies in the grains proves that during the explosion of a supernova, the material ejected by the star experiences enormous t urbulent mixing.
Isotopic anlaysis Measurement s of isotopic ratios can be made using any property, which depends on at omic mass. For example, spectroscopic measurements of isotopes in stars are possible because the frequency of molecular vibrations depends on the exact mass of the at oms and the optical spectra emit ted by cool stars show distinct lines for different isotopes. In the lab isotopes are counted in a mass spectrometer. The ion probe is a mass spectrometer that does not require chemical separations before the sample is analysed. The minerals can therefore be analysed in situ in a rock which allows us to choose the best spots, and make comparisons from one point to another in the rock. In addition, the ion probe is capable of analysing sub-micron samples, and thus individual pre-solar grains.
The interstel lar me dium Contains low concentrations of gas and dust . New stars can be born when a dense region undergoes gravitational collapse. Al and Si are absent from the gas but trappedin dust grains. At the extremely low temperat ures of these clouds, chemical reactions are still possible and lead t o unusual isotopic ratios for H and N, if the molecules are ionized. Ionization is guaranteed because of cosmic rays, very energetic particles probably accelerated in supernovae, which traverse the medium. In the clouds we detect large excesses of deuterium, which correspond to what has been measured in primitive meteorites, HaileyÕs comet and certain dust grains, which show that the early solar system has incorporated material from molecular clouds.
A clock that has stoppe d The silicon carbide grains are rich in Mg-26, formed by radioactive decay of Al-26. The initial quantity of Al-26 in these grains was much greater than that in the refract ory inclusions in the Allende meteorite; the highly variable Al-26/Al-27 ratio can at tain values close to unity while in the refract ory inclusions it is only on the order of 1/10,000! However, the Al-26 was totally transformed t o Mg-26 and the chronometer was thus extinct well before the arrival of the grains in the solar nebula.
The oxygen isotopic ratios in pre-solar grains
Supernovae and red giants which came before the formation of the solar system seeded the interstellar medium with pre-solar grains
Carbon isotopic ratios in pre-solar graphite grains from the Murchison meteorite
Tagish lake meteorite: Yukon, Jan.18,2000 --- among most chemically primitive --- undergone most spectacular aqueous alteration of any C2 chondrite --- two isotopically distinct lithologies
A. Carbonate-Poor lithology phyllosilicate-rich clasts with rimmed chondrules, amorphous carbon. Less than 100’C
B Carbonate-Rich lithology phyllosilicates with siderite grains. More than 300 ‘ C
--- among the most pristine in the world --- Abundant organic material --- nanodiamonds: dust of other stars --- unusual isotopic ratios
TAGIS H LAKE
METEO RITE
Yukon , Jan.18, 2000 59Õ42 N, 134Õ12E
Explosion (unusually long fireball about 15 sec, but a ÔslowÕfireball, 10X brighter than daylight, bluish t o greenish) with sizzling sounds, 2 sonic booms with dust cloud, peculiar smells (sulphurous, also of hot metal or rock). Dust lighted strong by the rising sun (just below the horizon). Hundreds of witnesses, recorded at seismic stations About 4 m big, pre-atmospheric 56-115 t ons, approached at 16 km/sec at an angle 18Õup from horizon. Only 1.3 t ons fell to the ground ! About 97 % of the meteor burned up in the atmosphere! Approx. 200 samples collected in situ, some situated deep within the ice. Total recovered weight of this lightweight , friable meteorit e is 5 Ğ10 kg (only 0.1 % of material that fell down) In contact with water, it dissolves turning int o black mud, slash. Exploded at about 46 km up, with energy of 1.7 kT of TNT (about 1/12 of the energy of the 1st at omic bomb) Porosity of 37 %
Satellite data (possibly the largest meteor ever recorded by satellite sensors over land) plot ted its accurate orbit (first time for a carbonaceous meteorite). The aphelion was found t o lie in the outer asteroid belt, possibly from the Apollo asteroid group, in particular the low-albedo D type asteroids Moved in a direction of 151.5 Ô P re-entry orbit has been plot ted (only 4 other meteor orbits are known) The strewn field is at least 16 km by 5 km. Among the most chemically primitive meteorites known, with a significantly higher carbonate abundance than any other carbonaceous chondrite. It has undergone the most pervasive aqueous alteration of any C2 chondrit e. Contains water structurally bound in hydrated minerals, adsorbed ont o mineral surfaces and absorbed within the layers of smectite-group phyllosilicates. There are two isot opically distinct lithologies- a dominant carbonate-poor lithology and a subordinate carbonate-rich lithology.
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CARBONAT E-P OOR LIT HOLOGY
Composed of phyllosilicate-rich clasts with rimmed chondrules (mainly olivine) enclosing Fe-Ni inclusions, anhydrous forsteritic olivine, grains with enstatite composition, magnetite, sulfides (mostly pyrrhotite), minor phosphides, chromite, spinel-rich spherules. Enclosed by fine-grained dust mantles composed of phyllosilicates, FeNi sulfides, FeNi metal, magnetites, pyroxenes and olivines. On a nano to micro scale globules of pristine, amorphous carbon are present at testing t o a persistently cold environment since the formation before or during the formation of the solar system. All these compoanents are contained within dense, fine-grained matrix of mostly Mg-rich saponite and serpentine along with Fe-Ni sulfides. Only rare carbonates are present within this matrix in the form of polycrystalline calcium carbonate grains. Micropores are partially lined with these carbonates. In the proposed alteration sequence, which occurred at very low temperat ures (less than 100 ÔC) olivine and metal were replaced by phyllosilicates and pyrrotites. Subsequent oxidation conditions resulted in the replacement of pyrrotite by magnetite. This was followed by the replacement of magnetite, precipitation of sulfides and growth of carbonates. Thereafter, sulfur was incorporated int o organic phases.
CARBONAT E-RICH LIT HOLOGY Phyllosilicates, mostly saponite with very limited clasts, CAIs & magnetite. Whitish Fe-Mg-Ca-Mn carbonate grains are very abundant , very lit tle calium carbonate occurs on its own. The Fe-Mg-Mn carbonates, called siderite, likely replaced existing Ca carbonate grains through the percolation of fluids following impact fracturing. In the proposed alteration sequence, which occurred at higher temperatures (more than 300 ÔC) than the carbonate poor lithology, olivine and metal were replaced by phyllosilicates and magnetite which were subsequently replaced by Ca carbonat e. Following impact fracturing, siderit e replaced some Ca carbonates, and coarse sulfides were deposited. It may be assumed that the carbonate-rich lithology succeeded the carbonate-poor lithology. A high bulk carbon content of about 5 wt %, higher than CI chondrites and much higher than CM chondrites with about 2.5 wt % of this carbon incorporated in organic components. The water-soluble organic component is carboxylic and dicarboxylic acids. The remainder as PAHs having an aromatic structure. Nitriles with low H content. The lower abundance of heavier organics compared t o those in Murchison is evidence of lower alteration temperatures and lower degrees of chemical evolution on this asteroid. Furthermore, micron-sized globules of amorphous carbon imply that a very cold environment persisted over its entire hist ory (a first).
A bulk density of 1.67 g/cc, much lower than CIs or CMs. The carbonates display a wide range in compositions and show heavy oxygen isot ope enrichment consistent with a high water/rock ratio (about 2) more similar t o CIs than t o CMs. The anhydrous silicate abundance is similar t o CMs but much higher than in CIs. Phyllosilicates are mostly Mg-rich saponite in contrast t o the Fe-rich serpentines found in CMs. Sulfides are more abundant than in either CIs or CMs. However, a roset te morphology is found exclusively in T agish lake. T race element data especially observed in a plot of Zn/Mn vs. Sc/Mn indicate that it is a carbonaceous chondrite distinct from any others. It is unique among the CM and CI chondrites. The first representative of a new C2 carbonaceous chondrite class. It is the first spectral match t o the D asteroid class. Among the D class asteroids 368 Haidea provides the best match. A prime candidate for the oldest known object on earth. Among the most pristine in the world as the fragments were protected from contamination when they became wedged in blocks of lake ice.
Using electron microscopy and isot ope tests showed unusual ratios of different forms of N and H. ratios of the isot ope N15 t o N14 were nearly twice those on earth, while the ratio of deuterium to normal H was between 2.5 and 9 times higher than usual. These isot opes could only arise from chemical reactions taking place in an extremely cold climate where temperat ures were as low as Ğ260 ÔC (near absolute zero). These conditions would only be found in molecular clouds before the formation of the solar system. These are real time machines, the material goes back t o the earliest formation of the solar system. It contains carbon, myriad clay minerals, amino acids. The clay layers, mostly silicates, can form protective pockets around the organic chemicals and act as reaction chambers where more complex molecules can form. The possible role of these pockets in the ultimate emergence of life has lead some t o refer them as wombs. These things tell us what kind of chemicals are out there in interstellar space.
Hollow spheres or bubbles could have provided a protective envelope for the raw organic molecules needed for life. They are the step in the right direction for making a cell wall. The globules are empty but have organic molecules on their surface. There are billions of them in the meteorite (they are extremely small). It contains by far more t otal carbon than either CI or CMs. Estimated that between 22 % and 47 % of the carbon was derived from carbonate minerals. A large fraction of the t otal carbon (about 44 %) is contained in organic molecules. Currently it is believed that organic compounds in primitive met eorites either are compounds formed in the int erstellar medium, or remnants of those compounds modified during alteration on asteroids. The presence of abundant organic material implies that a portion of this meteor was never strongly heated in the nascent solar nebula. A portion is contained in nanodiamonds (a few micrometers in size) more than in any other meteorit e. Nanodiamonds form in the expanding shell of a type II supernova. Hence, they are the dust of other stars that traveled through the int erstellar medium to end up in our forming solar system. Since this contains more of these grains of stardust, then it likely formed farther out.
REFRACTORY INCLUSIONS IN METEORITES
--- 24 RIs found in ordinary chondrites --- A-type --- spinel-pyroxene type
--- Genetic relationship between RIs and chondrules First chondrules & Ris formed 4.567 b.y. Transported to 1-4 AU
--- Ca-Al-rich inclusions: oldest solids in our solar system
Laboratory of Space Sciences, Washington, Univ. in St. Louis ---- Pre-solar Grain study
--- Inter-planetary dust particles --- meteorite geochemistry --- noble gas studies METEORITES CARRY ANCIENT CARBON --- primitive organic materials found in meteorites --- largely unaltered since solar system formed --- comets and asteroids (parent bodies) are similar in origin
--- remnants of precursor organic molecules formed in the interstellar medium before the solar system existed
REFRAC TO RY INC LUSIO NS in meteorites 24 R.I. ( 40 Ğ 230 mew.m) were found by X-ray mapping of 18 ordinary chondrites. All inclusions were heavily altered, consisting of fine-grained feldspathoids, spinel and Capyroxene with minor ilmenite. The presence of feldspathoids and lack of melilite are due t o alteration that t ook place under oxidizing conditions as indicated by FeO-ZnO-rich spinel and ilmenite. The pre-altered mineral assemblages are dominated by two types: one rich in melilit e, referred to as type A-like, and the other rich in spinel, referred t o as spinel-pyroxene inclusions. This study and previous data show similar type and size distributions of RI in ordinary and enstatite chondrites. A survey of RI on Allende and Murchison showed the predominant were type A and spine-pyroxene with average sizes of 170 new m in Allende and 150 in Murchison. The relatively large sizes are due t o common conglomerating of smaller nodules in both chondrites. Therefore, RIs had primarily formed under similar processes and conditions and were transported to different chondrite-accreting regions. Later alt erations were due t o secondary reactions during transportation
Genetic relationship between RIs and chondrules The first chondrules and RIs formed about 4,567 Ma ago during multiple evaporation, condensation and melting. P rob. formed in close proximity to the proto-sun at the inner edge of the accretion disk and subsequently transported t o 1-4 AU where chondrule formation and accretion of chondrit e parent asteroids t ook place. The formation of CAIs occurred within 0.5 M.year, but the formation of chondrules lasted for several million years. Most chondrules formed by melting of silicates, Fe-Ni-metal, and RIs mixed in various proportions. Some RIs were re-melted in the chondrule-forming regions with or without addition of chondrule material. Ca-Al-rich inclusions (CAIs) are mm-sized refractory objects commonly found in chondritic meteorites and are the oldest solids formed in our solar system. P rimary CAI formation may have occurred through condensation and evaporation processes near the P rot o-Sun, or alternatively, during localized event s in the asteroid belt . These objects provide us with a unique window int o the earliest development of the Sun and int o the evolution of the proto-planetary disk. A 26Al/ 26 Mg isochron for bulk CAIs from 4 CV carbonaceous chondrites which yield an initial 26 Al / 27 Al of 5.85 x 10 minus 5 and this suggests that primary formation of the CV CAIs may have occurred within an interval of just 20,000 years.
The fall of the Allende type 3 carbonaceous chondrite in Chihuahua on Febr. 8, 1969 marked the beginning of discoveries, which caused an unprecedented explosive increase in our knowledge of physical ad chemical processes that occurred at the birth of the solar system. P rior t o it s fall, studies of carbonaceous chondrites were severely limited by the rarity, small sizes and small sample sizes. While labs were waiting for the lunar samples t o arrive suddenly several t ons of a single carbonaceous meteorite were available for study. Most of the observations including mineral assemblages, mineral chemistry, texture, bulk compositions, O isot opic compositions, and REE pat terns of the Sahara inclusions suggest a common reservoir of RIs in enstatite, ordinary and carbonaceous chondrites. Chondrules and Ca-Al-rich inclusions (CAIs) are preserved materials from the early hist ory of the solar system, where they resulted from thermal processing of pre-existing solids during flash heating episodes which lasted for several million years. CAIs are believed t o have formed about 2 million years before the chondrules.
Laboratory of Space S ci e n ce s,W ash i n gt on Univ. in St. Lou i s Space Sciences, the study of the universe and our relationship t o it includes the formation of the solar system (carried out by ge ologi sts), measurements of isot opes in meteorites ( by ch emi sts) and a study of supernovae explosions (by th e ore ti cal ph ysi cists ). Some of the studies undertaken include : - P re-solar grains: isolation of grains of graphite, silicon carbide, aluminium oxide and silicon nit ride - Inter-planetary dust particles (IDP s) are among the most primitive materials in the solar system. Large enrichments of deuterium relative t o hydrogen have been found and are int erpreted as due t o material formed in chemical reactions in molecular clouds - meteorite geochemistry; trace element and isot opic compositions of meteorites. The trace elements and particularly the rare-earth elements (REE) are sensitive indicat ors of igneous differentiation processes as they typically partition strongly int o either the liquid or crystal phase of a magma system. - Noble gas studies; studies of isot opes in rare gases from nuclear reactions.
Pre-solar Grain Study: The morphology and surface properties of silicon carbide (SiC) grains gently isolated from the parent meteorites. They were first found in acid resistant residues of carbonaceous chondrites. The crystalline grains appearcorroded and their properties prior to chemical etching were unknown. T his issue resolved when a new Xray mapping technique was developed and it locat ed the grains on polished sections. Surprisingly, the grains were found to be isolated entities distributed at random in the fine gained meteorite matrix. High-resolution scanning electron images show distinct morphologies. Some are angular, most are rounded indicating various degrees of erosion. This rounding could be done by oxygen chemical at tach in the solar nebula or possibly by supernova shock wave in interstellar space.
METEO RITES C ARRYANCIENT C ARBON Meteorites contain some of the most primitive stuff of life. They are packedwith ancient carbon-rich, organic molecules. Until now, it was thought such mat ter, which formed before the solar system came int o existence, could only be found in interstellar dust (the object of missions like St ardust). Instead, primitive organic materials, essentially unaltered components of the original building blocks of the solar system can be found in meteorites. Researchers looked at the relative proportions of isot opes of N and H in organic mat ter of carbonaceous chondrites. They found regions where there were excesses of the heavier forms of these elements, something also found in interstellar dust grains. Therefore, the meteorites contained material that had been largely unaltered since the time when the solar system was formed from the collapse of a giant cloud of dust and gas, called the solar nebula.
It is amazing that pristine organic molecules associated with these isot opes were able to survive the harsh and t umultuous conditions present. It means that the parent bodies Ğ the comets and asteroids Ğ of these seemingly different types of extraterrestrial material are more similar in origin than previously believed. Before we could only explore minute samples from dust particles (IDP s). Our discovery now allows us t o extract large amounts of this material from meteorites, which are large and contain several percent carbon. We now (2006) have the potential to study pre-solar organic molecules in the lab. We are looking at the remnants of precursor organic molecules formed in the int erstellar medium before the solar system even existed, embedded in a complex that formed at a later time. Many pre-solar grains show excesses in 26 Mg due t o decay of the radioisot ope 26 Al (half life= 730,000 years). Low-density graphit e grains, SiC grains of type X and silicon nitride originated in the ejecta of supernova.
REFERENCES Lin, Kimura, et al, 2006, P etrographic comparison of R.I. from different chemical groups of chondrites Russell et al, 2007, The genetic relationship between Refract ory Inclusions and Chondrules, Chondrites and the P rot o-planetary Disk, Vol. 341, p.317 Thrane, et al, 2006, Extremely Brief formation int erval for RI and uniform distribution of 26 Al in the Early Solar System Lin, Kimura, et al, 2003, Unusually abundant RIs from Sahara 97159 (EH3) www.presolar.wustl.edu
This silicon carbide pre -solar grain probably comes from a red giant rich in carbon.
CHAPTER
8
Meteorites and Extinctions
Meteorite Impacts & Mass Extinction Events • • • •
Impacts & Mass Extinctions 25 extinction pulses in last 545 m.y. 5 major extinctions may be related to impacts Possible threshold of 100 - 150 km crater diameter • 7 extinction peaks correlated with stratigraphic impact markers
Impact and Extinction Events • • • • •
Impact signatures Shocked minerals Impact glass Microspherules Ni-rich spinels
Extinction Peaks • • • • • •
Pliocene, 2.3 m.y. Late Eocene, 35 m.y. Cretaceous / Tertiary, 65 m.y. Jurassic / Cretaceous, 144 m.y. Late Triassic, 201 m.y. Late Devonian, 365 m.y.
• • • • • • •
Periodicity in the Mass Extinction Record Evidence for periodic impact pulses Graph: Species Kill v. Age Graph: Crater diameter v. % Extinction Table: Stratigraphic Evidence of impact debris Table: Large dated impact craters Table: Impact craters
IMPAC TS and MASS EXTINC TIO NS Collisions with large bodies could be sufficient to explain the record of about 25 extinction pulses in the last 545 m.y. The same evidence implies that the 5 major mass extinctions are related to impacts of the largest bodies (more or equal to 10 km in diameter, more than 10 t o the eighth Mega t ons TNT events). T ests of Òkill curveÓ relationships for impact-induced extinctions suggest that a possible threshold for catastrophic global extinction pulses occurs with impacts that produce craters with diameters between 100 Ğ150 km. Seven of the recognized extinction peaks have thus far been correlated with concurrent stratigraphic impact markers and / or large impact craters. Statistical studies suggest a periodic component of about 30 m.y. that is associated with extinction events. These results could be explained by periodic showers of Oort cloud comets. The pacemaker for such comet showers may involve the SunÕs vertical oscillation through the galactic disk, with a similar cycle time between crossings of the galactic plane.
Introducti on Extinctions are periodic; might have a common cause; a 30 my periodic component; relation t o comet showers with solar system going through the central plane of Galaxy; flat distribution of clouds in galactic disk suggests periodic encounters with period of time equal to plane crossings. Last crossing was about 1 million years ago. Extinction event in the Miocene, 2.3 m.y. ago. Mean plane crossing period is about 44 m.y. and the peak to trough ratio in the comet flux is about 2.5 to 1. Good reasons t o believe that largest impact ors are comets, then largest craters should show the galactic modulations of comet flux. P ulses of increased comet flux could explain the stepped nature of some extinction events. Impact-exti nction hypothe si s We can estimate the expected times between collisions of bodies of various sizes with the earth based on observations of earth-crossing asteroids and comets and the cratering record of the inner planets.
Impacts an d exti n cti on e ve n ts: Evi de n ce One direction of research is the search for impact signatures at geologic boundaries marked by faunal changes. Among the materials considered diagnostic of impact are - shocked minerals (including shocked quartz, stishovite, zircon) - impact glass (microtektites/tektites) - microspherules with structures indicating high-temperature origin - Ni-rich spinels Shocked minerals and tektit e glass are quite rare in the geologic record and yet these materials have been reported in stratigraphic horizons close t o the times of six recorded extinction pulses. Thus far, 6 of the approx. 25 extinction peaks in Fig.1 - Pliocene 2.3 m.y. - Late Eocene 35 m.y. - Cretaceous / T ertiary 65 m.y. - Jurassic / Cretaceous 144 m.y. - Late Triassic 201 m.y. - Late Devonian 365 m.y. are associated with large impact craters and / or some form of stratigraphic evidence of impacts. Significant statistical correlation between impacts and extinctions has been demonstrated for the major biotic events of the last 250 m.y. Clusters of impacts could explain the difficulty in global correlation of some boundaries. For example, the Jurassic / Cretaceous boundary does not yet have as internationally accepted definition. The large Morokweng structure in South Africa (145 +-2) and the smaller Gosses Bluff in Australia (142.5+-0.5) are dated in the same J/K boundary interval.
Peri odici tyin th e Mass Exti n cti on re cord A 26.4 m.y. periodicity in extinction time series for the last 250 m.y. has been reported, also a 30 m.y. The int erpretation of these results has been a subject of considerable debate. The extinction record is likely t o be a mixture of periodic and random events. The 3 most severe mass extinctions ( 435, 250 & 65 m.y.) are separated by 180 m.y. The solar system undergoes a perigalactic revolution cycle with an estimated period of 170 m.y. This cycle may also modulate the flux of Oort cloud comets. Evi de n cefor peri odicimpact pulse s The original report of periodicity in extinction events (1984)found evidence for a possible 28 Ğ 32 m.y. period in impact crater ages. Others have argued however, that the number of well-dated craters is t oo small t o extract a consistent statistically significant periodicity.
C on clu si on s Only recently have we begun t o appreciate the importance of comet and asteroid impacts in earth hist ory. A further link between catastrophic impact events on the earth and the cycles of the Milky Way galaxy would represent a significant connection of terrestrial biological evolution with the larger astrophysical environment . The geologic data on mass extinctions and evidence of large impacts are thus consistent with a periodic modulation of the flux of Oort cloud comets with a mean period of about 30-36 m.y. as predicted by galactic models. REFERENCES Rampino, 1999, Impact crises, mass extinctions and galactic dynamics
Craters in Canada & Major Extinctions in the Past • 11,000 YBP • New Quebec, Haughton, Wanapitei, Mistastin, Montagnais • Chicxulub, 65 m.y. • Eagle Butte, Maple Creek, Steen river, Deep Bay, West Hawk,Carswell, Viewfield • 203 m.y. • Manicouagan, Saint Martin • 250 m.y.
Craters in Canada & Extinction Events (cont.) • • • • •
Gow,Clearwater, Ile Rouleau 355 m.y. Charlevoix, Elbow, Nicholson, Couture 435 m.y. Pilot, Slate Isles, Brent, Presquile,Holleford, Sudbury
WORST EXTINCT IONS 65 m.y.
Chicxulub
(Sepkoski, U. of Chicago)
70 % of marine families
end of T riassic Ğ 203 - 214 m.y.
lavas
18 % of vertebrate families
22 % of marine families
Manicouagan, St. Martin, Rochechouart 250 m.y.
asteroid lavas
364 m.y. ( others say 355) 439 m.y. (others say 435) 488 m.y.
95 % of all species 70 % of land 22 % of marine families
glaciation
25 % of marine families trilobites
EXT INCTIONS : T HE CRETACEOUS PARK The palaeont ological facts: 80 % of plankt on died along with ammonites, belemnites and rudists Animals bigger than 25 kg The last fossil ammonites & dinosaur footprints were found in the last few meters of the Mesozoic sediment s An important but ambiguous tracer : Iridium The t otal amount of iridium at the K-T boundary Ğ 500,000 t ons Ğ can only be explained by an exceptional supply of extra-t errestrial mat erial Shocke quartz grains with very fine lamellae of amorphous (non-crystalline) silica, which is very rare in nat ure We can only produce such lamellae by a very rapid pressure change Ğ to a pressure 10,000times atmospheric pressure The minerals talk : only a shock wave moving quicker than sound can cause deformation in the crystal lattice of quartz Nickel-bearing magnetite : formation requires melting at more than 1,300 ÔC Under extremely oxidizing conditions 500 million Hiroshima bombs fractured rock (breccias) soot means combustion of forests
a Òheat waveÓ
widespread layer of melt spheres has same age as impact melt spherule layer, up t o 100 cm thick near the crater, is only 6 cm in N. Jersey from reptiles t o mammals
Example: The Acraman-Bunyeroo Impact event, Australia • 580 m.y. • Evidence from Isotope & biomarker chemostratigraphy • First terrestrial event linked with distal ejecta • Satellite thermal infrared evidence • Diagenesis of ejecta horizon • Central uplift & magnetic anomaly
The ACRAMAN Ğ BUNYEROO Impact event (Ediacaran)
ch. 8
About 580 m.y., complex impact structure, 3 - 5 km of denudation since then Estimated impact energy of 5.2 X 10 to the 6th Mt (T NT) exceeds the threshold of 10 t o the 6th nominally set for global catastrophe. Evidence from isot ope & biomarker chemostratigraphy indicates major biotic change (two independent lines of enquiry on the impact site & ejecta horizon) A monomictic layer of fragmental red felsic volcanic rocks discovered in the Flinders Ranges, up t o 540 km away. They were ejecta by identifying P DFs in quart z grains from the volcanic clast. Four different sets of lamellae in the same grain that indicated shock pressures of up to 15 GP a for samples of both Acraman and the ejecta. For the first time a large terrestrial impact event has been linked with a distal ejecta horizon.
Satellite thermal infrared images revealed Acraman as a conspicuous ringed structure. Ejecta anomalous in Ir, Pt, Pd, Ru & Cr. Impact or had a chondritic composition. Impact or was 4.7 km in diameter and a density of 3500 kg/m3. Diagenesis had greatly altered the ejecta horizon. The clay fraction includes vermiculit e and kaolinite that probably were derived from alteration of glassy constituents. The Ediacarian acritarch diversification may be a recovery event following the Acraman impact. May have implications for the subsequent radiation of the Metazoa. Decrease in carbon isot ope ratios A four t o nine-fold drop in sterane/hopane ratios may indicate a sharp decline in marine algal productivity. The about 20 km-diameter circular magnetic low may mark the central uplift at the present level of erosion. A transient cavity up t o 40 km in diameter and a possible final rim of 85-90 km Arcuate features at 150 km diameter may mark disturbance beyond the final rim (a similar disturbance at 150 km occurs around the Manicouagan crater)
CHAPTER
9
Meteorites and Craters in Manitoba & Canada
SUMMARY ----------------------------------METEORITES AND METEORITE CRATERS IN MANITOBA Prairie Meteorite Search
Saint Martin meteorite crater : “Crater Chains” West Hawk meteorite crater
Other possible meteorite craters IMPACT STRUCTURES IN CANADA
P RAIRIE METEORITE SEARCH Red Deer Hill meteor shower : 30 fragments ~ 25 kg area 7 km long, 5 km wide The most numerous meteorite ÒfindÓin Canada The most numerous meteorite strewnfield in Sask. Study of radioactive nuclides during exposure to cosmic rays Delaire lake, Annaheim, Sask. : Abundant metal grains, must be H chondrite Site of another meteorite in 1914 Wilfrid Kunz 14th for Sask. Melvin Christensen : Kyle, Sask. Has a hole right through, the size of a quarter fusion crust : had melted 25 % Nibearing metals a very unusual texture of coarse fragments mixed with a fine grained melted matrix Wayne Langlois discovered in Churchill in 1955 Iron 72nd recovered in Canada
Elm Creek, man. 8.2.kg : second largest st ony ÒfindÓ Manit obaÕs largest by Tom Wood, 2001 Largest is Vulcan, Alberta Gilbert Plains, 200g Iron Group IIIAB (Steve Kissin) 4th for Manit oba 60th for Canada
7th of its type in Canada
Derek Erstelle was on a disability Bernic Lake Iron 6th for Manit oba, 64th for Canada Second Iron { Meteorites fetch $ 3 -4 per gram, but $ 10 for Irons} april 2004: Bernic Lake Erstelle 5.5 kg, + 4.3 kg
TWO !
June 2004 : Albert a, Belly River But tes fusion crust st ony 1.5 kg 15th for Alberta July 2005 : Erstelle again ! Pinawa 2.5 kg Sept. 2005 : Lake Eliza st ony 16th for Alberta Dec.2005 :Erstelle Lone Island lake rusty met eorite ~ 5 kg using a metal detect or & binoculars Iron Dec.2006 : 5th for BC found on a gravel road Eipper Iron 100 gr Aug.2007 : Loreburn, Sask. Iron (a very nice one!)
15th for Sask. 74th for Canada
ÒCRAT ER
CHAINSÓ 214 M.Y. ago
Imput Òshowered debris over BritainÓ No Atlantic ocean then. N. America & Europe were closer Rock found near Brist ol Evidence of shock wave carrying molten rock & dust that has left a thin layer of glass beads Shat tered mineral grains across ancient Britain 5 km big generated shockwave 40 million times Hiroshima bomb tiny green balls embedded in pinky-colored rock : was known, but could not been explained for 20 years but then they saw similar deposits from the crat er in Mexico. Rained hot dust & rocks
Rock in Canada hit normal rock, not salt or gypsum t o release poisonous gases A crater chain ( 5 craters) on two continents point to an impact at end of Triassic may have caused mass extinction ( one of the 5 greatest, with 80 % of species) Ar / Ar dating of glass Rochechouart, manicouagan, St. martin located at 22.8 ÔN, a 5,000 km chain of craters + Obolon (15 km), Ukraine, Red Wing, N. Dakota : parallel t o the 22.8 ÒN line most of the earth at that latitude was ocean St. Martin : 219 + /- 32 m.y. (uncertainty) St. Martin was equidistant from Manicouagan as was Rochechouart : A remarkable 4.462 km End of Triassic Extinction : ~ 202 m.y. wiped out dominant reptiles, so dinosaurs t ook over next extinction was 220 m.y. Manicouagan : at the center of the 5 pieces (craters)
it is the largest of the 5
WEST 2.44 km
HAWK LAKE (lake is 3.6. km )
deepest lake in Manit oba ? : 112 m water famous for its clarity soft sediment : 94 m water is cold, nutrient-poor, no algae badly broken Ğup breccia lens : 330 m usually this lens is eroded away in old craters fractured bedrock : 200 m with minerals from hydrothermal solutions (zeolite, zoisite) shock metamorphic effects, P DFs in quartz, many glassy & melted fragments hills in the surroundings may be part of the raised rim a single crater gravity low over crater :
shows there is a deep hole
Impact Structures in Canada • • • • • •
The terrestrial cratering record Spatial distribution Temporal distribution & Size distribution Formation of craters Shock metamorphism in the stratigraphic record Morphology, petrography, geochemistry, geophysics, age
Tables • • • •
Petrography of Canadian Meteorite craters Geochemistry of Canadian Meteorite craters Geophysics of Canadian Meteorite craters Age calculations on Canadian Meteorite craters
CRATER
BRECCIAS/ MELTROCKS SHATTER C. PDFs GPa MICROPHOTOGRAPHS
Brent
clastic
Carswell
dike-like
30 m dikes
Charlevoix Clearwater
yes
qtz
23
clouded feldspar
qtz
20
acicular feldspar
limestone/gneiss qtz,felds 23 swept by melt 15 m
poor
qtz
Couture
qtz
Deep Bay
suevitic
qtz
Eagle Butte
breccias
qtz
Elbow
breccias
Gow
clastic
yes
Haughton
90 m
porous
Holleford
breccias suevitic
Manicouagan
suevitic
limestone
coesite? fine-grained
suevitic
230 m
Montagnais
suevitic
New Quebec
columnar
poor
vesicular
suevitic
poor
Pilot
60
qtz
10
spinifex diopside, calcite globules quartz clast
qtz quartz clasts
qtz,felds.
maskelynite, checkerboard felds.
qtz
quartz grains
qtz
poikilitic pyroxene, maskelynite
qtz, felds.
feldspar laths
qtz
ballen quartz, glass coronae
qtz
crypto to micro-crystalline matrix
meta-basalt
Saint Martin
carbonate
Slate Isles
dikes 120 m
Sudbury
65 m
qtz,felds. basalt
vesicular
SIC
quartzite
Viewfield
qtz
yes 320 m
toasted quartz, highly zoned felds.
25 quartz with PFs
qtz,felds.
fine grained melt rocks
qtz
granophyric, micrographic
qtz
Wanapitei West Hawk
qtz
qtz,microcl. acicular pyroxene
Presquile
Steen River
quartz grain quartz grain
qtz
Maple Creek Mistastin
15
qtz
dolomite
Lamoinerie
toasted quartz quartz clast
qtz
Ile Rouleau
Nicholson
35
quartz
16
qtz
ballen quartz, flow texture
qtz
PFs wider space
Notes on the geology of Canadian impact craters
GEOCHEMIST RYOF CANADIAN IMPACT ST RUCT URES Notes on impact craters with geochemical data Brent
Enrichment in K, siderophiles, chondritic contamination
Carswell
No enrichment of siderophiles
Charlevoix
Decarbonation, Ni enrichment
Clearwater
Alt eration of impact rocks, enrichment of siderophiles
Gow
Enichment of siderophiles
Haught on
Shocked carbonate enriched in 13C & depleted in 18 O
Lamoinerie Manicouagan Mistastin Montagnais New Quebec Nicholson
No anomalies Some alteration, probable Ir anomaly Impact rocks have higher SiO2 & K2O, minor Ir enrichment Minor Ir enrichment Enrichment of siderophiles & Rh. Chondrite impact or Enrichment of siderophiles. Olivine-rich achondrit e impact or
Saint Martin No enrichments. Achondrite impactor. Volatilization of carbonates Slate Isles
No enrichments
Steen River
Alt eration of impact rocks
Sud bury Enrichment of SiO2, K2O, depletion of CaO, REE Wanapitei Enrichment of SiO2, siderophiles. Achondrite impact or West Hawk
No conclusions
Data from Canadian Impact structures
Age calculations • • • • • •
K - Ar in melt rocks 40 Ar - 39 Ar Fission track age from apatite Fission track of glass Rb - Sr U - Pb in zircon in melt rocks
SUDBURY WorldÕs largest Ni-Cu deposits over 15 m. t ons of Ni-Cu mined Long controversy. Very complicated. Large & complex hist ory Structure was circular, distorted by later deformation int o an ellipse At boundary between Superior province & Southern P rov. Of the Canadian Shield Evidence: ring fractures, shat ter cones, breccias, shocked minerals Sud bury Igneous Complex (SIC) + Ni-Cu was int ruded after impact : Now Ring-shaped Elliptical ring, 5 km thick, 25 km wide, 60 km across T wo types of breccias : Sud bury Breccia : Mixing of material, violent event Footwall Breccia : in thin sheets, mixture of rocks plus Ni-Cu In outer border of SIC Breccias have matrix of rock flour weakly recrystallized + fragments Inner border is Onaping Formation : granites extruded from magma chamber below Sud bury & was re-melted & released t o the surface like a volcanic rock Ğmelt rock with melted glass, pulverized target rock
Events - impact shock melting of t arget rocks, ejection of melt rocks + shat tered fragments + rock flour - minutes after floor of crater rebounds. Melt rocks around the crater. Rock, glass, dust blown int o the strat osphere. Footwall Breccia just exterior of the central lake of melt rock - hours later debris rains to form Onaping Formation. Magma chamber below crater is released & leaks t owards surface - later
sediments accumulate in the Basin
- much later geological activity along major fault zones separating provinces dist orts shape int o an ellipse
250 km shat ter cones ( up to 3 m long)
P DFs in quartz, feldspar, zircons
brecciation up to 80 km away multi Ğring core t oday is an OVAL impact melt sheet (SIC)
60 km X 30 km elliptical containing a 2.5 km thick layer of
transient crater was 10o km, 30 m deep in 10 minutes crat er rebounded 4 km of erosion obliterated raised rim collision of continents & folding dist orted crater shape metals are within a 2oo m thick impact melts isot ope analysis of metals show they come from the earthÕs crust , not the meteorite silicates & sulfides did not mix, easier t o mine separation int o layers (like oil and vinegar, with different densities)
SLAT E ISLAND 32 km shat ter cones up t o 10 m high : Largest on Earth !!!
a 20 m circumference at its base
impact glasses & breccias P DF in quartz Island: heavily eroded central peak
complex crater
Breccia has black color of a volcanic rock Water > 125 m deep in crater, surrounded by a rim with water At 25 m depth : a RING
WANAP IT EI Lake shaped like an ice-cream cone 37 m.y. 3 Ğ 7.5 km,
under study
lake is deep > 100 m with no islands impact breccia, coesite, glassy rocks a simple crater
LAC LA MOINIERE 8 km 400 m.y. east of New Quebec highly eroded complex islands are remains of central peak protected by later sedimentary rocks (found in crater) first land plants, first shark : at that time
LAC
COUT URE
8 km 430 m.y. near New Quebec rim eroded away 120 m deep with 25 m high central peak island Ğ free first life on land at the time
CLEARWAT ER WEST 290 m.y.
36 km
shock metamorphic : P DFs in quartz
shat ter cones K / Ar dates on melt rocks Classic example of impact of a contact BINARY Asteroid : RARE PHENOMENON Largest crater has prominent ring of islands with a diameter of 10 km (maybe uplifted rocks of central peak) Central peak is submerged Melt rocks are 100 m thick, under is 10 m layer of breccia Under the crater : 300 m fractured rock Common elevated rim t o both craters EAST Rb Ğ Sr dating of melt rocks
26 km
P DFs in quartz, no shat ter cones
A complex crater Melt t contains Iridium, Osmium,
therefore a CI chondrite
Central peak is submerged, can be seen from the air
MANICOUAGAN Shat ter cones rare, P DFs in feldspar, quartz 214 m.y. from melt rocks a complex, multi Ğ ring original diameter was 3 X ring lake erosion has removed 1 km of rock inner plateau made up of igneous / metamorphic along with a melt sheet, resistamt t o erosion melt sheet is 100 m thick transient crater 9 km deep outer disturbed zone : 150 km diameter inner fract ure zone :
100 km
Ò
annular moat
65 km
Ò
inner plateau
56 km
Ò
central uplift
25 km
Ò
400 m high, a bit displaced Mont Babel
Astronauts : one of the most conspicuous structure seen on earth
CHARLEVOIX, QUEBEC 342 m.y.
54 km
St . Lawrence River shat ter cones, PDFs
complex, multi-ring with central uplift transient crater 28 km, 10 km deep post impact crater collapse created central peak & peripheral modifications erosion has removed original crater rim, some of the central uplift and the crater-filled products, but the crater floor has been preserved preserved : terraces : created from the initial crater collapse & modification annular peripheral trough The central uplift consists of :
Annular plateau Inner ring of hills Inner valley Central peak : Monts des Eboulements, 780 m elevation
At the time, America was colliding with Europe / Africa The impact area is the most active earthquake zone in Eastern Canada because of fractured rock under impact St. Lawrence River is a rift , a break
UP HEAVAL DOME
UTAH
The internal structures of the central uplifts of impact craters are the most complex geologic features in the crust Geometry of central uplift High resolution mapping Inward & upward convergent flow of the crater flow during gravity-driven collapse of the transient crater cavity Original crater was 7 Ğ8 km
Now : 5 km
Dome was buried 2 km deep when impact t ook place Small t o medium complex crater Radial folds & stacking of imbricated thrust slices are prominent deformation features Fault planes are curved
CHAPTER
10
Survey of Notable Craters & Meteorites around the World
Other Notable Craters & Meteorites SUMMARY • • • • • • •
Tunguska Sound, light, tremor, forest devastation effects Sikhote-Atlin : the “queen of Irons” Popigai Ries Bosumtwi & Ivory Coast Tektite field Morokweng
SUMMARY cont. • • • • •
Vredefort Campo del Cielo Wolfe Creek Henbury Crater field Araguainha Peak-Ring structure, Brazil: shallow erosion, concentric rings
TUN G USKA SOUND EFFECT S Extraordinary underground roar like the sound of a number of trains passing t ogether over rails and then for 5 Ğ 6 min followed a sound like artillery fire. Between 50 Ğ 60 bangs becoming gradually faint er, the bangs were heard as soon as the t ongue of flame disappeared,the place where the fiery body had disappeareda t ongue of fire appearedfollowed by smoke 3 or 4 individual powerful bangs Violent wind blast The noise deafened one person and the shock caused him to suffer a long illness Strong acoustical phenomena 10 violent explosions great deal of noise & the forest around them blazing loud rumble, explosions were heard everywhere, the earth trembled & there was a loud crackling noise like gunfire last crash was the loudest the last crash was caused by the explosion following the end of the meteoriteÕs journey two bangs were heard & then a very loud bang followed by vibration the thunder was terrible, but the reindeer showed no agitation as they do during an ordinary st orm thunder became louder and louder finally there was a terrible crash and a small earthquake the thunder continued for 20 min counted 14 bangs a simple sharp bang so loud that one of the workmen fell into the water a fiery loud crash like thunder followed by a prolonged roar & a second fainter crash for at least 15 min 7:17 :11 sec a.m. 30 June 1908 from SE to NW unusual noise, frightful roar, deafening crash, loud crash, not like thunder but as if from the fall of large st ones or from gun fire 2 loud explosions, 5 Ğ 7 min later a second crash with a similar roar a fearful crash followed by a subterranean shock which caused buildings t o tremble
LIGHT EFFECTS
seen over radius of 600 Ğ 1000 km
by Evenki people
A flying star with a radiant body Shining very brightly, bluish-white light, when the flying object t ouched the horizon a huge flame shot up that cut the sky in two, the glow was so strong for 1 min extraordinary luminous effects observed in the atmosphere during the following days pillars of fire, a ball of fire strongest luminous & mechanical effects to the SE of place of fall Large cloud of black smoke, a cloud of ash disappearedby 2 Ğ 3 pm Smoke & fog from the burning falling trees The pillar of fire was seen by many, but a far greater number heard the bangs Strong earth tremor, horses & people were known t o have fallen, windows broken 700 km away the train driver was so frightened by the roar and the air vibrations that he st opped the train, thinking that it had gone off the rails his tent was carried away. T ent blown up int o the air t ogether with the occupants two lost consciousness water broke out from the earth in the flattened forest at one spot a pit was formed from which a stream flowed into the river Chambe
Forest radially flat tened A lake known as Cheko existed before the meteorite fell Thrown aside by a powerful jolt Ground shook & incredible prolonged roaring was heard from 7.15 t o 8.00 am As if a strong wind rocked him t owards the trunk A kind of swell came up the river T remendous roar for up to 20 min (depending on location of witness) Ending to a violent explosion accompanied by a pillar of fire and earth tremors 3 earthquakes air waves traveled twice around the world reached England 6 hours later air waves traveled at 318 m / sec at an altitude of 5.3 km optical phenomena in the atmosphere unusually bright nights over Asia / Europe for 10 days June 30 t o July 1st : no night at all !! One could read the newspapers at night
SIK HOTE - AL IN mountains in T aiga village of P aseko (bee garden) T HE HEAVIEST MET EORITE T HAT HAS BEEN OBSERVED TO FALL T HE QUEEN OF IRONS Massive impact event Febr. 12, 1947 10.30 am Fireball brighter than the sun 41 Ô deafening sound, up t o 300 km away Smoke train was 32 km long (painting) 14 km / sec largest mass broke up in a violent explosion meteor broke up violently or blasted apart during impact pre 900 t ons post 100 t ons some say 23 t ons recovered { Barringer was 25 t ons} coarse Iron octahedrite 5.9 % Ni 0.4 % Co + troilit e, chromite
0.4 % P
0.3 % S
Class IIB strewn field 2.1 km X 1 km
iridium, germanium
93 % Fe
N to S
Largest crater 26.5 m and 6 m deep First complete meteorite specimen was found April 29
was 11 kg
Area of brown bears, Siberian tigers and snakes Artist painted the fall (Medveder) became a stamp in 1957 T wo kinds of meteorites : those with ablation & fusion crust & regmaglypts Those violently torn apart in the air or blasted apart upon impact ÒThe BIGGER THEY COME, THE HARDER T HEY FALLÓ very peculiar
large pieces were found inside small craters, while large craters had very small meteorites no explosion during the fall : no seismic record (T unguska recorded 2 waves) 106 craters were described from 0.5 m to 26.5 m tree trunks were found pierced right through with fragments of meteorites no traces of explosions of the meteorites upon impact were found crater field > 2 km2
1100 m X 660 m
strongly marked sound, visual & mechanical phenomena one was stacking timber when he noticed a Òsecond shadowÓ appearing suddenly & looked up and saw the fireball when meteor landed the cloud of smoke of black smoke rose upwards
smoke pillar was gigantic with a whit e t op then, an explosion caused an earth tremor shook houses breaking window panes 10 Ğ 12 separate explosions, the loudest was the third, like machina gun fire lasting 10 minutes a mechanic was up in a pole at the time: felt a sharp electric shock from the wires although the power was out at the time doors were flung open, glass flew out of the windows, plaster came down from the ceilings & flames & cinders hurled out of lighted st oves color of bolide : 22 red, 7 yellow, 3 blue, 4 pink : kept changing the bolide left behind a huge dust trail, 33 km in height audibility zone of sound phenomena : radius of 120 km earth tremor felt over air wave (breaking windows)
Òof 50 km Òup to 180 km away
impacts craters with diameter from 9 m to max. of 26.5 m = 24 Ò
Ò
Ò
Ò
Ò 0.5 m to 9 m
= 98
Ò
Ò
Ò
Ò
less than 0.5 m
= 78
places of fall of minut e specimens on surface Ellipse
major axis Minor axis Area = 1.6 km 2
= 175
2.1 km 1.04 km
This area is extremely small ĞT HE SMALLEST OF ALL KNOWN MET. SHOWERS Is the second peculiarity of this shower There are 3 concentrations of points of impact of surface specimens in the rear part This is the third peculiarity of this shower 9 cases of meteorites that fell onto trees
FOUR CAT EGORIES OF
SPECIMENS
1. complete single specimens of different sizes
313
2. single specimens split & broken up int o large pieces t otaling 6 t ons
weighing 11 t ons 42 pieces from 27 specimens
3. Large fragments (> 5 kg) of broken up or split single specimens
185
2.7 t ons
4. small fragments (< 5 kg) of the same single broken up specimens 7,742 2.4 t ons 8,282 samples from 383 single complete or broken up specimens = 23 t ons Clearly marked channels were discovered in 9 craters of different sizes Some fragments rebounded from the holes they had formed and fell again some distance away Some st ones split trunks in half ( 9 cases)
# 66 is most interesting 344 kg struck the trunk of a fir tree 40 cm thick & split it in half. At the same time breaking up itself int o 2 parts. The st one not only cut through the tree, but split up the end of the trunk into a bunch of narrow, thin long splinters. T wo other pieces were inside crater Also numerous smaller pieces with sharp edges found stuck on trees Meteorites broken or split int o large pieces predominate in craters 2 m or more. Similarly in craters up t o 5 m Majority of whole single st ones lay in the southern areas of the small craters & hole sometimes under the southern edges That means that the large pieces fell from N t o S The more deeply buried pieces were in S E parts. This indicates movement of meteorite inside crater during excavation
Graph on page 319 shows : T HE LARGER THE CRAT ER, THE SMALLER T HE T OT AL WEIGHT OF THE ST ONES COLLECT ED DIAMET ERversus WEIGHT However, largest piece was 1,745 kg & should have been in a crater of 14.5 m, instead it was 3.5 m wide ! It had flat tened shape, its speed of descent was lower ( see Hoba)
CONCLUSIONS Intense fragmentation due t o its lumpy Ğ granular comparatively unstable structure From witnesses : 4 points of breakage at altitudes of 6, 16, 34, & 58 km Therefore, no explosions, only mechanical fragmentation SHAP ES : elongated, flat tened, crock-shaped, convex, concave, irregular, conical Axes of elements bisect at 60 Ô, 90 Ô,120 Ô SURFACE RELIEF Regmaglypts fracture irregularities covered by fusion crust 8-hedral structure
SURFACE RELIEF Regmaglypts fracture irregularities covered by fusion crust 8-hedral structure EDGES Most remarkable feature of the fusion crust : spat tering phenomenon (drop sprays) Droplets solidifying along lines on the surface Therefore, the DUST T RAIL consisted of spat tered droplets of melted mat t er blown off the surface Globules are magnetite in composition as the iron is oxidized in the air as molten iron is blown off Also, hollow flasks discovered made with iron & gases blown off through them Micro-meteorites : one was found to be at tached to a dead leaf hanging on a stalk of dry grass. It had all characteristics of meteorite including regmaglypts
COMP OSIT ION Inclusions of troilite, schreibersite, chromite Specific gravity = 7.2 Chief minerals : kamacite, taenite, plessite P recious metals : from spectra
Ru, Rh, P d, Ag, Pt, Au 5.7 0.9 6.9 6.2 4.6 1.8
grams / t on
tests of expansion, contraction & Curvature peculiar lumpy struct ure contributed t o its disruption in the air since it decreased the endurance of the meteoritic mat t er itself
P OP IGAI P rob. Same as Chesapeake& TomÕs Canyon in age 35 m.y.
multi Ğ ring inner structure
true bot tom relief : Asymmetry of deposits suggests an oblique impact annular uplift, annular trough, out er zone of deformed target rocks radial & concentric pat terns one of the best examples of complex impact structures crater filled with lithic breccias & melted rocks (tagamite, suevite) T agamites consists of glassy or crystalline matrix with numerous inclusions Sue vites consist of bombs, lapili or ash-size particles of impact glass & fragments 500 drill holes were studied central depression filled with breccia 2 km annular uplift annular trough 2 km outer flat slope of crater also, radial troughs flat central uplift suspected but not prominent central depression : 2 km deep Annular uplift group of small hills : breccia cemented by tagamite / suevite Crater floor Ğ during early modofocation stage before deposition of broken rock irregular relief caused by displacements of large blocks
Largest Cenozoic impact Underformed outline, lots of exposure in well-preserved state : prime place to study Complex origin, not a simple cratering model Expanding explosion cloud Impact melt rocks : products of quickly cooled impact melt Some impact melt is contaminated by the meteorite No other analogue among other impact structures, which are very old, eroded, buried , or partially exposed Graphite turned int o diamond within 13.6 km of the center Circular Basin with complex t opography system of hills in 3 concentric chains, separated by valleys There is no outer rim A deeply buried central uplift (not easy t o see)
An inner ring
50 km in diameter (not uplifted everywhere)
A central basin 2.5 km deep bounded by inner ring An annular trough
1.3 - 2 km deep surrounding inner ring
A tect onic rim zone Distal ejecta deposits very widespread as indicated by a strewn field of impact diamonds up t o 500 km from center. Also found in melt rocks inside crater Rim deposits have been eroded. Original rim was 350 m Topographic map showing inner ring Section of ring
RIES Diamonds in Canyon Diablo meteorite reported in 1891 Different than kimberlit e / lamproite diamonds : small size, poly-crystalline structure Frequently reported from iron meteorites esp. ureilites P roduced by shock compression FIRST described in Popigai impact structure. In graphitic gneiss clasts in suevite And impact melt rocks up to 1 cm
yellow, gray or black rarely colorless
Also found in alluvial around Popigai LAT ER found in Kara, Ukraine, Sud bury, Finland, Norway, K/T boundary layer in Mexico None are suitable for gems Ries : 24 km 14.3 Ğ 14.4. m.y. Base of ejecta deposits is a breccia 200 m thick up to 42 km away from center covered by fallout suevite ( fragments and glass bombs) Medium-sized complex Well-preserved, low erosion
ONE OF T HE BEST ST UDIED Flat inner basin, 12 km in diameter surrounded by inner ring : chain of hills ~ 50 m above the basin Outer rim ~ 25 km diameter T ransient cavity would have been coincident with the inner ring Crater floor less than 1 km below Basin It has its own T EKTITE FIELD : The Moldavit es Shape & size of medium Ğsize craters vary considerably Final shape depends on the make-up of target rocks Sedimentary material is assumed t o be less resistant against shear failure than ccrystalline basement rocks Craters in sedimentary rocks are FLAT Craters in crystalline rocks have more pronounced structural elements Diameter of the transient crater is about the size of the inner ring or rim The outermost ridge is used to determine size of structure. Depends on acoustic fluidization properties of target rocks
BOSUMTWI 10.5 km raise drim
1.3 m.y.
circular lake
250 Ğ 300 m above lake
water 80 m deep breccias on the rim & suevite is purple coesite & Ni-Fe spherules in glass suggests an Iron 300 m wide weighing 100 million t ons IVORY T EKTITE FIELD : one of the few in the world Formed ahead of the meteor by the shock compressed air layer picking up dust & sand & liquefying it & sending it 300 km to the west , even under the ocean Millions of but t on-sizes tektites
MOROKWENG 140 km t o 200 km
145 m.y. Jurassic / Cretaceous boundary
distinctive ring
a piece of the meteorite found inside the crater st ony, but different rich in Fe silicates & Fe Ğ Ni sulfides, but poor in metal
NEW T YPE ?
it should have been vaporized ! asteroid was 5 Ğ 10 km, now lies underneath the Kalahari desert sands drilling holes int o the impact melt discovered a piece ( 25 cm ) 770 m below surface , some bark blocks after tests it was a meteorite, a relic of the collision everything should vaporize in temperatures between 1,700 Ğ 14,000 ÔC composition is also unique a lit tle more radioactive more U, Na less Fe & Ni
VREDEFORT (RING OR DOME ) 1,970 m.y. complicated , controversial much erosion, dist ortion now : a series of concentric half-circles at the boundary of two groups ( continents) many shock Ğ produced features : shat ter cones, PDFs, coesite surrounded by Bushveld Complex (most of the Au in the world) : so, origin could be volcanic
CAMP O DEL CIELO, ARGENT INA oldest meteorite known to natives ancient tradition that a rock fell from the sky : Piguem Nonralta : Field of the Sky 12 small craters NE to SW with meteorites found in them crater field elongated 17 km long largest crater 100 m, smallest 20 m # 1 : Hoyo de la Canada mechanical excavations ( no explosions) # 7 is elliptical in outline st ones inside and outside of craters fossils found in the craters RIO CUART O CRAT ERS ( 100 years ago ? ) 1990 from air, 10 oblong from several km to 250 m size from NE t o SW low-angle impact , 2 small chondrites found, many tektites
WOLFE CREEK, AUST RALIA 0.88 km
< 0.3 m.y.
well Ğ preserved
most famous Australian crater, very mild erosion, retains most of its struct ure very similar to Barringer rim rises 35 m above plain : perfect ring crater floor 55 m below rim true crater floor is 150 m below present floor
ARAGUAINHA PEAK-RING S TRUCTURE, BRAZIL Abstract In trodu cti on Backgrou n d S trati graph y Crater morph ology Fi eld relation sh i ps Ge ome try of th e central uplift and con ce nri t c rin gs Post-impact erosi on Di scu ssi on
C on clu si on s Combined field-based observations and remote sensing analysis indicate this is a shallowly eroded peak-ring impact crater. The structure preserves a pervasive concentric geometry defined by a central peak, annular basin, and concentric rings. The target lithologies become progressively older and more intensely deformed t oward the central uplift . The annular basin has largely brecciated sandst ones and the outer annular ring has younger layered siltst ones and sandst ones. The origin of these features is directly linked t o the excavation and lat er collapse of the transient cavity. Reconstruction of the preimpact stratigraphy and transient cavity dimensions suggest that excavation was extensive in the annular basin but minimal t o nonexistent beyond the inner ring. The annular basin was probably part of the initial excavation crater, which was later modified by the rebound of the highly compressed rocks at the bot tom of the transient cavity. The inner ring exposes material that was temporarily uplifted during the expansion of the transient cavity and flowed inward during the collapse of the cavity walls. The origin of this feature is directly linked t o compressional stresses created during semicontemporaneous collapse of the central uplift and transient cavity walls. The structure underwent two periods of erosion. The first period occurred in the Late T riassic Ğ Early Jurassic, during which 50 Ğ1 30 m of the original crater was removed. The second period relates t o the recent removal of 200 Ğ 220 m of target rocks and overlying impact-related material. Our estimates suggest a relatively shallow level of erosion (250 Ğ 350 m) which explains the presence of impact breccia remnants and the good state of preservation of the stratigraphy and crater morphology. No evidence of ejecta occurs outside the structure.
CHAPTER
11
Threat of Asteroids / Comets in the Future
SUMMARY THE SPACEGUARD FOUNDATION THE TORINO SCALE
NASA’S STARDUST PROBE ROSETTA MISSION THE DEEP IMPACT SPACECRAFT
T HE SPACEGUARD FOUNDAT ION Established in 1996 by a group of astronomers Based in Rome, Italy : home of many of best researchers in the field of NEO dynamics Branches in Germany, Canada, Croatia and Japan One of the most active branches is in the UK Australia most vulnerable because most of the population lives in coastal areas, therefore a target for tsunamis generated by falls in the ocean T HE T ORINOSCALE Provides a mechanism for assessing the severity of the threat from any potential NEO impactor
ROSET TA A European mission designed t o follow a comet to see how its activity changes as it approaches the sun Original plan was to carry Champolion module, bring a sample of the nucleus of a comet for analysis It has been scaled down, because of high costs The target is comet Wirtanen, which was discovered in 1948 Water is the dominant component Icy constituents such as ammonia, carbon dioxide and many organic chemicals evaporate as tie sun heats them Water starts t o sublimate when the comet is about 3 AU from the sun This is why comets become brighter as they pass this threshold Roset taÕs mission is t o rendezvous with the comet when it is beyond that critical distance, and while flying alongside it gather data on how the nucleus springs t o life as solar radiation makes water and other icy chemicals turn t o gas Roset ta will get t o Wirtanen in late 2011 when the comet is about 3.5 Au from the sun Scheduled t o fly alongside the comet for 18 months as the comet comes t o perihelion in July 2013 It will drop an instrument on its surface t o make direct measurements of the composition and structure If successful, it will inspect several locations by hopping from one place o the next
DEEP IMPACT
(name st olen from Hollywood movie)
What is under the dark surface ? This is the fundamental question To blow a hole through the crust needs a big lump of inert metal traveling at 5 or 10 km per sec The spacecraft will release a half-t on cylinder of copper as it approaches comet T empel in July 2005 with the expectation that it would blow up a crater 120 m wide and 25 m deep This is monit ored from the mother ship at a safe distance
NEAR EART H AST EROIDS AST EROIDS WHOSE ORBITS ARE CLOSE TO EART HÕS ORBIT They pose a collision threat Over 4,500 are known, ranging in size up t o 32 km Those over 1 km in size is 800 Ğ 1000 NEAs only survive in their orbits for a few million years They eventually decay, collide with inner planets or are ejected further away by near misses with the planets There are 3 families of NEAs The Atens which have average orbital radii closer that 1 AU The Apollos, which have average orbital radii greater than that of the Earth 1. The Amors which have average orbital radii in between the Earth and Mars
Hist oric Impacts : 65 m.y ago, T unguska, June 6, 2002 : over the Mediterranean exploded in mid-air, energy was comparable t o a nuclear bomb Future Impacts : 1950 DA ; rediscovered in 2000 1 in 300 chance a collision in March 16, 2880 1 km in diameter Near Misses : March 23, 1989 : asteroid Asclepius missed by 700,000 km March 18, 2004 LINEAR announced that a 30 m 2004 FH would pass only 42,600 km near P rojects t o minimize the threat : LINEAR The Spacewatch project : Univ. of Arizona
