Apr 25, 1999 - coarse, pyroxene-rich basalt containing interstitial maskelynite ..... mode optical fiber is used to transfer the excitation laser beam to the probe ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E4, PAGES 8509-8519, APRIL 25, 1999
Raman spectroscopiccharacterization of a Martian SNC meteorite: Zagami Alian Wang, BradleyL. Jolliff, andLarry A. Haskin Department of EarthandPlanetarySciences andMcDonnellCenterfor theSpaceSciences, Washington University, St. Louis,Missouri
Abstract. To demonstrate theabilityof Ramanspectroscopy to determine themineralogical characterof a rockthatoriginatedonMars,we analyzeda smallslabof"normalZagami"by point analyses andmultipointscansusinglaboratoryspectrometers. Spectraof clinopyroxenes were dominant; theircompositions wereestimated froma calibration of Ramanpeakpositions with Mg/•g+Fe) basexl on lunarpyroxenes of knowncomposition, andtheseagreewith compositions obtainedby electronmicroprobe. A few spectraof orthopyroxene wereobserved. Thebroadspecmun of maskelynite wasobserved, butnotthatof plagioclase feldspar.Spectraof minorphosphates,magnetite, andpyrrhofitewereobtained,aswerespectraof an organiccontaminant andof hematite,bothapparently introduced duringsamplehandlingpriorto Ramananalysis.The modal analysis basedonthemultipointscansagreeswell withpublished values.If thespectra hadbeen obtainedon the surfaceof Mars by Ramanspectroscopic analysisasa stand-alone methodandno otherinformationaboutthe samplewasavailable(andby ignoringthe spurious hematiteandorganicmaterial),we couldroleoutsedimentary andplutonicrocktypesandconcludethatthe sample wasa pyroxene-phyric basalt. 1. Introduction
2. Sample and Experiments
We advocaterover-basedRamanspectrometers as a meansto
2.1. The Sample
obtain detailed, in situ mineral characterizationof materials on
Zagami,oneof thebasalticShergottites, fell in 1962in Nigeria planetarysurfaces[Wanget al., 1995, 1998]. Giventhat a series cut up anddistributed widely(summarized of landedmissionsto Mars is planned,with the intentto collect andwassubsequently samplesfor transportto Earth,we are investigating whata rover- by Meyer [1996]). Weighing18 kg, the Zagamimeteoriteis a rock that containsseveral texturally distinct basedRamanspectrometer couldtell us aboutmaterialsfoundon heterogeneous Overall,it is freeto mediumgrainedandhasbasaltic the Martian surface. We have previously shown that a lithologies. andmineralogy.Most of the meteoriteconsists of miniatudzedRamansystemcouldidentifymajorandminormin- composition eralsin rocks,determinemineralproportions in rocksand soils, so-called"normalZagarni"(NZ) lithology,which is pyroxeneenable identificationof rock types, determinecation milos in rich and has a diabasicor subophitictexturesimilarto that of certainminerals,providefirst-orderinformationaboutrock tex- Shergotty[Stolperand McSween,1979;McCoy et al., 1992]. roughly20% ture (the spatialdistributionof mineralphases,the presenceof Judgingfromoneof thelargeremainingspecimens, veins,etc.),and identifyalterationproductsof primaryminerals of the meteoriteconsistsof a dark, mottledlithology(DML), which is separatedfrom NZ by a sharpbut irregularcontact [Haskinet al., 1997]. In this study,we demonstrate that in an actualMartian rock, [McCoy et al., 1995]. The DML containsa fayalite-bearing the Zagami meteorite, major minerals (pyroxene and lithologyreferredto as ZagamiDN (originallyfoundin a sample maskelynite)and minor and trace minerals(merrillite (whit- fromDavidNew, reportedby Vistisenet al., [1992];seeMcCoy to represent a related,late-stage lockitc),apatite,pyrrhotite,and magnetite)can be identifiedby et al., [1995]),whichappears melt pocket. The DML also contains pockets of shockmeltthat usinga Ramansystemand a measurement proceduresimilarto [Mara'et al., 1995]. thosewe anticipateusingfor in situ Ramananalysison the sur- haveyieldedtracesof Martianatmosphere of veinsof faceof Mars.By Ramanpoint-counting, we haveobtaineda min- McCoy et al. [1992, 1995] alsoreportthe occurrence shock melt and of maskelynite that transect NZ. On the basisof eralmodeof the Zagamisamplethatis consistent with literature mineral compositions and assemblages, the three lithologies, NZ values.We have also obtaineda distributionof Mg/Fe ratiosin Zagamipyroxenesreflectingthe chemicalzoningof individual - DML - DN, appearto be relatedto oneanotherby a fractiona[McCoyet al., 1995]. The specimen that we anagrains.We were able to deducefrom the informationobtained tion sequence lyzed consisted entirely of the NZ lithology. that the Zagami rock samplehas a textureconsistent with a The ZagamiNZ lithologytypicallyexhibitsa foliatedtexture, coarse,pyroxene-richbasaltcontaininginterstitialmaskelynite consisting of a pronounced alignmentof pyroxenegrainsand and mesostasis. This studysupportsour contention that in situ Ramanmeasurements on Mars surfacesamplescanprovidegood maskelynite[Stolperand McSween,1979;McCoy et al., 1992], with maskelynite generally occupying interstices between pyroxfirst-order characterization of theselithologiccomponents. enegrains.Thecomposition of Zagamiis approximately thatof a Copyright 1999by theAmericanGeophysical Union Papernumber 1999JE900004. 0148-0227/99/1999 JE900004509.00
low-aluminabasalt.Stolperand McSween[1979], however, showedthatZagamihasan accumulation of pyroxenerelativeto an equilibfima,low-pressure magmaticliquid containingpigeoniteandplagioclase of appropriate composition. They inter85O9
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WANG ET AL.' RAMAN
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STUDY OF ZAGAMI
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Figure 1. Photograph of sawn,lightlypolishedsurfaceof Zagami,-• 1.5 cm across(2.5 g chip lent to us for this studyby Tom Wdowiak).Light-green,fracturedpyroxenedominates the assemblage, with somegrainsreaching34 mm in lengthand0.3-0.4 mm across.This specimenshowsclearevidenceof preferentialalignmentof pyroxene grainsandappearsto be representative of the "coarse,normalZagami"lithology.Ramanpoint-counttraverseswere madealongthethreelinesasshown,anda gridof spotanalyses wasdonein the areaoutlinedby therectangle.
preted thisaccumulation asaresult ofgravity settling. Thegrain portionof NZ. Pyroxenegrainsare typicallylight greenand
pigeonitefromaugitemacrosizeis variablein differentsubsamples or areas,with averages fractured;we couldnot distinguish
Initial Ramanmeasurements weremadedirectlyon ranging fromapproximately 0.2mm("frae-grained" portions) to scopically. 0.4mm("coarse-grained" portions) [McCoy etal.,1992]. Within theroughsawnsurfaceof therockslab.For latermeasurements, onesurface lightly.Thepolishing hadnonoticeable a given area,mineral proportions arevariable, asfollows: pyrox- we polished butenabled betterpeterie(augite, pigeonite): 70-80vol%;maskelynite, 10-20vol%; effectonthequalityof theRamanspectra, examination of thespecimen. oxides(titanomagnetite, ilmenite), 1.5-2.8vol%;sulfide(pyr- rographic rhotite), 0.2-0.6vol%;phosphate [Na-whitlockite (merrillite)],
0.5-1.3vol%;mesostasis, 1.7-3.7vol%,andshock melt,0.1-0.9 2.2. The Raman Spectrometers.
vol%[Stolper andMcSween, 1979;McCoyetal., 1992;](sum-
marizedin Table2). Mesostasis consists of vermicular inter-
Two differentRaman spectrometers with lasersoperatingat
growths ofglass andsilica withmagnetite, ilmenite, fayalite, and differentfrequencieswereusedfor this study.Both gavecomparableresults.The initial measurements weredoneusinga micropyrrhotite [Stolper andMcSween, 1979]. Minerals thatconstitute the mesostasis are concentrated andcoarserin the DN lithology Raman spectrometer model S3000 (Jobin-Yvon/ InstnunentS. than in NZ. In additionto men511ite, fluorapatitehas beenre- A.). It has a spectrograph of Czerny-Tumerconfiguration, couIt usesa diode ported inDN,butnotNZ [Watson etal.,1994]. Magmatic melt pled with a subtractivedouble-monochrometer.
detector andthe 514.5nmline of an Ar+ inclusions containing amphibole havebeenreported in thecores armyas multichannel
ofpigeonite grains [Treiman, 1985; McCoyetal., 1992];these laserfor excitation.The rock sampleto be studiedis placedon areOH-bearing (-*2wt%),withverylittleF orC1.
the microscope stageof this instrument. A microscope objective
anddirectstheexcitationlaserbeamontothe sampling Thesample thatweanalyzed isa 2.5g,flat,rectangular sawn condenses chipmeasuring roughly 1.5x 1.0x 0.5cm,asshown in the spot.The sameobjectivethencollectsthe Ramanscatteredradiafor bright-field photo, Figure 1. Visual examination oftheZagamition from the sampleand sendsit back to the spectrometer
specimen clearly shows a preferred orientation ofelongate pyrox- analysis.In thisstudy,an 80x ultra longworkingdistanceobjeceriegrains. ^ fewpyroxene grains are3-4mminlength and0.3- tive (0.85 NA) was used.It producesa condensedlaserbeam0.7) is slightlymoremagnesian (4).05 units)thananyobserved Zagamispectra. Thelunarpyroxene measurements weredoneon by othersusingthe electronmicroprobe.Our calibrationis selfa thinsection, anda highmagnification objective(80x, 0.85NA) consistentfor rapidly cooledpyroxenes,and we believeit is was usedwith a free focusingadjustment, producinga small accurate, butit appears to dependsomewhat onequilibration and,
WANGET AL.' RAMANSPECTROSCOPIC STUDYOFZAGAMI
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RamanShift(cm'•) Figure 3. Compositions ofZagami pyroxene: (a)Peak positions ofZagami pyroxene areplotted onthefieldof lunarpyroxenes of known structural types andcompositions. Values of Mg'determined forlunarpyroxene are
shown. (b)Histogram ofZagami pyroxene forthe third peak '•m spectral region 3,-325 cm 4.(c)Histogram of
Zagamipyroxene forthefirstpeakin spectral region 2, -670 cm'.
presumably, cationordering. Theexactcalibration of theRaman dottedlines.It is obviousthat the positionsof the two Raman (4370cm4 and--325cm4) varysympathetically; this peakpositions for Mg', stillunderstudy,is somewhat complex peaks occursbecause theybothare affectedby Mg' in a similarway. andbeyond thescopeof thispaper. asMg' decreases, Becauseindividualcoarsepyroxenegrainsare readilydis- Thepeakshitltendstowardlowerwavenumber tinguishable by visualobservation in thissample of Zagami,we so that the classiczoningpatternof Mg-rich coresgradingto are ableto draw conclusions aboutthe variationin spectralpeak more Fe-rich rims shouldbe observable.This zoningpatternis positions (andthusMg')astheRaman traverse crossed individual seenclearlyonlyfor the grainat the far rightof thediagram(in grains.Figure4a shows the variation of Ramanpeakpositionsthe 3500-4000Ixmrange).Thatgrainhasa corewith Mg'- 0.7, on bothsidesof the core,increasing Fe among themeasurement pointsonpyroxene grainsalonga linear andin the overgrowths traverse of 40 points(100 gm apart)on theroughsawnsurface. enrichmentwith distancefrom the coreis seen,extendingas far Peakpositions for spectra fromsuccessive pointson the same asMg' 4).5. Such a clean-cutzoning pattern startingfrom a highly grain(as determined by visualobservation) are connected by
8514
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What distribution of Mg' would the Raman microbeam encounter? Only rarely would the path of the traversecross perpendicular to a grainedgesothattheclassical zoningpattern of initially increasingMg' througha core of constantMg' to decreasing Mg'be encountered. Moreoften,thebeamwouldmiss thecorealtogether (therimsareasthickor thickerthanthecores) and,at interpointdistances of 100 or 200 grn,wouldcross,for example,a comerof a tilted grain,samplingtwo sidesof the overgrowth unevenlyandnot seeingequalamountsof the two sidesor obtainingthe samerange of Mg', still showingsome systematic changein Mg' with distance. Complicated patterns of variationthus occur,with mostcrossings not reachingthe most magnesian compositions. In addition,the laserbeampenetrates into the rock matrix and scattersthere in a complexway that involvesindicesof refraction,specularreflection,andabsorption, and theseaffectthe volumethat is sampled.Effectsof this type areexacerbated if thegrainsarenotrandomly oriented,asin the caseof the Zagamislab.Experience in observing rocksof many texturesand zoning patternswill be requiredto draw the maximum in accurateconclusionsabout a rock specimenfor which no informationother than a set of Raman spectrais available.
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All of the plagioclasein Zagami has been convertedto maskelynite[StolperandMcSween,1979],a glassof plagioclase composition formedby high shockpressure[Tscherrnak, 1872; Stuffier, 1974]. Compositionsrange from An57Ab42Or]to Am3Ab•3Ora with significantFeO (0.5-0.8 wt%) andMgO (0.10.2 wt%). The elongation andapparentalignmentof maskelynite grainswas interpretedby Stolperand McSweento resultfrom crystallizationof plagioclasefrom intercumulusmelt trapped betweenaligned,cumuluspyroxenegrains. No RamanspectnLm characteristic of plagioclase wasobtained from the Zagami sample.The secondmost abundanttype of spectra(Figure2c) obtainedwere from dark, glassygrainsthat yieldedtwo weak, broadRaman spectralbands.TheseRaman bandscenterat ---510cm'] and ---1000cm'•. Such featuresare typicalof glassesconsisting of frameworksilicates[McMillian, 1984].We attributethis spectrumto maskelynite, the amorphous form of plagioclaseproducedby shock.The spectrumin Figure 2d was obtainedfrom a lunarmaskelynitegrainin a thin section of 14161,7080rockchip,andshowsverysimilarfeatures. The Raman spectralfeaturesof maskelynitefrom different sources(a lunar meteorite,Martian meteorites,and a terrestrial impactstructure)were studiedby Treimanand Treado[1998] andMikouchiet al. [1998].In the studyby Triman andTreado, maskelynite from Martian meteorites ALH84001,
Figure4. Spatialdistribution of pyroxene Mg': (a) fromspectra ALHA77005,137, and EETA79001,367 all showed broad and takenalonglineartraverse1 in Figure1 (40 points)and(b) from weakspectralbands,whereastheterrestrialmaskelynite fromthe
spectra takenin rectangular field1 (5 rows,22 points each)in
Figure 1.
Manicouagan impactstructurehad sharpRamanpeaksnear 500
cm'], similar tothose ofreference feldspar samples. In thestudy
by Mikouchi, most maskelynitegrains from the Martian meteorites Zagami,EETA79001,QUE94021,ALH77005, andYmagnesian coreandcontinuing monotonically to a moreferroan 793605 gave broad and weak Raman bands like those we rim cannotbe seenfor otherpyroxene grainsin Figure4a or observedin Zagami maskelynite.Only the recrystallized Figure4b. Figure4b showsthe equivalent variations in peak plagioclase rims in ALH77005 yieldeda Ramanpeaknear 500
position of-670 cm'] peakfromthespectra takenona two- cm'],typical ofplagioclase. A similar plagioclase spectral feature dimensional grid(22 pointsx 5 lines).Theobserved irregularity was observedfrom the maskelyniteof lunarmeteoriteY-793169 isto beexpected. Consider anelongate pyroxene grain,randomly butnot fromthatof A-881757,whichshowsonlythebroadbands oriented relativeto the surface of theZagamislab,eachgrain asMartianmaskelynite. In an experiment[Mikouchiet al. 1998], witha magnesian coreandanoutward-grading, moreferroan tim. Zagamimaskelynite wasannealed by heatingit at 900øCfor 1, 4,
WANG ET AL.' RAMAN
SPECTROSCOPIC STUDY OF ZAGAMI
8515
8, 24,and72hoursat anfo2of iron-wustite +2. TheRaman pe• near500cm'x typicalof plagioclase appeared after4 hoursof annealing, andthe peakintensity increased as the duration of annealing increased. The plagioclase-maskelynite relationship reflects thedegree of vitrification of shocked plagioclase andthe degree of subsequent annealing andthusoffersinformation about thehistory of plagioclase-bearing materials subjected to shock, although it mayprovecomplex to interpret. Also,according to Ramanstud/es doneby Heymann[1987,1988],theincrease of bandwidth of plagioclase Ramanpeaksandthevariation of the
a. pyrrhotite
fluorescent background canbe usedto evaluateshockpressure.
Amongthe morethan 60 Ramanspectratakenfrom the maskelynite grains inourZagami sample, noresults of annealing weredetected. Also,Zagami isreported tocontain mesostasis that consists of intergrowths of sihcaandK-richglass(shocked K, Na-feldspar) whereinthe sihcamaybe eithercristobalite or "amorphous" [Duke,1968].We d/dnotobserve spectra of any glasses thatwerenoticeably moresihceous thanthemaskelynite.
b. standard pyrrhotit from Durango, Mexiccl
3.3. Olivine
•
•
c.standard pyrrhotite
Fayaliticohvine(Fa90.•)hasbeenreported at tracelevelsas part of an "intergrowth' assemblage in the late-stage DN lithologyof Zagami,first notedby M0ssbauer spectroscopic study[Vistisen et al, 1992].In our study,Ramanspectra were takenfrommorethan300 pointson the Zagamispecimen, and we did not observe the characteristic peaksof olivinein anyof them.As ohvineis a strongRamanscatterer, we conclude thatit is eitherabsentfromthe areaswe investigated or presentat less
d. magnetite -_
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200
400
600
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Raman Shift(cm -•) Figure 6. TypicalRamanspectraof Fe-beahngmineralphases found in Zagami: (a) pyrrhotite, (b)a standardpyrrhotite, Durango,Mexico,(c) a standard pyrrhotite,(d) magnetite, and(e)
b. a lunar whitlockite(high REE)
hematite.
3.4. Phosphates
In the Zagamisamplestudiedhere,merrillReoccursas lightcolored,narrowgrainsmoldedalongthe edgesof maskelynite grains.The identification of phosphates by theirRamanspectrais straightforward. Ramanspectrawereobtainedfor two accessory phosphates, merrillire(whitlockite),and apatite[seePrewittand Rothbard, 1975; Jolliff et al., 1996]; Figure 5 showstypical examplesof their spectra.The spectrumof Zagami metallite
c.apatite
shows a doublet at 975and962cm'xasthemainpeaks (Figure 200
400
600
800
1000
RamanShift(cm'•)
1200
1400
5a), typical of merrillitewith low concentrations of rare earth elements(REE). In contrast,merrillitestud/edin lunarsamples (Figure5b, 15273,2lunarrockchip)hasonlya singlemainpeak
near962cm4 andpeaks at higherwavenumbers fromrareearth
[Jolliff et al., 1996]. The fluorescence peaksof the FigureS. Typical Raman spectraof phosphates found in fluorescence REE are not Raman signals but have wavelengths independent of Zagami:(a) Zagamimerrillite(whitlockite), (b) lunarwhitlockite,
and(c) Zagamiapatite.
the excitingwavelengths. Their positionon the scaleof Raman
8516
WANG ET AL.' RAMAN
SPECTROSCOPIC
shifts, however,dependson the laser wavelength.Only two grainsof apatitewere encountered (Figure5c), and both were presentwithin merdllite grains.Merrillite grainsin the Zagami meteoritecommonlycontainelongateglassinclusionsthat have eutecticcompositions, and severalhavebeenobservedthat also containeuhedmltitanomagnetite and pyrrhotitegrains[Stolper andMcSween,1979]. 3.5. Pyrrhotite
Figure6a showsRamanspectraobtainedfromtheFe-beahng mineralin the Zagamisample.Pyrrhotiteis the only Fe-sulfide phase found among them. Pyrrhotiteis a complicatedFe-S mineralwith three differentstructures (hexagonal, monoclinic, andpseudo-rhombohedral), and12 superstructures, depending on its Fe/S ratio and on conditions of its formation such as the tem-
STUDY
OF ZAGAMI
StolperandMcSween,1979;Brearley,1991;McCoyet al., 1992, 1995],sowe arepuzzledby its appemmace. Heatingof pyrrhotite in air would yield hematite,and the laserbeamof the Raman spectrometer certainlyaddsheat,especially to an opaquemineral, butthisdoesnotseemto beresponsible for theappearance of the hematitespectra,asgoodpyrrhotite(andmagnetite)spectrawere observed.The low powerdensityandred (632.8 rim) laserexcitationwavelengthwouldnot be expected to oxidizepyrrhotiteor magnetite readily.Allowingthe laserbeamto dwellonpyrrhotite grainsdid not inducean observablechangewith time in the specmuu for that mineral. Possibly, the hematite is ,a metamorphic oxidationproductresultingfromconditions thatthe Zagamimeteoriteexperienced, or it may have beenproduced during cuttingand preparationof the sample.The frequent appearance of thespectra of hematitein theZagamimeteorite and someothermaterialswe havetestedremainsa topic for flirther
peratureand coolinghistory[Scott,1974]. In orderto identify study. pyrrhotitefrom its spectrumfoundin the Zagamisample,we measuredseveralgrainseachof two standard pyrrhotitesamples that gavetwo very differentspectralpatterns.The spectrumof 4. Mineral Proportions Figure6b,takenfroma pyrrhotitesamplefromDurango,Mexico, For thosemeasurements in whichpointsalonga lineartraverse is similarto that from Zagami. This contrastssharplywith the or on a grid were sampled,the mineralidentifications from the spectrum in Figure6c, whichshowssharppeaksbetween300 of a petrographic point450 cm'l, withthe strongest at 341 cm'l. ThelatterspectralRamanspectraareroughlytheequivalent madeon patternand the positionof its main peak are consistent with count[Haskinet al., 1997].Of theRamanmeasurements expectations fromthe Ramanspectraof otherS-bearing minerals the unpolishedand the lightly polishedsawn surfacesof the with highlyregularcation-Sbondingsuchas pyrite,marcasite, Zagami sample,302 were collectedin this manner.The total sphalerite, andchalcopyrite, all of whichhavesharpandnarrow scanningdistancesfor each line or grid traversewere long of the Raman peaks, mostly at Raman shifts below400cm'l. The341 enough(0.5 -1.0 cm) to exceedthe scaleof heterogeneity cm'l peakofpyrrhotite isconsistent withvibration ofFe-Sbonds, sample,so the mineralmode obtainedfrom the measurements within the statistics of thatnumberof basedon its similarityto the strongestpeaksfrom marcasite, shouldbe representative, points observed. Spectral quality for mineral identification was sphalerite,and pyrite. Sucha spectnnurequiresgoodordering essentially the sameon the rough-sawn surfaceas on the lightly amongthe atomsin themineral. Bothspectraof Figure6a and6b haveonlya strongandbroad polishedsurfaceof thisZagamisample. Raman bandcentered near430cm'l. Thetentative interpretation Table 1 shows the results of the Raman point-counting Theyare separated intotwo categories: with laser for the spectralpattemof theZagamiandDurangopyrrhotites is measurements. point and thattheyhavethe superstructure 1c of the hexagonalstructure, in beam focusingadjustmentat each measurement alongthreetraverses(90 which occupationof the octahedralsitesby Fe atomsis dis- without.The scanningmeasurements points in total) were taken using a carefully focusedlaserbeam ordered [Scott, 1974]. The consequentirregularityof Fe-S and high magnification and high numerical aperturesampling bondingyieldsa broaddistribution of vibrationalfrequencies, and objectives(80x, 0.85 NA; 50x, 0.85 NA). For thesemeasurethebroad band near430cm'l istheenvelope oftheiroverlapping peaks.Hexagonalpyrrhotiteis the only high-temperature struc- ments, the sampledspotswere smaller in diameterand the samplingdepthswere on averageshallowerthan for the unturalformof pyrrhotite. focusedpoints,whichwere takenwith a 20x, 0.4 NA objective. Therestof therectangular grid-stylemeasurements (212 pointsin 3.6. Magnetite total), taken in two separateareas,were obtainedwith the less laser beam by using a 20x, long-workingdistance Magnetitewasidentifiedthroughits Ramanspectrum(Figure condensed sampling objective. Thislatterobjectiveis thetypethatwouldbe 6d),whichshows a majorbroadpeakat -670 cm'l plusa few used by a simple planetary instnnuenthavinga fixed sampling weakerpeaksat lowerwavenumbers. Titanomagnetite is the form observed petrographically[Stolper and McSween, 1979; distance.As seenin Table 1, the two methodsgavebasicallythe Brearley, 1991]; we have not yet determinedwhetherRaman same results.The propo•iionsof pyroxene(74% and 76%) The spectroscopy can readily distinguishamongdifferentforms of obtainedby the bothmethodsare identicalwithin statistics. sameis true for maskelynite(21% and 20%). The modesare magnetite. slightly different for the minor or accessorymineral phases (phosphates, oxides, and sulfides), which is statistically 3.7. Hematite reasonable.
The mineral mode obtainedfrom our traverseanalysesis Raman spectraof hematite(Figure 5e) were obtainedfrom with thosefrom otherpetrographic studiesthat were many bright grainsadjacentto someof the pyrrhotitegrains consonant asseenin Table2. TheRaman (indistinguishablein binocular micrescopic observation). doneon differentZagamisamples, because the specHematitewas also encountered in the point-counting measure- spectrado not includea category"mesostasis" mentson boththe rough-sawnsurfacxand the lightly polished trometerreceivessignalonly from mineralsthat might be in surface.Hematitehasnotbeenreportedin theZagamimeteorite, mesostasis (discussed above).The phosphate mineralsarea good of phosphate is similarto theproalthoughits mineralogyhas beencharacterized carefully[e.g., exampleof this;theproportion
WANG ET AL.' RAMAN SPECTROSCOPICSTUDY OF ZAGAMI
8517
Table1. Resttits of RamanPoint-Counting Measurements
Pyroxene Maskelynite Merrillitc ApatiteMagnetite Hematite Pyrrhotite Organic* Totalpoints Finebeamwithfocusing adjustment 40points traverse 1/S/Sunp**/100 Innstep 24points traverse 2/H1/Sunp/500 Innstep 26 pointstraverse 3/H1/Sunp/100 Innstep Subtotal (frequency of encounter) Percentage (excludesorganic) Coarserbeamwithoutfocusingadjustment 22x5pointsgrid1/H2/Sp/200Innstep 17x6pointsgrid2/H2/Sunp/100 pmstep Subtotal (frequency of encounter) Percentage
Total(frequency of encounter) Mineralproportions (exclude organic)
29 17.5 16 62.5
7 5 6 18
0.5
74.0
21.3
0.5 0.6
88.5 73 161.5
18 24 42
2.5 2 4.5
76.2
19.8
224 0.755
60 0.202
2
2 2.4
1.5
1.5 1.8
40
1.5 4 5.5
1
2.1
2 3 1.4
1 1 0.5
5 0.017
3 0.010
3 0.010
24 26 90 100.1 110 102 212 100.0
1.5
5.5
302
0.005
* Theorganic phase intheZagami sample was animpurity from handling; itwas removed bylight polishing ofthespecimen. ** S=S3000, 80xobjective, HI=HoloLab, 50xobjective, H2=HoloLab, 20xobjective, Sunp= unpolished surface, Sp=polished surface portionof mesostasis reported in thepetrolgraphic measurements. a bimodaldistributionif it crystallizedboth low- and high-Ca Nor is therea category for "shockmelt."We presumed in our pyroxeneor if it had exsolvedto producelamellae,and sucha tabulation thatall glassyspectra weremaskelynite. We concludepatternwasnotobserved. thatthe Ramanpointcountsgivebasically the sameresultsas Fromtheabundance of the glass,we wouldknowthe rockhad petrographic pointcounts, asobserved forothermaterials [Haskin eitherquenchedor beenshocked.The combinationof abundant et al., 1997].
feldspathicglassalong with pyroxenewould suggesteither a pyroxene-phyric lava or a pyroxene-plagioclase rock suchas a gabbroor basaltwith shockedplagioclase(maskelynite).The 5. Stand-Alone Characterization of a Martian broadrangeof pyroxenecomposition is consistent with chemical Rockby RamanSpectroscopy zoning preservedduring rapid crystallizationand supportsa The Raman spectroscopic measurements were made in the basaltictexture;however,the frequencyof adjacentpyroxenelaboratory usinga spectrometer with an opticalperformanceonly spectraalong a linear traversesuggestsrelativelycoarse The relativelylow Mg' of the most similarto onethatwouldbe suitable for on-surface planetarypyroxene,i.e., phenocrysts. magnesian pyroxene makes basalta better choicethan a more application [Wangetal., 1998].Theyweremadeusinga similar rockandthe modesupports this,as described below. measurement procedure, point-counting ona gridor alonga line, feldspathic mineralsis consistent with asanticipated formineralanalysis onplanetary surfaces [Haskin The presenceof scatteredphosphate et al., 1997]. If we had obtainedthesedatafrom a rock on the the presenceof areas of crystallizedmesostasis.From the surface of Mars,usingRamanspectroscopy only,andwe knew approximatepositionof the broadpeakin the Ramanspectraof nothing elseaboutthisrock,whatcouldwemasonably conclude glass,we would distinguishit as feldspathic,not an Fe-rich justfromanalysis of theRamanspectra abouttheminerals and silicatemelt as would be expectedof glassybasalticmesostasis. Thuswe wouldconclude thattherockwassufficientlyshocked to the rocktype? destroy its plagioclase. From the abundantpyroxeneand the absenceof lower-temFromthe modalanalysis,we wouldknowthatthe rockis rich perature minerals, we wouldknowthe rockwasigneous, and fromthe absence of quartzandK-feldspar, we wouldconclude in pyroxeneand thereforemafic. It is considerablyricher in thatit wasnota felsicigneous rock. Fromtherangeanddistri- pyroxenethantypicalmaficmelts,whosechemicalcompositions cotecftc.This wouldmean butionof pyroxenecompositions, we wouldknowthatthe rock lie on or neara plagioclase-pyroxene one,well into a pyroxene did notcrystallize slowlyin a plutonic setting. An equilibratedthat the melt was a high-temperature plutonic rockwouldhaveeithera single pyroxene composition or field of thephasediagran•,or a coolermeltthathadaccumulated pyroxene.That nearlyall of the pyroxeneis monoclinicrestricts thenatureof the rockandits parentmelt.Basedon the Earthand Moonasanalogs, thehighabundance of pigeoniteimpliesa high Table 2. Comparison of MineralMode Obtainedby Raman Ca/A1ratio typicalof basalts,ratherthan the lower Ca/A1ratio Traverses With Modes From Previous Studies characteristic of plagioclase-orthopyroxene plutonicrockssuchas riorites.Pyroxeneis the only mafic mineralobservedand thereStolperand McCoy et al. [ 1992],vol% McSween foreapparently the onlymaficmineralthatformedduringcrystalThis Study,[1979], vol% Fine Grained Coarse Grained lizationof the parentmelt. If we were to concludethat it is a vol% Pyroxene Maskelynite
75.5 20.2
76.3 18.8
69.7 24.7
2.0
1.7 2.7 0.5
2.6 2.8 0.2
Mesostasis Oxides Sulfides
0.5
Phosphates
1.7
Shock melt
77.7
74.3
76.0
80.4
17.6 1.8 1.5 0.6 0.5 0.1
18.8 3.0 1.8 0.4 0.6 0.9
18.6 2.1 2.0 0.4 0.5 0.3
10.3 3.7 2.6 0.6 1.3 0.9
basalt, the absenceof olivine would constrainthis rock to the
field of tholeiites. Theparentmelthada moderate, butnothigh Mg', because theMg' of someof thepigeonitereaches 4).7. Supposecrystallizationof the melt parentalto the rock had progressed slowly, at least for a while, so that the Mg' of the pyroxeneremainedin equilibriumwith that of the residualmelt. As longas crystallization progressed in thatmanner,pyroxeneof
8518
WANG
ET AL.: RAMAN
SPECTROSCOPIC
STUDY
OF ZAGAMI
a single compositionwould be produced.The histogramof locationsalonga pointmounttraversemightbe thoseof the dust, pyroxene(Figure3b) is notthatof a narrow,equihbrium compo- but manyor mostwouldnot,andoncetheRamanspectraof the sition. If crystal dimensionsand diffusion rates prohibited dusthadbeendetermined,the affectedpointscouldbe ignored. to relativelythick completeequihbrimn,yet coolingwas still slow, the crystal More seriouswouldbe coatingscorresponding wouldbecomezoned.As this occurred, the Mg' of the newly desertvarnish[Israelet al., 1997].Suchcoatingsarelikely to be formingpyroxenewould changeonly slowlyat first becausea thin or absentin at leastsomeareas(notethe flutingon someof relativelylarge volume of the melt would have to crystallize the rocks in the images from the Patlff'mdermission;M.P. beforethe Mg' of the melt changedappreciably. The rate of Golombek, personal communication, 1998),andthecoatings can change of Mg' wouldthusbecome morerapidastherelativepro- alsobe characterized andtheireffectscanbetakenintoaccount. portionof melt in the systemdecreased. The zoningwould Besides, information onthenatureof the coatings is valuableas approach thatof surfaceequihbrium. Theresulting histogram of information onpasthistoryandenvironments.
pyroxene compositions wouldshowa relatively highproportion Givensamples of the qualityof the Zagamimeteorite to of corepyroxene anda monotonically decreasing volumeof analyze, Raman spectroscopy asa stand-alone technique provides pyroxenewith decreasing Mg' for as long as the crystallizationconsiderableinformation for future planetary missions.The gainedfromthatmethodcanbe enriched, however,by progressed smoothlyand the Mg' was controlledprimarilyby knowledge with othermethodssuchas M6ssbauer pyroxenecrystallization. This alsois not observed(Figure3b). usingit synergistically thermalemissionspectroscopy, and alpha-protonInstead,most of the pyroxenehas a relativelylow Mg'. This spectroscopy, suggestsrapid crystallizationof the pyroxene,without an x-ray spectroscopy. approachto equilibriumbetweenmelt and crystals.Suchrapid Acknowledgments.We are gratefulto T. Wdowiakof the University crystallizationcouldaccompanyeruptionof the parentmagma of Alabama at Birmingham for lending us the sample of the Zagami onto the surfaceof Mars. The fairly steepbut still somewhat meteorite,and for partial supportof this work by NASA under grants gradualrise from a low valueof Mg'to the mostcommonvalue NAG 5-4642 and NAG 5-7140. (Mg' •0.45) suggests crystallization ontomagnesian cores,with crystallizationdepartingslowlyfrom equilibriumcrystalli7.ationReferences to surface equilibrium (Rayleigh) crystallization, then acceleratingto precipitateadditionalpyroxeneat an increasing Binder,A. B., R. E. Arvidson,E. A, Guinness,K. L. Jones,E. C. Morris, T. A, Mutch,D.C. Pied, andC. Sagan,The geologyof the Viking rate and withoutsurfaceequihbrium,and f'mallyrapidcrystalhlander1 site,J. Geophys.Res.,82, 4439-4451, 1977. zation of the remainingpyroxenecomponentin the melt, much Brearley, A. J., Subsolidusmicrostinctures and coolinghistory of too rapidly to approachequilibrium between the growing pyroxenes in the Zagamishergottite (abstract),LunarPlanet.Sci.,22, pyroxeneandthe residualmelt. This zoningpatternis consistent 135-136, 1991. Dele-Dubois, M. L., P. Dhamelincourh and H. J. Schubnel, Etudepar with the histogramof Figure3b. spectroscopic Raman d'inclusionsdans les diamonts,saphirset Fromthe detailsof themerdlhtespectrum, we wouldconclude emeraudes/2, Rev. Gemre., no. 64, 13-16, 1980. that the rock was not rich in REE, as no effect on the spectral Duke,M. B., The Shergotty Meteorite:Magmaticandshockmetamorphic peaksand no additionof rare-earthfluorescence was observed. features,in ShockMetamorphism of NaturalMaterials,editedby B. Furthermore, the Ramanpeaksare sharp,notbroadas in the case M. Frenchand N.M. Short, pp 612-621, Mono Book Corp., Baltimore,1968. of lunarwhitlockitewherehigh concentrations of U andTh cause and S. K. partial metamictization and peak broadening. From the Ghose,S., N. Choudhury,S. L. Chaplot,C. P. Chowdhury, Sharma,Latticedynamicsof Ramanspectroscopy of protoenstatite appearance of magnetite,hematite,andpyrrhotite,the fugacities Mg2Si206, Phys.Chem.Miner., 20, 469-477, 1994. of oxygenand sulfur can be constrained. If the hematitewere Golombek,M.P. et al., Overviewof the Mars pathfindermissionand foundon Mars, andwe wereableto establish thatit waspartof assessment of landingsitepredictions, Science,278, 1743-1748, 1997. the igneousrock and not contamination by red dust,we would HaskinL. A., A, Wang K. M. Rockow,B. L. Jolliff, R. L. Korotev,and K. M. Viskupic,Raman spectroscopy for mineral identificationand concludethat the samplehad experiencedan oxygenfugacity quantification for in situplanetarysurfaceanalysis: A point count consistent with the hematite-magnetite buffer.(We suspectthe method,J. Geophys.Res.,102, 19293-19306, 1997. hematitein our sampleis a productof samplepreparation,so Heymann, D., Raman study of expedmentallyshockedplagioclase (abstract),Lunar Planet. Sci., 23, 421-422,1987. sucha conclusion is thusnot justifiedbasedon our laboratory Heymann, D., Luminescence of expedmentallyshockedplagioclase Ramandata.) The absenceof hydratedmineralsconstrains the feldspar(abstract),LunarPlanet.Sci., 19, 489-490, 1988. Israel,E. J., R. E. Arvidson,A. Wang, J. D. Pasteds,and B. L. Jolliff, which it subsequentlyresided; for example, no hydrated LaserRamanspectroscopy of vanishedbasaltandimplications for insitu measurements of Martianrocks,J. Geophys. Res.,102,28705amphibolewasobserved, indicatingit wasabsentor scarce. conditions under which the rock formed and the environments in
Thuswe would concludethat the rock waslikely a pyroxenephyricbasalt,in a generalsense,a commonrock type on Earth and probablyon Mars. Overall,this studydemonstrates that considerablebasic,fast orderinformationabouta rockandits origin and environments can be obtainedjust from a Ramanspectroscopicstudyof its minerals.Obtainingsuchinformation in situon a planetarysurfacerequiresa capableRaman spectrometer, a goodmeansfor its deployment, andthatthe surfaceof therockbe accessible.Dust was present on the rocks observedby the PathfinderandViking missions[Golornbek et al., 1997;Binderet al., 1977;Mutch et al., 1977].This dustdid not covermostrocks uniformly,and the sensorof the spectrometer couldpossiblybe guided to a relatively dust-poorlocation. Spectraat some
28716,1997.
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8519
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Treiman, A, H., Amphibole and hercynite spinel in Shergottyand Zagami:Magmaticwater,depthof crystallization, and metasomatism, (ReceivedNovember2, 1998;revisedJanuary26, 1999; Meteoritics,20, 229-243, 1985.
acceptedJanuary27, 1999.)
