Sep 10, 1995 - Duggan, and George Ulrich, and of Jeffrey Judd of the National Park ... Robert Tilling, Margaret Mangan, Katherine Cashman, Harry Pinker-.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. B9, PAGES 17,637-17,657, SEPTEMBER
10, 1995
Comparative geothermometry of recent Hawaiian eruptions Rosalind Tuthill Helz, • Norman G. Banks,2 Christina Heliker, 3 Christina A. Neal, 4 and Edward
W. Wolfe 2
Abstract. In this paper we comparelava temperaturesmeasuredusingCr-A1 thermocouplesor infrared spectrometrywith estimatedquenchingtemperaturesbasedon the glassgeothermometrycalibrationof Helz and Thornber (1987). Comparativedata are availablefor the April 1982 and September1982 summit eruptions,the Pu'u 'O'o east rift eruption(1983-1986), all three eruptionsat Kilauea, and the 1984 Mauna Loa eruption. The resultsshowthat quenchingtemperatures,basedon the MgO contentsof Kilauean
glasses (TMgo), lie within_+10øCof fieldmeasurements usingtheinfraredspectrometer for 85% of the samples.Where a Cr-A1 thermocouplewas used,90% of the field
measurements lie within+1øto -11øCof TMg o for samples withTfiel d > 1130øC. Samples whereTfiel d < 1130øC showlargerdivergence. The uncertainty in TMgo byitselfis _+10øC, sothelevelof agreement betweenfieldmeasurements andTMgo isverygoodfor Kilauean lavas.Systematiccomparisonof field measurementsof temperaturewith glass geothermometryfor the 1984 Mauna Loa eruption suggests that, althoughthe field and
glasstemperatures lie within+ 10øCof eachother,theKilaueanTMg o calibration is neverthelessnot appropriatefor Mauna Loa glassesand that actual quenching temperaturesfor Mauna Loa sampleswill lie 10ø-20øChigher than would be predicted from the Kilauea calibrationcurve.Considerationof possibleeffectsof variablevolatile content suggestthat in most casesthese are small. Sampleserupted early in an eruption
mayreflectpreeruptive watercontents differentenoughto affectTMgo significantly, but later spattersamplesand all flow samplesappear to have equilibratedat low enough water contentsfor the calibrationto be applicable.We concludethat the MgO-based geothermometercan be appliedto glassyKilauean samplesto give temperaturesthat generallywill lie within _+10øCof a field measurement.Plots of glassMgO contentversus time, if a suitablesamplebaseis available,shouldgive a thorough,quantitativerecord of the thermal historyof any Kilauean eruption. Kilauea summit eruptions,the early Pu'u 'O'o east rift eruption, and the 1984 Mauna Loa eruption. Kilauea Volcano has long servedas a natural laboratoryfor After makingthe initial comparisonbetweenthe methods, the direct observationand measurementof the propertiesof we evaluatethe possibleeffectsof varyingvolatile H20 content activemagma,aswell as a testinggroundfor the development on the usefulnessof glassgeothermometry.Last, we consider of new techniquesin volcanology.One of the parametersof the thermal history of several small Kilauean eruptions to greatestinterest is the temperatureof erupting lava, because evaluate the congruencebetween the thermal signature of its thermal conditiongivesimportant insightinto the state of those eruptions,as definedby glassgeothermometry,and the the magma reservoir.Also, lava temperature,combinedwith available field observations. It is essential to assess the intertemperatureprofilesin this way, its crystallinity,determineslava rheology,which controlsmuch pretabilityof the glass-based as extensive field measurements of temperature are not rouof the posteruptivebehavior of the magma. tinely available for other sample suites. Glassgeothermometrywas developedby Helz and Thornbet [1987] in order to determine the quenchingtemperaturesof glassycore samplesfrom Kilauea Iki lava lake. The purposeof Historic Background this paper is to evaluatethe utility of this calibrationfor erupPreviousattemptsto determinethe temperatureof lava and tion samplesfrom Kilauea and Mauna Loa usingcomparative gases eruptedfrom Kilauea can be dividedinto three periods: temperature data obtained during the early eighties. This 1911-1917, 1950-1968, and later than 1968 (Table 1). The unique database includes the April and September 1982 earliest classictemperature determinationswere all directed towardmeasuringtemperaturesin the activelavalake in Hale•U.S. Geological Survey,Reston,Virginia. maumau (Figure 1) and its associatedgasjets and hornitos. 2U.S.Geological Survey,Cascades VolcanoObservatory, Vancouver, Washington. Jaggat[1917a] could already refer to two previous attempts, 3U.S.Geological Survey, HawaiianVolcanoObservatory, Volcanoes that of Perret and Shepherd[Perret,1913], who in 1911 obNational Park, Hilo. tained a thermocouplereadingof 1050øCon a lava fountain in 4U.S.Geological Survey, AlaskanVolcanoObservatory, Anchorage. the summitlava lake, and that of Day and Shepherd[1913],who This paper is not subjectto U.S. copyright.Publishedin 1995 by the in July1912obtaineda temperatureof 1185øCfor fountainsby American GeophysicalUnion. usingan opticalpyrometer.Thesetwo methodshavedominated Introduction
Paper number 95JB01309.
all subsequent attemptsat fieldmeasurement of temperature, and 17,637
17,638
HELZ ET AL.' HAWAIIAN ERUPTION GEOTHERMOMETRY
Table1. Reported FieidTemperatures ofHistoric (1911-1977) Kilauea andMauna LoaEruptions Time of Observation
Reference Perret[1913] Day and Shepherd[1913] Jaggar[1917a] Jaggar[1917b] Finch and MacDonald [1953]
Summer
1911
Location of Eruption
Temperatures Reported, øC
Halemaumau
1050
active lava lake
July1912
Halemaumau
1070-1185
activelavalake
Jan. 1917 April-May 1917
Halemaumau Halemaumau
up to 1130 700-1170
activelava lake (surface) activelava lake (interior)
June 1950
Mauna Loa
1030-1110
vents, flows near vents
southwestrift
MacDonald [ 1955] MacDonald and Eaton [1964]
Method
Object Measured
summer
1952
March 1955
850-980
Halemaumau
1100-1155
Kilauea east rift
1025-1082
May 1955
1145 1959
Kilauea
Iki
flow fronts, edges lava fountains
lava fountains, flows
flow from last vent
Richterand Murata [1966]
Nov.-Dec.
1065-1190
lava fountains
RiChter andMurata[1966]
Jan.-Feb. 1960
Kilauea east rift
1020-1125
lava fountains(?)
Richteret al. [1964]
Feb. 1961 March 1961
Halemaumau Halemaumau
1095-1125 1096-1114
lava fountains lava fountains
Peck et al. [1966] Wrightet al. [1968]
July 1961 Aug. 1963 March 1965
Halemaumau Alae Makaopuhi
1085-1127 1140 1087-1160
lava fountains lava fountain surfaceof lava lake
Kinoshitaet al. [1969] Graeberet al. [1979]
1967-1968
Halemaumau
1110-1125
lava fountains
Sept. 1977
Kilauea eastrift
1062-1095
flow
of
Measurement
Pt-PtRh thermocouplea optical pyrometer Seger cones Seger cones optical pyrometer optical pyrometer optical pyrometer optical pyrometer thermocouple optical pyrometer optical pyrometer optical pyrometer optical pyrometer optical pyrometer optical pyrometer opticalpyrometerand Cr-A1 thermocouple optical pyrometer thermocouple
aPt-PtRh, Platinum-Platinum Rhodium.
the rangetheseearliestinvestigators found encompasses almost all subsequent observedtemperatures(seeTable 1). Jaggar himself attempted to use Seger cones, commonly usedto determinetemperaturesin kilns.His two mostsuccessful experimentsappear to have been a determinationthat the temperatureof the interior of the summitlavalake wasgreater than 1130øC[Jaggar,1917a] and that the temperature at the bottom of the lake was about 1170øC [Jaggar,1917b]. The
disappearanceof Kilauea's summit lava lake in 1924 and a subsequentlongperiod of quiescenceendedtheseearly efforts at temperature measurementat Kilauea. The middle period of temperature measurementsextends from 1950 to 1968 (Table 1) and was dominatedby the appearanceof a new generationof opticalpyrometers.Finch and Macdonald [1953] useda new "disappearingfilament" optical pyrometer during the 1950 Mauna Loa eruption, reporting
155ø15'
•
4000 ••
HAWAIIAN
155ø10'
1•19
KILAUEA CALDERA
VOLCANO OBSERVATORY APRIL 1982
19ø
KILAUEA
3750 SEPTEMBER 1982 HALEMAUMAU
PAUAHI
MARCH 1980
HAWAII
, •
I
5• MAUNA ULU NOVEMBER •t x• 2MILES • SHIELD
I I 1 • KILOMETER
MAKAOPUHI
CONTOURINTERAL 250 FEET=76METERS
I
Figure 1. Index map of the summitand upper eastrift of Kilauea Volcano showingthe locationof the April 1982 and September1982 summiteruptions,of the November1979 eruptionnear PauahiCrater on the upper east rift zone, and of the March 1980 eruption east of Mauna Ulu. Hachured lines showthe locationof the eruptive fissuresfor the varioussmall eruptions.
HELZ
ET AL.: HAWAIIAN
ERUPTION
GEOTHERMOMETRY
1'7,639
maximumvent temperaturesof 1070ø-1100øC.Thesewere uncorrectedtemperatures,and Finch and Macdonald indicated that the true temperaturesmight be 20øC higher. During the 1952 summiteruption of Kilauea, whichwas confinedto Halemaumau, Macdonald [1955] made a special study of optical pyrometrymeasurements.He noted that even under optimum viewing conditions,the apparent fountain temperature decreasedas the distancebetweenthe pyrometer and the fountain increased.Attributing this to the effect of absorptionby fume, as well as distance,he correctedhis publishedtemperatures for these effects(see Table 1). By contrast,the 1955 flank eruptionof Kilaueawaspoorlysuitedto opticalpyrometry,
mometry resultsfor the April and September 1982 eruptions, the Pu'u 'O'o eruption(January3, 1983,to July 1986), and the 1984 Mauna Loa eruption. In addition, glassgeothermometry data for the brief 1979 eruption are presentedfor comparison with the data from the 1982 eruptions. Comparative data for all but the Pu'u 'O'o eruption are summarizedin Table 2. The two brief 1982 summit eruptions were describedby Bankset al. [1983] and Baker [1987].The first eruptionstarted about noon on April 30 along a northeasttrending fissureon the floor of Kilauea caldera(see Figure 1) and endedshortly
1967-1968 Halemaumau eruption, for which Kinoshita et al. [1969] reported lava fountain temperaturesof 1087ø-1125øC, with the remark that they were probablytoo low. This period also saw the first attemptsto use the resultsof laboratory experimentsto calibrate temperatures at Kilauea. Both Richter and Murata [1966] and Wrightet al. [1968] attempted to use the early resultsof Tilleyet al. [1963, 1964] to cross-checkfield determinations of temperature. Estimates basedon the phaseequilibria data were much higher than the field measurements, generallyby 50ø-75øC.In addition, Wright and Weiblen[1967] analyzedcoexistingilmenite and magnetite from the 1965 Makaopuhi lava lake and used the Fe-Ti oxide geothermometerof Buddingtonand Lindsley[1964] to obtain a temperatureof 1030øCfor the first appearanceof magnetitein that bulk composition;this estimatewassimilarto the temper-
volumeof lavaproduced was3 x 106m3 (Table2), butroughly
aftermidnightonMay 1 afterproducing about0.5 x 106m3 of
lava. Fountain heights were generally low, and remarkably as fountainswere low and weather conditionsoften unfavorable, little sulfurousfume was detected. Temperatures of 1151øso both the optical pyrometerand a thermocouplewere used 1129øCwere measured,and many sampleswere collected.The duringthis eruption,with resultsas indicatedin Table 1. samplescontain small, sparseolivine phenocrystsand microThe 1959 summit eruption, with 18 episodesof high founlites of olivine,augite,and plagioclasein a glassymatrix. Glass taining, was well suited to optical pyrometer measurements analysesand supportingdata are presentedin Table A1. Ta[/lult et al., 1961;Richterand Murata, 1966]. The temperature blesA1-A4 are availableasan electronic supplement. • range observedwas 1065ø-1190øC,but a commentby/tult et al. The second 1982 summit eruption began the evening of [1961] suggeststhat only temperaturesin the range 1180øSeptember25 and endedabout0830 (all timesHawaiian Stan1190øCwere regardedas reliable. The optical pyrometerwas usedroutinely during the sixties,with data being publishedfor dard Time, HST) the next morning.In this case,the eruptive the 1960 flank eruption [Richterand Murata, 1966], the 1961 fissureopened up along the southernrim of Kilauea caldera summiteruption [Richteret al., 1964], the 1963 east rift erup- (Figure 1), forming a curtain of fire up to 1 km long with tion that producedAlae lava lake [Pecket al., 1966], and the fountains typically 20-40 m high. Flows moved both north 1965 Makaopuhi eruption [Wrightet al., 1968], as summarized (into the caldera)and south(out of the caldera)from the vent in Table 1. The lasteruptionof the 1950-1968 periodfor which system.For most of the eruption the main vents were surfield measurementsof temperature were published was the rounded by, and erupted through, a temporary lava lake. The half of this drained back down the fissureafter the eruption. From its abundantfume, more explosivebreaching,and somewhat higher fountains, the September lava appeared to be considerablymore gas-richthan the April eruption. Temperatures of 1160ø-1170øC were obtained on the main fountains,
with valuesof 1157ø-1122øCobservedin peripheral fountains and flows.An extensivesuite of glassysampleswas collected, thoughspattersamplescouldbe obtainedonly from the small, relativelycool, peripheralvents.The samplesare aphyric,with unusual,highly skeletal olivine _+ augite + plagioclase,in a glassymatrix. Glassanalysesand supportingdata are shownin Table
A2.
The November 1979 eruption, which occurredon the upper eastrift zone of Kilauea near Pauahicrater (Figure 1), wasin ature of magnetite-in(1020øC)that Wrightand Weiblen[1967] many respectsrather similar to the April 1982 eruption (see had inferred from thermocouple measurements in the Table 2); in particular, observersreported that the lava was Makaopuhi lava lake. relativelydegassed.Like the 1982 eruptionsit wasvery closely Interest in attemptingto determinelava temperature in the observed,though no reliable field measurementsof temperafield waned as it became apparent that many field tempera- ture were obtained.The early activitywas sampledintensively; tures were much lower than those observedin melting exper- however,becauseof the inaccessibilityof the singlevent active imentson Hawaiian basalts.Thus there are no publishedtemperature measurementsfor the 1968 eastrift eruption,for any few later 1979 samples.The lava containssmall olivine pheof the 1969-1974 Mauna Ulu activity,or for the 1971 and 1974 nocrystswith microlitesof olivine, augite, and plagioclasein summiteruptions.Graeberet al. [1979]obtainedthermocouple measurements in someof the Pu'u Ki'ai flows(see Table 1), of this materialmay be obtainedon a but, with that one exception, over a decade lapsed before •An electronicsupplement observersat Kilauea and Mauna Loa began to measure lava disketteor AnonymousFTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and temperaturesin the field again. GUEST as the password.Go to the right directory by typing CD
Eruptive Activity at Kilauea and Mauna Loa, 1979-1986 The presentpaper is a productof this latest period of interest in eruption temperatures,reportingcomparativegeother-
APEND. Type LS to seewhat files are available.Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95JB01309,Comparativegeothermometryof recent Hawaiian eruptions,RosalindTuthill Helz, Norman G. Banks,ChristinaHeliker, ChristinaA. Neal, and Edward W. Wolfe). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington,DC 20009;$15.00.Paymentmustaccompany order.
17,640
HELZ
ET AL.: HAWAIIAN
ERUPTION
GEOTHERMOMETRY
the glassygroundmass. Glassanalysesand other data are presented in Table o
A3.
Another Kilauea eruption that forms part of the database for comparativegeothermometryis the Pu'u 'O'o eruption, especiallythe first 20 episodes.This long-livedeastrift eruption beganon January3, 1983,with the initial fissuresextending from Napau Crater nearly 8 km downrift. The eruption then becamefocusednear the midpoint of the initial fissure andbuilt the hugecinderandspatterconesubsequently named Pu'u 'O'o. Wolfeet al. [1988]providean excellentdescription of the first 20 episodesof the activity(from January3, 1983,to June8, 1984), and Helikerand Wright[1991] give a brief summary of the subsequentactivity.Of the samplesfor whichwe have both field and glassgeothermometry,38 are from episodes1-20 and 11 are from later episodes;the relevant field data on the samplesfrom episodes1-20 are presentedby Neal et al. [1988]. The last of the eruptionsfor which comparativegeothermometrydata existis the 1984 eruptionof Mauna Loa, which beganon March 25, 1984,and lastedfor 3 weeks[Lockwoodet al., 1985;Lipman and Banks, 1987]. The lavaswere sparsely olivine-phyric, with microphenocrysts of plagioclase andpyroxene.Petrographicstudiesperformedto date includeLipman et al. [1985],Duggan[1987],and Crispet al. [1994].Lava temperatureswere determinedrepeatedly[Lipmanand Banks,1987], and many correspondingglassysampleswere collected,again making this eruption suitable for comparativegeothermometry. Glassanalysesand supportingdata are givenin Table A4. Methods
Used for Field
Determinations
of Temperature Infrared Pyrometry Measurements
.•
Temperaturesof lava fountainsand fast-flowing,uncmstcd lava flowswere measuredusinga hand-held,two-color,infrared pyrometer(brand name Hotshot). Measurementat two wavelengthsallows instant correctionfor emissivityto yield accuratetemperaturesfor the radiating source.Measurement in the infraredis preferableto the useof opticalwavelengths, especiallyfor objectswith temperaturesabove1100øC,as discussedby Pinkerton[1993], so it was anticipatedthat this pyrometerwould be an improvementover earlier generationsof opticalpyrometers.In order for a successful temperaturemeasurementto be made, the followingconditionsmust be met: (1) the targetareamustbe largeto eliminateinterferencefrom skyor crustedlava, and (2) there mustbe only minimal fume or steambetweenthe pyrometerand the target to avoid an altered absorptionresponseat one of the two wavelengths used. In general,the Hotshot pyrometerwas most useful for fissure-fedfountains,such as were available during the two 1982Kilauca summiteruptionsand the 1984Mauna Loa eruption. Details of the techniqueare givenby Lipman and Banks [1987]. Thermocouple Measurements
o
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Two typesof thermocoupleswere used during this period. The typemostcommonlyusedwasmadewith 1/16inch (0.159 cm) diameter, Inconel-cladChromel-Alumelthermocouple. For temperaturemeasurements in a'a flowsa stronger1/4 inch (0.635 cm) diameterthermocouplewas usedin order to penetrate the highlyviscouscrustof the a'a. These commercially availablethermocoupleshavean operatingrangeof 0ø-1250øC in moderatelyoxidizingenvironments.Testsperformedin an
HELZ ET AL.: HAWAIIAN
ERUPTION
independently calibratedfurnaceshowedthe thermocouples to be accurateto 1ø-3øCunder laboratoryconditions.Field mea-
....
METERS
surements havea somewhat largeruncertainty, butmultiple measurementsperformedwithin one episodeof eruption had a maximumscatterof 20øC[Neal et el., 1988], suggesting that the uncertainty of the field measurementsdoes not exceed _+10øC.In the field, when the original shielded tip of the thermocouplebecamedamaged,investigators would strip the wires back slightlyand twist the wires together. The twisted wire wouldthen be immerseddirectlyin moltenlava.Temperatures obtained in this way were similar in magnitude and reproducibilityto those obtained using thermocoupleswith original tips. As describedby Neal et el. [1988]and by Lipman and Banks [1987] measurementswere made by preheatingthe thermo-
GEOTHERMOMETRY
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;.?.;i•'. ?.-.•:..-. .•'"-.• :::;:•.......:, •.:-¾::':':'•.-:--" .• ....•-•-:--•:;•,, ....... :.•......:.: :.:..:•... ............ :,,..=.,.•:• . •...:....•:•....._...-•.•..:• Figure 13. SEM imagesof groundmassaugite + plagioclase in samplesfrom the April 1982 eruption. (a) Sample KS-4S, from the early part of the April 1982 eruption, showingfeathery or irregularlyformed augite intergrownwith acicularplagioclase.Magnificationis 160X. (b) Densespatterfrom sample KS-20S, from the latter part of the April eruption, showing augite + plagioclaseof more euhedralcharacter,typicalof the later samples.Magnificationas in Figure 13a. (c) Vesicular scoriafrom sampleKS-20S, showinglessabundantand finergrainedbut still euhedralaugiteand plagioclase.Magnification is 450X.
Figure 14. SEM imagesof samplesfrom the September1982 eruption.(a) SampleKS-5S,from early in the eruption,showing a small,highlyskeletalolivine characteristicof the September samples.(b) SampleKS-5Sagain,showingsmall,irregular or acicularcrystalsof augite+ plagioclase.(c) SampleKS-16S, from the latter part of the Septembereruption, showingmuch coarser,euhedralaugite + plagioclasetypicalof the later samples. Scalebar is 100 •m in each photo.
HELZ ET AL.: HAWAIIAN
ERUPTION
as definedby glassgeothermometry,are shownin Figure 15. Two of these(the 1982eruptions)are part of the comparative geothermometrydatabase;the third curve,for the 1979 eruption (Table A3), is includedhere becausethis eruption was close to the others in scale and was sampled with similar frequency.In all casesthe samplingwas so frequent that the shapesof the curvesare overdetermined.More important, in spiteof the similaritiesamongthe three eruptions(Table 2), eachhasa differentthermal signature,implyingthat the shapes contain information unique to the individual eruptions.The curvesin Figure 15 can be divided readily into three time segments:an early stageof 5-8 hourslength, a brief transition period, and a waning stage,which lasted to the end of the eruption. Early Stage
The moststrikingfeature of Figure 15 is that eacheruption has a differentslopeduringits first 5-8 hoursof activity.We suggestthat the differencesin initial slopereflectsthe interaction of two competingeffects.The first factor is the cooling experiencedby the earliestmagma,as the feeder dike propagatesthrough cold wallrock to the surface,an effect that will alwaysbe present in any eruption and that should result in cooler lava comingout first. The secondfactor, differencesin preeruptive devolatilizationhistory of the magma body, will vary from one eruptionto the next. In the 1979 eruption,MgO in the melt risesduringthe first 5 hours,showingthat the effectof coolingof the initial magma in the new dike is dominant.This impliesthat there wasvery little drop in H20 activityin the melt asthe 1979dike reached the surface;hence the 1979 magma had already undergone extensivedegassingin shallowstorage.This suggestionis consistentwith the presenceof plagioclasein sample79-11S(Table A3) in a glasshavingMgO = 7.12%. In samplesfrom the 1983-1990eastrift activity,plagioclase-in hasbeenobservedto lie at suchhigh MgO contentsonly in the Kupaianahapond samples,whichhad beensubjectto extensivedegassing prior to being sampled [Helz et al., 1991]. Further support for prior equilibrationof the 1979magmaat very low water contentslies
GEOTHERMOMETRY
17,653
EXPLANATION
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Figure 15. MgO in glass(left ordinate)and equivalentglassquenchingtemperatures(right ordinate)versustime (HST), for three brief eruptionsat Kilauea. Solid circlesare spatter samples,and opensquaresare flow samples.Solidverticallines connectreplicate analyseson the same material; dashedverin the goodagreement betweenTMgo andTeao for the early tical lines connectdifferent subsetsof scoriaor flow fragments 1979 samples(Table A3). from a singlesample.Early samplesfrom the April and SepIn contrast,the April 1982 slopeis slightlynegative,and the tember 1982 eruptions may not have equilibrated at water September1982slopeis steeplynegativeduringthisperiod.As contentslow enough for this calibration to be applicable,in discussed in the previoussection,there was a significantdrop which case their actual quenchingtemperature could be as in H20 in the top of the dikesthat fed the 1982 eruptions,with much as 10øClower than the right-handordinatevalue (see in text). The apparentlyanomalousresultsin Figure the amount of water lost being as much as 3 times higher for discussion the Septembermagmathan for the April lava.The variationin 15b, where the spattersamplesplot lower than adjacentflow samplesreflectsthe fact that the September1982 spattersamslopesseen in the first 5-8 hours of these three eruptions ples all come from relatively cool, small, peripheral vents, appearsto correlate directlywith relative preeruptivewater whereas flow sampleswere from the main flows, fed by the contentsand may serve as a qualitative indicator of this van- hotter main vents. able in other eruptions.
Transitional Stage
The brief transition period that occursin each of these eruptionsis marked in severalways.First, unusuallydifferentiated spattersampleswere erupted8-9 hoursafter the onset of the April 1982 and November1979 eruptions.Second,the calibrationshift seenin the 1982eruptions(Figure 12) occurs
beganto closedown after 1740 HST (Figure 15a), and major drainbackwas observed.Very cool, oxidized spatter began appearingat the still-survivingventswithin half an hour of that time and continuedfor about an hour. Similarly in 1979, the westernmost ventsbeganto declineat 1540HST (Figure 15c); at thispoint.Last,thesizeof thegapbetween Tcao andTMgo their last productswere cool, dense,oxidized spatter (e.g., changesfor the September 1982 and November 1979 erup- sample79-15 in Figure 16). Note that in both eruptions,most tions, also within this same narrow time window. of the materialeruptedstill later washotter than the reerupted The anomalouslycoolspatteris probablyregurgitateddrain- drainbackand that reerupted drainbackhas not been otherback.in April 1982the westernmost part of the eruptivefissure wise encountered,in spite of intensivespatter sampling.The
17,654
HELZ ET AL.' HAWAIIAN
ERUPTION
GEOTHERMOMETRY
Figure 16. Photomicrograph(plane light) of sample79-15, from the November 1979 eruption.The field of view is 2 cm across.The samplecontainsmany dark streaksof oxidizedmaterial within the body of the glass; theserepresentsurfacesformerly in contactwith air that have been folded back into the melt. The abundance of former air contactsurfacesin this sample suggestthat 'it is probablyreerupted drainback.
steep V shape in Figure 15a correlatesexactlywith the step- external stresson the magma rather than by processesin the wise shift seenin Figure 12a. Both of thesephenomenaappear feeder dike. to reflect a fairly abrupt drop in effusionrate observedat this point in the eruption, and we suggestthis is the underlying The 1984 Mauna Loa Eruption causefor all transition-stagephenomenain all three eruptions. We concludedthat the Kilauea-basedcalibrationof the glass geothermometersis not quite appropriate for Mauna Loa Waning Stage glasses.Nevertheless,as MgO would be expectedto vary linThe latter part of each eruption is distinct from the earlier early with temperature in Mauna Loa glasses,the profile of stagesof the sameeruption.In the two 1982 eruptionsall later MgO versustime for this eruption shouldstill be interpretable. material erupted had achievedan advancedstate of thermal Figure 17 showsa rapid initial drop in MgO content of the and textural equilibriumat low water contents,prior to erup- glasseswithin the first 12 hours of the eruption. This presumtion, as discussedabove. The overall decline in glass MgO ably reflects initial degassingfollowed by reequilibration at contentwith time seenin both 1982eruptionsat this stagethus lower water contents,after the dike breached the surface,by representsa true decline in the temperature of the dike. The analogywith the 1982 Kilauean eruptions. No field determitemperature decreasewas 1.3øC/hfor April and løC/h and nationsof temperaturewere made during this first 12 hours,so 1.3øC/hfor the vent and later flow samples,respectively,from the effect cannot be demonstratedas neatly as in Figure 12. the Septembereruption.Curiously,this declineis independent However, the similarities in pattern and length of time inof the effusionrate (Table 2), whichwasan order of magnitude volvedsuggestthisprocesswasinvolved,aswasinferred earlier greater throughoutthe Septembereruption than in the April by Lipman et al. [1985] and more recentlyby Crispet al. [1994]. eruption.The parameter of dike geometrythat most strongly Montierth et al. [1995] present evidence that crystallization controlscoolingrate is dike thickness[seeDelaneyand Pollard, triggeredby devolatilizationmay have extendedover an even 1982].The similarityin apparentcoolingrate in the latter half longer period for this eruption. Figure 17 and Figure 15a resembleeach other in that glass of the two 1982eruptionsimpliesthat the two feederdikes were of similar thickness,consistentwith the limited range of MgO rangesfrom a maximum of 6.8% at the beginningof the thickness(1-2 m) observedin dikes in Hawaiian volcanoes eruptionsto 5.8-6.0% at the end in both cases.The volume of lava erupted at Mauna Loa was 2 ordersof magnitudegreater [Delaneyand Pollard, 1982; Walker,1987]. The MgO content of the 1982 eruptionsdeclined to 6.1- (Table 2), and the eruption lasted 26 times longer, so the 6.0% MgO, at which point the eruptionsstopped,with termi- recurrenceof the same MgO range is striking. The total temnation presumablycontrolled,at least in part, by coolingand perature interval involved would be 15øC in the absenceof crystallizationin the feeder dikes.By contrast,MgO contentin volatile loss effects.It was recognizedearly [Lipman et al., the later 1979 glassesplateaued at 6.2-6.5% MgO (Figure 1985] that the 1984 lavas were uniform in bulk composition 15c),with the terminationof the eruptionnot beingobviously and general petrographiccharacter.The data in Figure 17 and related to progressivecooling of its feeder. The absenceof a Table A4 emphasizethat they were thermally uniform as well. declinein TMgo towardthe endof the 1979eruptionsuggestsThis uniformity implies the existenceof a reservoir of wellthat its terminationmay havebeen controlledmore by relief of mixed magma. If the 1984 lavaswere only slightlybelow pla-
HELZ
ET AL.' HAWAIIAN
7,2 --
ERUPTION
Mauna
GEOTHERMOMETRY
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Loa 1984 EXPLANATION
•1• Spatter, channel fluff I--I Pahoehoe / tube / channel dip samples
6.4
