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SIDEROPHILEELEMENTABUNDANCESINKARAVANNOE:IMPLICATIONSFORTHEORIGINOFTHEEAGLESTATIONPALLASITES
M. Humayun1, S. N. Teplyakova2, C. A. Lorenz2, M. A. Ivanova2 and A. V. Korochantsev2
1Department of Earth, Ocean & Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA; 2Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow, Russia 119991.
Introduc)onThelinkbetweenchondri)cmeteoritesand
differen)atedmeteoritesisanimportantgoalinmeteori)csthatiso9enelusive,withtheexcep)onofisotopicfinger‐prin)ng[e.g.,1‐2].Tounderstandthepaleomagne)smofCVchondritesmodelsofapar)allydifferen)atedasteroid,withacarbonaceouschondri)csurfaceandadifferen)atedinterior(Fig.1),havebeenproposed[3‐4].ChemicaltestsofthismodelsupportedtheideathatCVchondritesmaybelinkedtotheEagleSta)onpallasitetrio(ESP)onthebasisofsiderophileelementmeasurementsofmetalfromEagleSta)onandColdBay[5].Therangeofchemicalfrac)ona)onintheESPmeteoritesisratherlimited,sotherecentdiscoveryofanewmember,Karavannoe[6],providestheopportunityforanimprovedtestusingsiderophileelements.OlivinefromtheKaravannoepallasitehasoxygenisotopecomposi)onsandmineralchemistry(Fe/Mn~97,Fa~20)thatiden)fyitasthefourthmemberoftheESP[6].Further,themetalinKaravannoeisplessitewithswathingkamacitearoundtheolivines[6].
Analy)calMethodsA19mmx12mmsawnslabofKaravannoe
(Fig.2)withminimalweatheringwasreceivedfromtheRussianAcademyofSciencesCommi\eeonMeteorites,mountedinepoxy,andpolishedononesideexposing>50%metalwithweatheringrestrictedtotheolivine‐metalcontacts.Siderophileelementsweredeterminedbyrasteringthreeolivine‐freemetallicareaseach~3‐4mm2.Laserabla)onICP‐MSanalysiswereperformedwithaNewWaveUP193FXexcimerlasersystemcoupledtoaThermoElementXR.A100µmbeamspotscannedat10‐25µm/s,50Hzrepe))onrate,2GW/cm2fluence,wasused.Otheraspectsofthemeasurementareiden)caltothosepreviouslydescribed[5].
References[1]ClaytonR.N.andMayedaT.K.(1996)GCA,60,1999‐2017.[2]BurkhardtC.etal.(2011)EPSL312,390‐400.[3]CarpozenL.etal.(2010)PNAS108,6386‐6389.[4]Elkins‐TantonL.etal.(2011)EPSL305,1‐10.[5]HumayunM.andWeissB.P.(2011)LPSCXLII,Abstract#1507.[6]KorochantsevA.V.etal.(2013)LPSCXLIV,Abstract#2020.[7]Schmi\W.etal.(1989)GCA,53,173‐185.
ResultsThesiderophileelementabundancesfor
threemembersoftheEagleSta)onPallasites(ESP)areshowninFig.3,comparedwithmetallicliquidderivedfrommel)ngofaCVchondrite[5].Thesuccessofmeasuringsmallareasbylaserabla)onandes)ma)ngthebulkmetalcomposi)oninapallasiteisjudgedintermsofobtainingmeaningfulrepresenta)onofkamaciteandplessite(inthecaseofKaravannoe)ortaenite.KamaciteconcentratesCoandAs,whiletaeniteconcentratesNiandAu.Fig.4showsthecorrela)onbetweenthe(Co/Ni)CIandthe(As/Au)CIra)os.IndividualanalysesofColdBaycoverabroadrangeofcomposi)on,whileanalysesofEagleSta)onandKaravannoearenearlyconstantatchondri)cra)osofCo/Ni.TheNi/Cora)oobtainedinthisstudyis
iden)caltothatofEagleSta)on(20.6)andtothatofCVmetal(20.4,[5]).BecausethebulkICPdataof[6]aresystema)callylowerincompa)bleelementsandsimilarinincompa)bleelements,tothenewLA‐ICP‐MSdatathedifferencebetweenourdataandthatof[6]hastobeascribedtochemicalzona)onwithinthelargeKaravannoemass(132kg,[6]),whichmayprovideanopportunitytobe\erdefinetheinternalfrac)ona)oninKaravannoe.
Discussion
Thesiderophileelementpa\ernofKaravannoeexhibitsthehighcompa)blesiderophileelementabundancesandthemarkeddeple)onsofW,FeandGarela)vetoaCVchondritecomposi)onthatalsocharacterizeothermembersoftheEagleSta)ongrouplet(Fig.3),confirmingthepriorclassifica)on[6].Thesefeaturesarerelatedtoforma)onunderoxidizingcondi)onsrequiredtostabilizehighNialloy.KaravannoefallsalmostdirectlyontheCVchondritemetalcomposi)online[5],exceptthatithaslowerPdandotherincompa)bleelements(Au,As,etc.)asexpected.Intermsofestablishingagene)crela)onshiptotheCVchondrites,Karavannoenicelyfollowsthecalculatedpa\ernofasolidmetalformedfromaCVmetallicliquid[5]a9ersomedegreeoffrac)onalcrystalliza)ontoremoveearliersolidsrepresentedbyEagleSta)on.Theexactdegreeoffrac)ona)onrequiredtomatchtheKaravannoepa\ernisdeterminedbytheScontentoftheliquid,whichisnoteasytoconstrainapriori.TomatchtheCuabundanceswerequireabout3‐6wt%Sintheini)alliquid,otherwise,thecalculatedsolidmetalCuabundancesformedfromCVmetallicliquid[5]aretoohighforESPmetal.Thusfar,afittoGeremainsdifficulttoachieve
sinceGedoesnotvarysignificantlyinitssolidmetal‐liquidmetalpar))oncoefficient.However,thetemperaturedependenceofthemetal‐silicatepar))oncoefficientisnotwellknown[7].Thefittothemostvola)leelementsbetweenacalculatedCVmetalcomposi)onandtheEagleSta)onPallasitesissufficientlyclosethatfurtherrefinementofthemodelisdesirable.AnimportantaspectoftheEagleSta)on
groupletisthatthehighcontentsofcompa)blesiderophileelementsimplyahighdegreemeltoftheCVparentbodyconsistentwiththemodelof[3‐4].
0.01
0.1
1
10
W ReOs Ir Ru Pt RhMoNi Co Pd Fe AuAs Sb CuGaGe Sn
CV3metalEagleStaMonColdBayKaravannoeF=1F=0.8F=0.6F=0.4 0.0
0.5
1.0
1.5
0.0 1.0 2.0 3.0 4.0
(As/Au)
CI
(Co/Ni)CI
EagleStaMon
ColdBay
CV3metal
Karavannoe
Karavannoebulk
Fig.4:Kamacite‐taenitefrac)ona)oninESPmetal.
Kama
cite
Taenite
Fig.3:CI‐,Ni‐normalizedsiderophileelementabundancesinEagleSta)onPallasitemetal.
Fig.1:Par)allydifferen)atedparentbodymodelforCVchondrites‐EagleSta)onPallasites,a9er[3‐4].
Fig.2:Polishedsec)onofKaravannoepallasite,newestmemberoftheESPgrouplet[6].