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].