Hydrogen isotopic composition of water in CV-type carbonaceous chondrites

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Hydrogen isotopic composition of water in CV-type

carbonaceous chondrites

Laurette Piani, Yves Marrocchi

To cite this version:

Laurette Piani, Yves Marrocchi.

Hydrogen isotopic composition of water in CV-type

car-bonaceous chondrites.

Earth and Planetary Science Letters, Elsevier, 2018, 504, pp.64-71.

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Hydrogen

isotopic

composition

of

water

in

CV-type

carbonaceous

chondrites

Laurette Piani

∗

,

Yves Marrocchi

CRPG,UMR7358CNRS,UniversitédeLorraine,54500Vandoeuvre-lès-Nancy,France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received11July2018

Receivedinrevisedform21September 2018 Accepted24September2018 Availableonlinexxxx Editor: F.Moynier Keywords: chondrites protoplanetarydisk water organicmatter hydrogenisotopes SIMS

Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organicmatterare observedthat mightberelatedtothe timeand/orplaceofformationoftheir asteroidalparentbodies.However,theprecisedistributionofthesevolatile-bearingcomponentsbetween chondritegroupsandtheirchemicalandisotopiccompositionsremainfairlyunknown.Inthisstudy,we used anovelsecondaryionmassspectrometry analyticalprotocoltodetermine thehydrogenisotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the firsttimetheD/HratioofCVchondritehydrousmineralstobedeterminedwithouthindranceby hydrogencontributionsfromadjacentorganicmaterial.WefoundthatwaterinthealteredCVchondrites Kaba, Bali,andGrosnaja hasanaverageD/HratioofD/HCV-water = [144₋+821] ×10−6(or δDCV-water =

−77+₋54₁₃₁h,2

σ

),significantlyhigherthanwaterinmostCM-typecarbonaceouschondrites(D/HCM-water

= [101 _± 6]_× 10−6 _orδD

CM-water = −350± 40h,2

σ

). We show that becauseorganicmatterin

CV chondrites is depleted in deuterium compared to that in CMchondrites, such differences could result fromisotopicexchangebetweenwater and organics.Anotherpossibility isthatthe CMandCV parent bodiessampled differentreservoirsofwater iceandorganics characterizedbyvariableisotopic compositionsduetotheirdifferenttimeand/orplaceofaccretion.

©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The distribution of volatile elements and water in the Solar protoplanetarydiskisafundamental constraintontheconditions ofwaterdelivery toterrestrialplanets(Marty, 2012), yetremains poorlyknown.The differenttypesofcarbonaceouschondrites ac-creted water in varying concentrations (Marrocchi et al., 2018) thatmakethemsuitableforconstrainingthedynamicalprocesses that operatedduring asteroid accretion.Hydrogen isotopes (here-after D/H) represent a powerful cosmochemical tracer that pro-vides constraints on the origin and distribution of water within thedifferentSolarSystemobjects(Ceccarellietal.,2014) butfew precise estimations of the D/H compositions of water accreted by asteroids have been reported so far (Alexander et al., 2012; Pianietal.,2018).Thisisdueto thecomplexsub-micrometer in-tergrowthsofhydrated minerals andorganiccompounds (e.g.,Le Guillou et al., 2014)—the two main H-bearing phases in primi-tive chondrites—that impedethe estimation of water retainedin hydrated minerals without any contribution from organic

hydro-*

Correspondingauthor.

E-mailaddress:[email protected](L. Piani).

gen(Bonal etal., 2013; DelouleandRobert,1995; Remusatetal.,

2010).

The abundances andD/H compositionsof waterinchondrites have long been approximated by their bulk D/H ratio or esti-mated by mass balance using bulk and isolated organic matter measurements (e.g., Alexander et al., 2010; Robert and Epstein,

1982). However, these estimatesare affected by uncertainties on the organic content and its hydrogen abundance and isotopic composition(especially solubleandunknownorganiccompounds; Alexander et al., 2010, 2015). Based on bulk D/H and C/H mea-surementsofCR- andCM-typecarbonaceouschondrites,Alexander et al. (2012) showedthat the D/Hratio ofthe meanC-free end-membercanbeestimatedforchondritetypesofwhichnumerous samplesexist, suchasCR- andCM-typechondrites (D/Hinitial water

=

[171

±

17/10]

×

10−6 and [87

±

4]

×

10−6 (1

σ

), respec-tively).Forothertypesofcarbonaceouschondrites(CV, CO,orCI) thoughttorepresentdifferentparentbodies,thelimitednumberof hydrated samples available for bulk measurements precludes de-terminationoftheD/Hcompositionoftheirwater.

Recently, a new method has been developed to estimate the D/Hratioofwaterinchondritesfromin-situ measurementsofthe C/H and D/H ratios by secondary ion mass spectrometry (SIMS; Piani etal.,2018).Thenoveltyofthemethodwastomeasurethe

https://doi.org/10.1016/j.epsl.2018.09.031

0012-821X/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

D/HandC/Hratios indifferentareasofa givenchondritematrix containing varying proportions of hydrated minerals andorganic matterandusethe zero-intercept ofthecorrelation betweenthe two ratios to estimate the D/Hratio of hydrated minerals (with aC/H

=

0) (see Pianiet al.,2018 formoredetails).Thismethod revealed that all hydrated CM chondrites accreted water with a similar D/H ratio of [101

±

6]

×

10−6 ₍

_δ

_D

CM/SMOW

= −

350

±

40

h

,2

σ

),consistentwiththemeancompositionofCMchondrite water determined from bulk measurements. However, the least-alteredlithologiesoftheleast-alteredCMchondriteParis(Hewins etal., 2014; Marrocchi etal., 2014) showsignificantly higherD/H ratios(D/HParis

=

[145

±

25]

×

10−6 or

δ

DParis/SMOW

= −

69

±

163

h

,2

σ

)thatcould tracktheaccretionofwatericegrainsthat originatedintheouterSolarSystem(Pianietal.,2018).

Vigarano-typecarbonaceous(CV)chondriteshavebeenaffected by variable,butsignificant, secondary alteration processes(Doyle etal., 2015; Ganino andLibourel, 2017; Marrocchi et al., 2018), revealingthat the CV chondrite parentbodies accreted water ice grainsatan estimated water/rockratio of0.1–0.2 (Zolotov et al.,

2006). CV chondrites are thought to have formed earlier during theevolutionoftheprotoplanetarydiskthanhydrated CIandCM chondrites(SugiuraandFujiya,2014),andconsequentlycouldhave accreted water ice grains characterized by unique hydrogen and oxygen isotopic compositions.Indeed, CV chondrite bulk, matrix, andsecondarymineraloxygenisotopiccompositionsdefinea con-tinuoustrend resulting fromisotopic exchange between 16_O-rich anhydrous silicates and 17,18_{O-rich fluid (Marrocchi et al., 2018).} Based on those results,it has been proposed that CV chondrites accreted water with

δ

18O

>

60

h

(i.e.,



17O

= δ

17_{O – 0.52}

_×

δ

18O

>

15

h

;Marrocchietal.,2018),whichcouldresultfromthe contributionof 17,18O-richwater icefromthe outer Solar System (Sakamotoetal.,2007).

CVchondrites are subdividedintoa reducedsubgroup (CVred) and two oxidized subgroups, Allende-like (CVoxA) and Bali-like (CVoxB),reflectingtheircomplexalterationhistory(Ganinoand Li-bourel,2017; McSween,1979).Bali-type CV3chondrites (CV3oxB; Weisberg et al., 1997) contain abundant H-bearing phyllosili-cates(saponite,Na-rich phlogopite)intheir fine-grainedmatrices (Howardetal., 2010), allowing the D/Hcompositionof CVwater to be determined. Here we report in-situ SIMS measurements of theD/HandC/HratiosofthematricesoftheoxidizedCV3 chon-dritesKaba,Bali,andGrosnajaanddiscusstheirconsequencesfor thedistributionofwaterisotopiccompositionsintheasteroid for-mationregion.

2. Samplesandmethods

Samples of Kaba (NHMV_#ID_13236_[3a]), Bali (NHMV_

#ID_524_[C]), and Grosnaja (NHMV_#ID_2745_[C]), were kindly provided by the Natural History Museum Vienna. Submillimeter-sized pieces of each chondrite were handpicked and pressed in indiumfoil.Scanningelectronmicroscopemappingwasperformed to select flat areas of fine-grained matrix forSIMS analyses. For comparison,theCM2chondriteSayama(ahighlyalteredCM chon-drite;Yoneda etal., 2001) andtheleastalteredlithologiesofthe CM2.7chondriteParis(Hewinsetal.,2014) wereanalyzedduring thesameSIMS session(Pianietal.,2018,andreferencestherein).

SIMS measurements were performed with the IMS 1280HR at

Centrede Recherches Pétrographiqueset Géochimiquesin Nancy, France.A 10 keV Cs+ primary beamwas used forthe measure-ments.Prior toanalyses, 6 minutesof high-current(2.5 nA) pre-sputtering was applied to clean the sample surface and reach a steadystateforcesium implantation andsputtering. Thesamples werethen sputteredwitha primary currentof2nA and250pA forCVandCMchondrites,respectively,toobtainabout1.5

×

105 countsper second onH− forall samples.Theprimary beamwas

rastered over an area of 50

×

50 μm2 _{on the sample surface.} A normalincidentelectrongunwas usedtoimprovecharge com-pensation. The analyzed area was restricted to 15

×

15 μm2 in the center of the rastered area by using a 30% electronic gate. A high magnificationmode(MaxArea 40)anda smallfield aper-ture(FA2000)were usedto minimize Hcontamination fromthe borderofthebeam.H−,D−,13_C−_,and29_Si− _{ionswerecollected} successivelybychangingthemagneticfieldandcountedwiththe monocollectionelectronmultiplier.Themassresolutionpowerwas settoM/



M

=

3300toavoidinterferenceson13_C− ₍12_CH−_)and

29_Si−₍28_SiH−_{).Foreachanalysis,40cycleswerecollectedwith2 s} ofcounting time per cycleforH−,13C−,and 29Si− and20 sfor D−foratotaldurationof31 min.

Aserpentineandfourwater-bearingglasses(in-housereference basaltic glasses with 0.85, 1.45, 3.64, and 4.29 wt.% H2O) were chosenasreferencesforhydratedminerals.Organicreference ma-terials were atype III kerogen,insoluble organicmatter (IOM) of the Antarctic Grosvenor Mountains 95502(GRO 95502) ordinary chondrite,andIOMofthecarbonaceouschondriteOrgueil(for de-tails,seePiani etal.,2012,2015).Allsamplesandstandardswere pressed in indium and gold-coated before analyses. The statisti-cal error on the D/H ratio for one analysis is

≤

27

h

(2

σ

) and thereproducibilityis

≤

68

h

(2

σ

standarddeviation)onthe water-bearingglasses.ThezerointerceptoftheD/Hvs.C/Hcorrelationin thefine-grainedmatrixisusedtoestimatetheD/Hratiosof inor-ganic phases(hydratedminerals) followingthe methoddescribed inPianietal. (2018).

3. Results

Althoughthe29Siand13Cionicemissionsweresimilar forCV andCMchondrites,hydrogenionicemissionsofCVchondrite ma-triceswereaboutoneorderofmagnitudelowerthanthoseofthe ParisandSayamaCMchondrites(Tables1andS1).Positive correla-tionsbetweenthemeasuredD/HandC/Hratioswereobtainedfor bothCMandCVchondrites(Fig.1).KabaandBalihaveC/Hratios

∼

7–8timeshigherthantheCMchondrites,andtheirD/HandC/H ratiosare positivelycorrelated withPearson correlation factorsof 0.76and0.58,respectively(Fig.1,Table1).Theco-variationofD/H andC/Hratiosisnot clearforGrosnaja,asitsPearsoncorrelation factoris0.04.AlthoughtheSi/HratiosofGrosnajaareofthesame orderastheother CVchondrites,its C/Hratiosareabout3times lower,indicatingthatGrosnajacontainslesscarbonthantheother CV3chondrites (Table1,Fig.1). Inadditiontolinearfitsforeach CV chondrite, we calculated a linear fit for all the CV chondrite data combined (Pearson coefficient 0.81) (Fig. 1, Table 1). Linear fitsandparameterswereobtainedusingtheIgorProsoftware.

4. Discussion

4.1. HandCcontentsandD/HratiosofwaterinCVchondrites

The hydrogen signal measured in CV chondrite matrices is aboutone order of magnitudelower than that in CM chondrites Sayama and Paris (Table 1). Although it is difficult to quantita-tively estimate the H content of chondrite matrices (i.e. Deloule andRobert,1995),suchresultsprovidequalitativeinformationon thelowHcontentofCVchondrites.Asshownbyanalysesofglass andserpentinestandards(Fig.2),H−/29_Si−_{linearlycorrelateswith} H2O/SiO2.Assuming thatthe SiO2 concentration issimilar inCM andCV chondritematrices, we thus estimate that theCV matri-cescontain aboutone orderofmagnitudelesshydrogenthan the CM matrices. This estimate is consistent with (i) bulk measure-ments reporting that the mass fraction of H is 830–2250 ppm in Kaba, 470 ppm in Bali, 1560 ppm in Grosnaja, and between 0.53 and 1.46 wt.% in CM chondrites (Alexander et al., 2012;

Table 1

MeasuredH−(incountspersecondperpAofprimarycurrent),29_Si−_/H−_,and13_C−_/H−_{ratiosandhydrogenisotopiccompositionsofhydratedmineralsdifferentCVand}

CMchondrites.D/HratiosarecorrectedforinstrumentalmassfractionationusingglassstandardTan25_0.N=numberofanalyses.SE=standarderror. H− (counts·s−1_·pA−1₎ 29_Si−_/H−_(×10−2₎ ±2SE 13_C−_/H−_(×10−3₎ ±2SE N Pearson coeff. D/H(×10−6₎ ±2SE δD(h) ±2SE Kaba (CV3oxB) 60±42 59.9±3.5 67.5±4.4 42 0.76 139±14 −108±88 Bali (CV3oxB) 56±31 80.9±9.2 62.0±4.9 28 0.58 153±13 −20±86 Grosnaja (CV3oxB) 94±226 53.8±8.1 19.3±4.1 10 0.04 154±31 −11±199 All CVs – – 81 0.80 144±7 −77±44 Sayama (CM2) 427±89 6.6±0.3 7.9±1.7 16 0.81 105±9 −329±56 Paris (CM2.7/2.9) 443±213 6.9±1.9 9.1±3.9 7 0.94 127±12 −182±78

Fig. 1. MeasuredD/Hvs.13_{C/HratiosinthematricesofCVchondritesKaba,Bali,}

andGrosnajaandCMchondritesSayamaandParis(measuredduringthesame an-alyticalsessionforreference).Linearfits(solidlines)and95%confidenceinterval bands(dashedlines)werecalculatedforthecombinedCVdatasetandthe individ-ualCMchondrites.Errorbarsrepresentinternalerrorat2σ.

Fig. 2. Relationshipbetween thesecondaryionintensityratioH−/29_Si− _andthe

knownH2O/SiO2contentsoftheserpentinestandardSerp-12A(H2O=12.2wt.%,

SiO2=43wt.%)andfourhydrousglassstandardsTan25_0andTan25_4(H2O=

0.85and4.29wt.%,respectively,SiO2=60.2wt.%),ETNA4(H2O=3.64wt.%,SiO2 =46.8wt.%),andGlass47963(H2O=1.45wt.%,SiO2=48.2wt.%).Therangesof

H−/29_Si−_{ratiosobtainedinthisstudyforCMandCVoxBchondritesareshown.}

Kerridge,1985),and(ii)thelowerabundanceofhydratedminerals inCVcomparedtoCMchondrites(Howardetal.,2010,2014;Krot etal.,1998).

Contrary to hydrogen, the ranges of carbon emission in CV and CM chondrites are of the same order (0.4–16.4 and 1.5– 7.2 counts

·

s−1

_·

_pA−1_{,respectively;TableS1).ThebulkC}

concentra-tion in CV chondrites (1.07 wt.% in Kaba, 0.64wt.% in Bali, and 1.1 wt.% in Grosnaja; Alexander et al., 2012; Kerridge, 1985) is aboutafactoroftwolowerthaninmostCMchondrites.However, because carbonis mainly presentinthe matrix,andthe amount ofmatrixinCVs(

∼

35vol.%)isabouthalfthatinCMs(

∼

60 vol.%), ourresultsindicatethatthecarboncontentsofCVandCM chon-drite matrices are of the same order (Alexander et al., 2018; Zandaetal.,2009).Wemeasuredalowercarboncontentin Gros-naja compared to the other CV chondrites (Table 1) that is not consistentwithreportedbulkmeasurements(Kerridge,1985).Itis thus possiblethat Grosnaja contains aheterogeneous distribution of organic particles andhydrated minerals, asreported forother CVchondritessuchasBali(Bonaletal.,2006; Kelleretal.,1994).

The positive correlations between the C/H and D/H ratios of chondrite matricescan beinterpreted astheresultofmixing be-tween twoisotopicallydistinct H-bearingend-members:aD- and C-rich organic material and a C-free, D-poor inorganic material (Alexander etal., 2012; Pianiet al.,2018). Thezero interceptsof theC/Hvs. D/Hcorrelationscanthusbeused,aftercorrectionfor instrumental massfractionation,to estimatetheD/Hratiosofthe inorganicphases(Pianietal.,2018).D/Hratiosarecorrectedfrom the instrumentalmassfractionation usingglassstandard Tan25_0 (

α

=

0.87

±

0.02),whichhasthesameHcontentandwas mea-suredwiththe sameprimary currentconditionsastheCV chon-drite matrices. Because isotopic fractionation between waterand hydratedmineralsattheCMparent-bodytemperatureislow com-paredtotheprecisionofthemeasurements(Pianietal.,2018),the inferredD/Hisaproxyofthehydrogenisotopiccompositionofthe wateraccretedbytheparentbody.TheD/Hratiosofwaterinthe Sayama andParisCMchondrites are D/HSayama-water

=

[105

±

9]

×

10−6 ₍

_δ

_D

Sayama-water

=

–329

±

56

h

,2

σ

) andD/HParis-water

=

[127

±

16]

×

10−6(

δ

DParis-water

= −

182

±

78

h

,2

σ

),respectively (Table 1, Fig. 3). These values are similar within error to previ-ousestimatestakingtheSayamaD/Hratioasrepresentativeofthe main water componentin CM chondrites (

δ

DCM-water

≈ −

350

h

; Alexander et al., 2012; Piani et al., 2018) and the least altered lithologiesofParisaspreservingadeuterium-richwatersignature presentintheCMrockbeforealteration(Pianietal.,2018).InCV chondrites, theconditions (temperatureand/or abundance of wa-ter)inwhichhydratedmineralsformedarepoorlyconstrained, re-sultingin(1) uncertaintiesonthepossiblewater/hydratedmineral fractionationcoefficientsand(2)possiblynoorverylow fraction-ationasmostofthewaterisprobablyconsumedtoformhydrated minerals.SmectiteinKabawasproposedtoforminatemperature range of0 to100◦C by analogy to terrestrialoccurrences(Keller andBuseck, 1990andreferencetherein), butarecent mineralog-ical study by Ganino and Libourel (2017) proposed the hydrous phases inCV chondrites could witness thelow temperaturepart (

∼

200◦C)ofthefluidassistedmetamorphismtrendpossibly dur-ingparentbodycooling.Giventhefullpossibletemperaturerange (0–200◦C), theisotopicfractionationfactorbetweensmectiteand water can thus be containedbetween 87

h

at 0◦ and

−

10

h

at 200◦C(Méheutetal.,2010) orbecloserto0

h

ifmostofthe wa-terwasconsumedtoformhydratedminerals.Takingourcombined

Table 2

Hydrogencontentandisotopiccompositioninorganicmatter(OM)andhydratedminerals(Hydr.Min.)forCVoxBchondriteKabaandCMchondrites(meanvalue).

Phase Modal ab. H content ModalH

(wt.%) HOM/Hbulk Hbulk (wt.%) D/H (×10−6₎ BulkD/Hcalc (×10−6₎ BulkD/Hmeas. (×10−6₎ CV (Kaba) OM 0.68 wt.% Ca _{H/C (wt.)=0.03}a _{0.018 0.28 0.063 188}a _{153 158}h Hydr. Min. 2.3 wt.%b _{2.0 wt.%}e _{0.045 139±14}f CM (average) OM 0.9 wt.% Cc _{H/C (wt.)=0.05}c _{0.049 0.05 0.913 272±9}c _{110 175}h

Hydr. Min. Cronst. 31.0 wt.%d _{1.0 wt.%}d _{0.864 101±6}g

Serp. 38.3 wt.%d _{1.4 wt.%}d

a _{TotalCcontent,H/C,andD/HratiosofinsolubleOMinKabafromAlexanderetal. (2007).} b _{ModalabundancesfromHowardetal. (2010),calculatedusingdensitiesfromwebmineral.com.} c _{AveragedvaluesforallCMchondrites(exceptBells)fromAlexanderetal. (2007).}

d _{ModalabundancesfromHowardetal. (2014);averagedensitiesandHcontentsofCM2chondritecronstedtite(Cronst.)andFe/Mg-serpentine(Serp.)fromwebmineral.}

com.

e _{HcontentinKaba’sphyllosilicatesestimatedfromKellerandBuseck (1990).} f _{Thisstudy,cf.Table1.}

g _{Pianietal. (2018).}

h _{Averagevaluesforallnon-heatedCM(exceptanormalBellsandEssebichondrites)fromAlexanderetal. (2012).}

Fig. 3. D/HratiosofwaterinthemeasuredCVandCMchondritesaftercorrection forinstrumentalmassfractionation(errorbarsat2σ).Theaverage compositions ofCMchondritewater(Pianietal.,2018) andCVchondritewater(thisstudy)are distinct(graydomainswith2σ standarderrorintervalsforhydratedminerals de-limitedbythedashedlines).Thedoted-lineshowstheminimumD/HvalueforCV watercorrespondingtotheextremecasewereCVhydrousmineralsformedat0◦C (seedetailsinSection4.1).Theless-alteredlithologyofParis(purplediamond)is notrepresentativeofCMchondritewaterbutofadeuterium-richwatersignature presentintheCMrockbeforealteration.(Forinterpretationofthecolors inthe figure(s),thereaderisreferredtothewebversionofthisarticle.)

CVchondritedataset,weobtainedaD/HratioforCVchondrite hy-dratedmineralsofD/H

=

[144

±

7]

×

10−6 (

δ

D

= −

77

±

44

h

, 2

σ

;Table1).Weobtainsimilarestimatesbyconsidering eachCV chondriteseparately,althoughwithhigheruncertainties (Table1). Giventhewhole rangeofpossibleformation temperaturefor hy-dratedminerals,thisleadstoaD/HratioforCVchondritewaterof D/HCV-water

=

[144+₋8₂₁]

×

10−6 (or

δ

DCV-water

= −

77+₋54₁₃₁

h

,2

σ

; Fig.3).ComparedtothemainwatercomponentofCMchondrites, waterinCVchondritesissignificantlyenrichedindeuteriumwith anenrichmentfactorof

∼

1.4(Fig.3).

4.2.ComparisonofwaterandorganicsinCVandCMchondrites Determining the D/H composition of the C-rich end-member (organics) would require assumptions on the 13C−/H− ratios of the CM or CV organic components, inducing large uncertainties. Nevertheless,thecleardivergenceoftheCM andCVmixinglines (Fig.1) indicatesthat theC-rich end-member inCVchondrites is significantly depleted in deuterium compared to organics in CM chondrites.Indeed,IOMisolatedfromCVchondriteswasfoundto havealower D/Hratiothan CM chondriteIOM (Alexander etal.,

2007) and solubleorganiccompounds(Pizzarelloetal., 2006).To furthercompare theD/Hcompositions ofCM andCV chondrites,

we have estimated the respective contributions of organic mat-terand hydrated silicatesto the bulk hydrogen budget based on theirmodalabundancesandHconcentrationsinCMandCV chon-drites(Table2andreferencestherein).AmongCVchondriteswith well-characterizedorganicmatterandhydratedsilicates,the least-metamorphosedKabachondrite (Bonal etal., 2006) was takenas the mostrepresentative. The contributionof hydrogenin organic matterto thebulkHcontent ismuchhigherinCV(29%)than in CM chondrites(5%).Using theseestimatesandassuming thatthe organicmaterial correspondsto IOM thatcan be isolatedby acid treatments, we estimate the CV bulk D/H to be 153

×

10−6_{, in} goodagreementwiththebulk D/Hof156

×

10−6 reportedfrom gas-phasespectrometry(Alexanderetal., 2012). Thisimpliesthat the organic matter component in CV chondrites can be approxi-matedfromIOM. Incontrast,the calculatedCMbulk D/Hratiois 0.6timesthe reportedD/Hratio(Table2). Thisdiscrepancy indi-cates that (i) IOM in CM chondrites is not representative of the entireorganic matter component, and(ii) the total organic com-ponent currently present within CM chondrites is enriched in D comparedtotheorganicmaterialsknownsofarinCMchondrites. 4.3. OriginofthedifferencesintheD/Hratiosofwaterandorganic matterinCMandCVchondrites

The water-derived H content of CV chondrites corresponds to about 5% of that ofCM chondrites (Table 2). Thisdifference can eitherbeshapedbyHlossduringparent-bodyprocessesor inher-itedfromdifferentwatericecontentsintheCMandCVaccretion zones.Duringaqueousalteration,waterisbelievedtoactasan ox-idizingagent formetal,resulting inthereduction ofwatertoH2 (Alexander etal., 2010). Theloss ofvolatile H2 wouldbe associ-atedwithRayleigh-typedistillation,enrichingtheremainingwater indeuterium.Based ontheiron valencesestimatedinCI,CM,CR, and OC chondrites, Sutton et al. (2017) proposed that all these chondritetypesaccretedwaterwithasimilar deuterium-depleted isotopic composition of D/H

≈

60–180

×

10−6 (

δ

D

≈ −

650 to

+

154

h

) depending on the reaction temperature(0–200◦C). Fol-lowingtheirestimation,we fixedtheinitialD/Hratioofwaterin CV chondrites as that in CM chondrites, D/HCM

=

101

×

10−6 (Piani et al., 2018), and calculated the isotopic evolution of CV chondrite waterduring Rayleighdistillation at0,100, and200◦C usingthe equilibriumfractionation coefficientsbetweenH2Oand H2 vapordefinedbyLécluseandRobert (1994) (Fig.4).The differ-encebetweentheD/HratiosofCVandCMwatercanberelatedto thelossofabouthalfoftheirinitialwatercontent(fractionof re-mainingwaterof0.66at0◦C,0.58at100◦C,and0.51at200◦C), butsuchalosscannotexplainthestrongdifferenceinwater con-tentbetweenCMandCVchondrites(Table2,Fig.4).

Fig. 4. CalculatedD/HratiosoftheremainingwaterfractionafterRayleigh fraction-ationat0,100and200◦CforwaterwithaninitialD/HratioequaltothatofCM carbonaceouschondritewater,D/H =101×10−6_{.Rayleighfractionationcannot}

explainboththeremainingfractionofwaterinCVchondritesrelativetoCM chon-dritewaterandtheD/HcompositionofCVwater(graylines).

Assuming that carbonaceous chondrites heterogeneously ac-creted water icegrains withsimilar D/Hratios would requireto findaparent bodyprocess capableofenhancingthe D/Hratioof waterinCV chondritesbutnot inCM chondrites.Giventhe high proportionsofC-bound(organic)tosilicate-bound(water-derived) HinCVrelativetoCMchondrites(Table2),Hisotopicinteractions between water and organic matter—if kinetically efficient—could havesignificantlyaffectedtheD/Hratiosofbothwaterand organ-icsinCVchondrites. Theefficiencyofhydrogenisotopicexchange betweenaqueousfluidsandextraterrestrialorganicmaterials has long been discussed in literature (Alexander et al., 2007, 2010; Obaand Naraoka,2009; Piani et al., 2015; Remusat etal., 2010; Sessionsetal., 2004; Yabutaetal., 2007). Themain challengesin constrainingsuchhydrogenisotopicexchangeliein:(1)the multi-ple typesof C–H bonds inorganic materials, resulting ina large range of possible reaction constants and rates; (2) the complex natureofchondriticorganicmaterialforwhichnosatisfactory ter-restrialorsyntheticanaloguesareavailable,precluding straightfor-wardisotopic exchangeexperiments; (3)thepresenceofpossible catalyticmineralsurfacesthatwouldmodifythechemicalreaction efficiency;and(4)theoverlappingofseveralchemicalmechanisms thatcannotbeunraveledwithoutcompound-specificD/Hanalyses ofknownorganicmaterials, i.e., pureisotopic exchange,chemical

reactionsthatmodifythemolecularstructurebutnotthechemical formula,andchemicalreactionsthatmodifythechemicalformula (Sessionsetal.,2004).

Consideringonlypureisotopicexchangebetweenwaterand or-ganicmaterials, theextentofisotopicexchangewoulddependon thedurationandtemperatureofalteration.Theoreticalcalculations (Bottinga, 1969; Remusat et al., 2010) and experiment-based es-timates (e.g., Sessions et al., 2004) indicate that, at equilibrium, organicsshould bedepletedindeuteriumrelativetowater.Given our calculated bulk D/H ratio and fractions of hydrogen in or-ganic matter (HOM/Hbulk; Table 2) and water (1

−

HOM/Hbulk) in the CV chondrite Kaba, we calculated that the equilibrium D/H ratios of organics and water should be about D/HOM

=

137

×

10−6 _andD/H

water

=

159

×

10−6,respectively(timetEq inFig.5; see Appendix A for details). The presence of D-poor water rel-ative to organics in both CM and CV chondrites indicates that, if isotopic exchange occurred between water and organics dur-ing aqueous alteration on asteroids, equilibriumwas not reached (Fig. 1, Table 2). Hydrouspyrolysis experiments involving D-rich IOMisolatedfromCMchondritesandD-poorwaterhaveshowna strongdecreaseoftheorganicmatterD/Hratios(

δ

Ddecreasingby

∼

800

h

)atT

=

270–330◦Cinafewdays(ObaandNaraoka,2009; Yabuta etal.,2007),consistent withpreviousexperiments involv-ingterrestrialkerogens(Schimmelmannetal.,1999).ForCM chon-drites, such a rapid isotopic effect is difficult to reconcile with: (i) thelackofvariationofIOMD/Hratiosmeasuredinthese chon-drites(Alexander etal.,2007);(ii)theabsence ofmodificationof the IOMaromaticmoietiesafteraqueousalteration (Busemannet al., 2007; CodyandAlexander,2005),contrarytopyrolysis exper-iments (Sephton et al., 2000); (iii) the absence of self-diffusion fronts in the D-poor hydrated minerals surrounding the D-rich organicparticles(Remusatetal.,2010);and(iv)identicaland well-definedmatrixC/Hvs.D/HcorrelationsindifferentCMchondrites despitetheirdrasticallydifferentdegreesofalteration(Pianietal.,

2018) and,assumedly,differentextents ofmodificationby pyroly-sis.

In CVoxB chondrites, the presence ofscattering inthe D/H vs. C/H trends (Fig. 1) and therange ofD/H ratiosmeasured for CV IOM (Alexander et al., 2007) could be the consequences of par-ent body alteration and/or metamorphism. The amount of water available toreactwiththeorganicmaterialsislowerthanthat in CM chondrites and could havelimitedthe efficiencyof exchange reactions, explainingwhyequilibriumwasnot reached. Assuming the bulk D/Hratio ofKaba andthat the amountof H inorganic matterandwaterwaspreservedduringtheexchangereaction(i.e.,

Fig. 5. Simplifiedmodelofisotopicexchangebetweenwaterandorganicmatterduringparentbodyalteration,assumingpureisotopicexchangeinaclosed-system.(a) The D/Hratiosofwaterand organicmatter(OM)evolveduntilreachingequilibriumattimetEq.(b)CoupledevolutionoftheD/Hratiosoforganicmatterandwateruntil

reachingequilibrium(Eq.)from(a)comparedtotheD/HratiosofwaterandinsolubleorganicmatterinCMchondrites(average)andCVchondritesKabaandBali(Table2

sinceaccretion),wecalculatedtheinterdependenceoftheD/H ra-tiosofwaterandorganicmatterthroughouttheisotopicexchange reaction until equilibrium was reached (Fig. 5 and Appendix B). Thestartingcompositionsrepresenttheunrealisticcasewhereall availableDatomsinCVchondritesareassociatedwithorganic ma-terial(Fig. 5a), corresponding to a maximumorganic matterD/H ratio of 545

×

10−6 (

δ

D

=

2500

h

). Interestingly, this value is belowthatreportedforIOMintheCRandanomalousBells chon-drites(

δ

D

∼

3000

h

; Alexanderetal., 2007), indicating that the D/H composition of CV organic matter before isotopic exchange withD-poorwateron asteroidscannot be identicalto thatofCR chondrite IOM as proposed by Alexander etal. (2012). The cou-pledevolutionofthewaterandorganicmatterD/Hratiosisthen comparedtothoseofKaba,Bali,andtheaverageCM composition (Fig. 5b). The goal of this exercise is to see if CV chondrite wa-terandorganicmattercouldhaveasimilaroriginasorbederived fromwaterandOMin CM chondrites.The averaged composition ofCM chondritewaterandorganicmatter fallsonthe CV evolu-tion trend(Fig. 5b), indicating that pure isotopic exchange could explaintheevolutionofwaterandorganicD/HratiosfromCMto CVcompositions.

Weacknowledge that there are several issueswith usingthis simple mechanism to understand the data. First, it is unclear why water and organics in CV chondrites would have experi-enced a greater degree of hydrogen isotopic exchange than in CM chondrites, which would have been a more favorable envi-ronmentforisotopicexchangegiventheirhigherwater/rockratios compared to CV chondrites (e.g., Marrocchi et al., 2018). How-ever, this could be linked to the higher alteration temperature experienced by CV chondrites (i.e., temperature range from 200 to 600◦C; Ganino and Libourel, 2017) relative to CM chondrites (i.e., 110◦C; Verdier-Paoletti et al., 2017). Consequently, the ca-pabilityofC-boundhydrogen inextraterrestrialorganicmatterto exchange with water-bound hydrogen (exchangeability) could be limitedattemperaturesrelevanttoCMparentbody hydrothermal-ism(Schimmelmannetal.,1999,andreferencestherein).

ItisalsopossiblethatthedifferentD/Hratiosofwaterand or-ganicsinCMandCVchondriteswere inheritedfromthedifferent place and/or time ofaccretion of their respective asteroidal par-entbodies(JacquetandRobert,2013; Yangetal.,2013).Thiscase couldprovideanexplanationfortheoxygenisotopiccompositions ofCV chondrite secondary phasesthat suggest that the aqueous fluidfromwhichtheyformedwasenrichedin17,18Oisotopes com-pared to CI-, CM-, and CR-type carbonaceous chondrites (Fujiya,

2018; Marrocchi etal., 2018).WaterinCV chondriteswouldthus havebeen enriched in the heavy isotopes of both hydrogen and oxygen,suggestinga contribution ofouter SolarSystem waterto theCVchondriteparentbody(YurimotoandKuramoto,2004).The factthatCVchondritessampledahighercontributionofouter So-larSystemwatercomparedtomostofthehydratedcarbonaceous chondrites(CM,CR,CI),withtheexceptionofParisCMchondrites (Pianietal.,2018; Vacheretal.,2016),couldbeduetotheslightly earlieraccretionoftheCVparentbody(SugiuraandFujiya,2014).

5. Concludingremarks

Usinga recentlydeveloped SIMS analyticalmethod,we deter-minedtheD/HratioofwaterpresentinCVoxB-typecarbonaceous chondritesKaba,Bali,andGrosnaja.TheD/Hratioofwaterinthese CV chondrites (D/HCV-water

=

[144+₋8₂₁]

×

10−6 or

δ

DCV-water

=

−

77+₋54₁₃₁

h

,2

σ

) issignificantly higherthanthat ofwater in CM-type carbonaceous chondrites (D/HCM-water

=

[101

±

6]

×

10−6 or

δ

DCM-water

= −

350

±

40

h

,2

σ

; Piani et al., 2018) and simi-lartothatoftheleastalteredlithologiesoftheCMchondriteParis (D/HParis

=

[145

±

25]

×

10−6 or

δ

DParis/SMOW

= −

69

±

163

h

, 2

σ

; Piani etal., 2018). Onthe other hand, organicmatter in CV

chondrites is depleted indeuterium relative toorganic matter in CMchondrites.Assumingthat(1)thedifferencesbetweentheD/H ratiosofwaterandorganicsinCVandCMchondriteresultedfrom parent body processes and (2)both typesof chondrites accreted waterandorganicswithsimilar D/Hratios,thepresentD/H com-positionscanbeexplainedbypure Hisotopicexchange.However, thelikelihoodandefficiencyofsuchanisotopicexchangehasnot been demonstrated, and may be linked to the higher metamor-phismtemperatureoftheCVrelativetotheCMparentbody.Itis alsopossiblethattheCVandCMparentbodiesaccretedwaterand organicswithdifferentisotopiccompositionsduetoadifferencein theirtimeand/orlocationofformation.

Acknowledgements

TheauthorsaregratefultotheNaturalHistoryMuseumVienna andtoLudovicFerrièreforprovidingpiecesofthethreeCV chon-drites Kaba, Bali, and Grosnaja, to the French National Museum ofNatural History andB. Zanda forproviding pieces ofthe Paris chondrite,andtothe JapaneseMuseum ofNaturalHistory andS. YonedaforprovidingtheSayamasample.AndreiGurenkoandthe membersoftheIonProbeTeamNancy(IPTN)arethankedfortheir helpontheSIMS.TheauthorsthankB.Marty,H.YurimotoandD. Bekaert forfruitfuldiscussions. We thankWataru Fujiyaandone anonymousreviewerfortheir constructivereviewsandcomments andAssociateEditor FredericMoynierforhiscarefulediting.This workwassupportedbytheJapanesegrant-in-aidforScientific Re-searchonInnovativeAreas(Grantnumber16H00929,L.Piani)and bytheEuropeanResearchCouncil(PHOTONISproject,grant agree-ment No. 695618 to Bernard Marty). This is CRPG contribution #2597.

Appendix A. Isotopicfractionationatequilibrium

Using the fractionation factor estimated by Remusat et al. (2010) between ethyl benzene and water at 100◦C (1000

×

ln

α

OM -water

= −

150 or

α

=

0.86),weestimatedtheD/H composi-tionoforganicmatter(OM)andwateratequilibriumconsidering thefractionsofHinOMandwatertobexOM

=

28%(Table2)and 1–xOM

=

72%,respectively,as:



D

/

HOM

=

α

D

/

Hwater D

/

Hbulk

=

xOM

×

D

/

HOM

+ (

1

−

xOM

)

×

D

/

Hwater

,

(A.1)

⇒



D

/

HOM

=

α

D

/

Hwater

D

/

Hbulk

=

xOM

×

α

D

/

Hwater

+ (

1

−

xOM

)

×

D

/

Hwater

,

(A.2)

⇒



D

/

HOM

=

α

D

/

Hwater D

/

Hwater

=

_xOMD_.(/_αH₋bulk₁₎₊₁

.

(A.3)

Using our calculated bulk D/H ratios for Kaba and

Equa-tions (A.3), we calculated the equilibrium D/H ratios for organic matterandwatertobeD/HOM

=

137

×

10−6andD/Hwater

=

159

×

10−6_{,respectively(timet}

Eq inFig.5).

Appendix B. Kineticisotopicexchange

Beforereachingequilibrium,theD/HratiosofOMandwaterat agiventime t areafunctionofthetotalamountofHandD,and shouldthusbelinkedbythelinearrelation:

D

/

Hbulk

=

xOM

×

D

/

HtOM

+ (

1

−

xOM

)

×

D

/

Htwater (B.1)

⇒

D

/

Ht_OM

=

D

/

Hbulk xOM

+

(

1

−

xOM

)

xOM

×

D

/

Ht_water

.

(B.2)

Using equation (B.2) and the xOM and D/Hbulk values of CV chondrite Kaba, we calculated the coupled evolution of the D/H ratios of organic matter and water until reaching equilibrium at 100◦C(Fig.5).OnFig.5,wereporttheD/Hisotopiccompositions ofKabaandBali,thetwoCVoxchondritesforwhichtheD/Hratios of water and organic matter are known (Tables 1 and 2). Inter-estingly, the average isotopic compositions of water and organic matter in CM chondrites also fallson the kinetic evolution line, indicating that theCM composition could have beenthe starting compositionforCVchondrites,oranearlierstage ofthesame ki-neticallycontrolledisotopicexchange(Fig.5).

Appendix C. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.epsl.2018.09.031.

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