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The chlorine isotopic composition of the Moon: Insights from melt inclusions
Alice Stephant, Mahesh Anand, Xuchao Zhao, Queenie Chan, Magali Bonifacie, Ian Franchi
To cite this version:
Alice Stephant, Mahesh Anand, Xuchao Zhao, Queenie Chan, Magali Bonifacie, et al.. The chlorine
isotopic composition of the Moon: Insights from melt inclusions. Earth and Planetary Science Letters,
Elsevier, 2019, 523, �10.1016/j.epsl.2019.115715�. �hal-02392227�
Contents lists available atScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
The chlorine isotopic composition of the Moon: Insights from melt inclusions
Alice Stephanta,∗,Mahesh Ananda,b, Xuchao Zhaoa,Queenie H.S. Chana, Magali Bonifaciec,Ian A. Franchia
aSchoolofPhysicalSciences,TheOpenUniversity,MiltonKeynes,MK76AA,UK bDepartmentofEarthSciences,TheNaturalHistoryMuseum,London,SW75BD,UK
cInstitutdePhysiqueduGlobedeParis,SorboneParisCité,UniversitéParisDiderot,UMR7154CNRS,F-75005Paris,France
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received22November2018 Receivedinrevisedform7July2019 Accepted15July2019
Availableonlinexxxx Editor: R.Dasgupta
Keywords:
Moon meltinclusion NanoSIMS chlorineisotopes
TheMoonexhibitsaheavierchlorine(Cl)isotopiccompositioncomparedtotheEarth.Severalhypotheses havebeenputforwardtoexplainthisdifference,basedmostlyonanalysesofapatiteinlunarsamples complementedbybulk-rockdata.Theearliest hypothesisarguedforClisotopefractionationduringthe degassingofanhydrousbasalticmagmasontheMoon.Subsequently,otherhypothesesemergedlinking ClisotopefractionationontheMoonwiththedegassingduringthecrystallizationoftheLunarMagma Ocean (LMO). Currently, a variantof the LMO degassingmodel involving mixing between two end- membercomponents,definedbyearly-formedcumulates,fromwhichmaremagmasweresubsequently derived,and aKREEPcomponent,whichformed towardstheendoftheLMOcrystallization,seemsto reconcilesomeexistingClisotopedataonlunarsamples.To furtherascertainthehistoryofClinthe Moon and to investigate any evolution ofCl during magma crystallization and emplacement events, whichcouldhelpresolvethechlorineisotopicvariationbetweentheEarthandtheMoon,weanalysed theClabundanceand itsisotopiccompositionin36olivine- andpyroxene-hostedmeltinclusions(MI) infiveApollo basalts(10020,12004,12040,14072and 15016).Olivine-hostedMI haveanaverageof 3.3±1.4 ppm Cl. HigherClabundances(11.9 ppmonaverage)aremeasuredforpyroxene-hosted MI, consistentwiththeirformationatlaterstagesinthecrystallizationoftheirparentalmeltcomparedto olivines.Chlorineisotopiccomposition(δ37Cl)ofMIinthefiveApollobasaltshaveweightedaveragesof +12.8±2.4hand+10.1±3.2hforolivine- andpyroxene-hostedMI,respectively,whicharestatistically indistinguishable. Theseisotopic compositions are alsosimilar to those measured inapatite inthese lunar basalts,with the exception of sample 14072, which is known to have a distinct petrogenetic historycomparedtoothermarebasalts.Basedonourdataset, weconcludethat,post-MI-entrapment, nosignificantClisotopicfractionationoccurredduringthecrystallizationandsubsequenteruptionofthe parentmagmaandthatClisotopiccompositionofMIandapatiteprimarilyreflectthesignatureofthe sourceregionoftheselunarbasalts.Ourfindingsarecompatiblewiththehypothesisthatinthemajority ofthecasestheheavyClisotopicsignatureoftheMoonwasacquiredduringtheearlieststagesofLMO evolution.Interestingly,MIdatafrom14072suggeststhatApollo14lunarbasaltsmightbeanexception andmayhaveexperiencedpost-crystallizationprocesses,possiblymetasomatism,resultinginadditional Clisotopicfractionationrecordedbyapatitebutnotmeltinclusions.
©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
LunarsamplesexhibitawiderangeofmeasuredClisotopicra- tio(Barneset al.,2016;Boyceet al.,2015;Pottset al.,2018;Sharp et al.,2010;Tartèseet al.,2014;Treimanet al.,2014)comparedto theEarth(Bonifacieet al.,2008;Manziniet al.,2017;Sharpet al., 2013).Indeed,incaseoftheMoontheClisotopicratio,reportedas
* Correspondingauthor.
E-mailaddress:alice.stephant@open.ac.uk(A. Stephant).
δ37Cl values(inpermil),rangesfrom−4 to+18hinmarebasalts (Barnes et al.,2016; Boyce et al., 2015; Sharp et al., 2010), +14 to +40h inKREEP-rich basalts(Barnes et al., 2016; Potts et al., 2018;Sharpet al., 2010;Tartèse et al., 2014) and+25 to +36h inhighlands materials (Barneset al.,2016; Treimanet al., 2014), while Earth’smantle is ≤ −1.6h (Bonifacie et al.,2008) and no significant Clisotopic variations is observedamong major terres- trialreservoirs(i.e.∼ +0.1±0.4h;Sharpet al.,2007). Earthand the Moon show little orno variations for manyisotope systems https://doi.org/10.1016/j.epsl.2019.115715
0012-821X/©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
such as silicon (Armytage et al., 2012), oxygen (Wiechert et al., 2001;Spicuzzaetal.,2007;Hallisetal.,2010;Youngetal.,2016;
Greenwoodet al.,2018),titanium(Zhanget al.,2012)andtungsten (Kleineetal.,2005;Dauphasetal.,2014;Kruijeretal.,2015).This general feature has significant implications for the origin of the Moon (Canup,2012; Cuk andSteward, 2012;Canup et al., 2015;
Rubie et al.,2015). Interestingly,heavy isotope enrichmentin lu- narrocks comparedto theEarthare observedforseveralvolatile tomoderatelyvolatileelementssuchaszinc(Moynieret al.,2006;
Paniello et al., 2012; Kato et al., 2015), rubidium (Pringle and Moynier, 2017), potassium (Wang and Jacobsen, 2016) and gal- lium(KatoandMoynier,2017),arguingforavolatilelossfromthe Mooneitherduring thegiant impact(Paniello et al.,2012; Wang and Jacobsen, 2016) or during the LMO differentiation (Day and Moynier, 2014; Kato et al., 2015). Several hypotheses havebeen proposed to account for this large spread in δ37Cl composition inlunarsamples.Sharpet al. (2010) performed thefirstanalyses forClisotopesinlunarbasaltsandsuggestedthatvolatilizationof metal chlorides (e.g.FeCl2 orZnCl2)during the eruption ofmare magmas was responsible for the observed Cl isotopic fractiona- tion.Thishypothesiswasfoundedonthefactthatthelowestδ37Cl measuredinlunarsamplesoverlappedwiththeterrestrialvalues.
Theheavierδ37Cl valuesrelative to∼0hwere arguedto bethe resultofisotopic fractionationduring degassingoflunar magmas upontheireruptionandemplacement.Thishypothesisimpliesthat theMoon isanhydrous,asany significantpresence ofH inmare magmas would have resulted in the degassing of HCl instead of metal chlorides, which would not have produced the Cl isotopic fractionationobservedin latecrystallising phasessuch asapatite.
Indeed,inhydrousmagmaticreservoirsinterrestrialsettingwhere degassingofHClispredominant,lossof35Clisaccompaniedbya similarlossof37Cl,resultinginnonetisotopicfractionation(Sharp et al., 2010). However, for the Moon, this scenario goes against numerousstudiesarguing forarelativelywetMoon (Hauriet al., 2011; Saal et al.,2008), albeitstill volatile-depleted comparedto theEarth(Albaredeet al.,2015).
Furtherstudies,basedoncorrelatedanalysesofHandClabun- dances and their isotopic compositions in lunar apatite (Barnes et al.,2016;Boyceet al.,2015,2018),havechallengedthehypothe- ses of an anhydrous Moon and the Cl isotopic fractionation by degassingduringthemagmaemplacement stage(cf. alsoreferred aslava-outgassing(Boyce et al.,2015)).As such, (i) thepresence of OH-rich apatites (Barnes et al., 2013; McCubbin et al., 2015;
Tartèse et al., 2013), (ii) the absence of negative correlation be- tweenClcontentandδ37Cl (Barneset al., 2016), (iii) theabsence of anycorrelation between δD and δ37Cl in lunar apatite (Boyce et al., 2015), and (iv) the abundance of Cl relative to refractory incompatible elements such as U and Th in low-Ti and KREEP- rich basalts (Boyce et al., 2018) argue against lava-outgassing as beingthemajorcontrolover Clisotopicfractionation.Analterna- tive scenario is envisaged to explain the elevated δ37Cl in lunar basalts,whichincludesmixingbetween2reservoirs:a lighterone (∼0h), representative ofmare-basaltsource regionsandan ele- vatedone(i.e.25–30h) fromurKREEPreservoir,thefinalproduct of the LMO crystallisation. The heavy Cl isotopic composition of theurKREEPreservoiristhoughttohavebeenacquiredeitherdur- ing theincremental degassingof theLMO(Boyce et al.,2015) or by the degassing of the KREEP-rich layer exposed to vacuum of spaceduringcrust-breachingimpactevent(s)(Barneset al.,2016) orinsomecases,a vapor-inducedmetasomatism∼4 Ga ago(Potts et al.,2018;Treimanet al.,2014).
A potential shortcomingwith lunar Clmeasurements to date, withthe exception of few bulk measurements on glasses(Sharp et al.,2010), isthattheyhavebeenperformedonapatite(Barnes et al.,2016;Boyceet al.,2015;Pottset al.,2018;Sharpet al.,2010;
Tartèse et al., 2014). During the crystallisation of mare magmas,
apatitetypicallyformsafter>95% fractionalcrystallizationandas such,itisnotpossibletoconfirmwhethertheClisotopefraction- ation occurs prior to or during the crystallization of the parent magma. In orderto evaluate the possibility ofisotopic fractiona- tionduringcrystallizationanderuptionoflunarbasalt,andexplore other alternatives,a more pristine,early-crystallisedphase should be targeted inthe same samples from which apatite were anal- ysed.Moreover,Clabundanceinparentmagmacanbemoreeasily determined frommelt inclusionsasthey providedirect measure- mentoftheparentmelt.
Silicate-hosted MI are small blebs of silicate melts trapped withinphenocrystsatmagmaticpressuresandtemperatures(Can- natelliet al., 2016; Lowenstern,2003; Métrichetal., 2008; Roed- der,1979;Walkeret al.,1976).Thephenocrysthostactsasapres- surevesselduringeruption,keepinganyprimaryMIisolatedfrom subsequent degassing or crystal fractionation effects during the eruptionandemplacementofamagma.Thesilicate-hostedMIare of particularinterestasthey trapmagmaticliquid atthetime of their hostcrystallizationandassuch,record thecompositionand theevolutionofthemagmaatthetimeoftrapping.Moreover,MI representthe onlytool that candirectlyprovide thepre-eruptive volatile(suchasH2O,Cl,CO2,S,F)contentofamagma,although diffusion (Bucholzet al., 2013; Gaetani et al.,2012; Hauri, 2002) and post-entrapment crystallization (PEC) (Danyushevsky et al., 2002; Steele-MacInnis et al.,2011) could alter the initial H bud- get.
There have been numerous studies investigating volatile con- tentsinMItoinferthevolatilebudgetofvolcanicsystemsonthe Earth(e.g.Espositoet al.,2012;Hauri,2002),theMoon(e.g.Chen et al.,2015;Hauriet al.,2011;Saalet al.,2013;Singeret al.,2017) andMars(e.g.Giesting et al.,2015;Usuiet al., 2012). Pioneering workonlunarMIwascarriedoutbyRoedderandWeiblen (1970;
1971) whomadeanexhaustivelistofMIpetrographiccharacteris- tics.A fewstudieshavemeasuredtheClcontentsofMIfromlunar samples,includingtheApollo17sample,74220(Chenet al.,2015;
Hauri et al., 2011), which is a high-Ti volcanic glass produced by pyroclastic eruption, aswell asinlunarbasalts 10020,12008, 12040and15016(Chen et al.,2015;Niet al.,2019).However,no data havebeenreportedpreviously onClisotopiccomposition of lunarMI.
Here,forthefirsttime,wereportClabundanceanditsisotopic composition in a large selection of silicate-hosted MI from five Apollobasalts.Thisselectionincludesglassy andpartiallycrystal- lizedMIinbotholivineandpyroxenehosts.Theaimofthisstudy isto:(i) determinetheClabundanceofthemantlesource-regions ofmarebasalts;(ii) totracktheevolutionofClabundanceandiso- topiccompositionduringthecrystallizationofmaremagmas;and (iii) toinferprocess(es)influencingClisotopicfractionationinlu- narsamples.
2. Materialsandmethods 2.1. Samples
The selection ofsampleswas basedon severalcriteria.Firstly, we selected five mare basalts from four Apollo landing sites, in order to document any lateral heterogeneity for Cl in the lunar mantle assampledby mare magmas. Indeed,it hasbeenargued previouslythatlunarvolatileabundances,andespeciallywater,are associated withlocalheterogeneitiesinthe lunarmantle andare not globally representative of the Moon (Albarede et al., 2015).
The samplesselectedare10020, whichisalow-Kilmenitebasalt (Krameretal.,1977;Meyer,2009);vesicularolivinebasalts12004 and 12040 which are low-Ti basalts; KREEP-rich olivine basalt 14072 and low-Ti vesicular olivine basalt 15016 (Meyer, 2009).
Secondly, we selected thesemare basalts based on their cooling
rates,inordertoevaluatetheeffectofcoolingonClsignaturesin lunarmagmas.TwosampleswereselectedfromApollo12collec- tions, to investigate the evolution ofvolatiles during the cooling history of magmas derived from the same basaltic reservoir, as hasbeendone previouslyforHisotopes (Singeret al.,2017).The morerapidlycooled samplesare10020 (3◦C/h—BeatyandAlbee, 1978; Rhodes andBlanchard, 1980), 12004 (Walker et al., 1976) and14072while thetwo samplesmoreslowly cooled are12040 (Walkeret al.,1976) and15016(0.6◦C/h—Tikooet al.,2012). Fi- nally, Cl abundances forthree samples that we selected (10020, 12040 and 15016) were previously also measured in olivine- hosted MI by Chen et al. (2015);while Cl isotopic compositions and abundances have been reported for apatite in mare basalts 12040(Boyce et al., 2015), 14072(Potts et al., 2018) and15016 (Barnes et al., 2019). Thus, we are able to compare and cross- check our data on Cl abundances with previous measurements whileextending thedatabaseofvolatiles forlunar MI.Finally,in order to monitor the evolution of Cl abundance andits isotopic composition in the lunar magma, we analysed both olivine- and pyroxene-hostedMI.Indeed,olivine- andpyroxene-hostedMIwere trapped at different times during the crystallization of the par- entmagmaandthusprovidesnapshotsofmagmacompositionat the instant of melt trapping. Therefore, our approach should al- lowustoidentifyanymagmaticprocessesthatwouldhavealtered the Cl content and/or δ37Cl in lunar samples during crystallisa- tion.
2.2.Identificationofmeltinclusions:SEMandRaman
Olivine and pyroxene phenocrysts containing MI were iden- tified by optical microscopy and the FEI Quanta 200 3D Dual Beam scanning electron microscope (SEM) at The Open Univer- sity(OU). Sampleswere also analysed usinga Jobin-Yvon Horiba LabRamHR RamanmicroprobeattheOU,withagreen(514 nm) laser excitation source delivering 1 mW at the sample surface, and a 600 grooves/mm grating to provide a spectral resolution of∼3 cm−1. The laser beamwas focused through a microscope equippedwitha×50 objective(N.A.0.75),providingaspatialres- olutionof ∼1 μm.Spectra werecollected foreachsample inthe range of 100–4000 cm−1. The exposure time for each spectrum was60 sandthreeaccumulationswereobtainedforeachanalyti- calspot.Peakpositionwas calibrateddailyagainstasiliconwafer priortosampleanalyses.
2.3.NanoSIMSprotocol
Clabundanceandits isotopiccomposition were measured us- ing the Cameca NanoSIMS 50L at OU using a protocol modified afterTartèse et al.(2014) and Barneset al.(2016). Analyseswere carriedoutusingaCs+ primarybeamwithadiameterof∼1 μm and an accelerating voltage of ∼16 kV. A 300–500 pA primary currentwas rastered over the sample on areas ranging between 10 μm×10 μm to 30 μm×30 μm, depending on the sizes of MI.Eachanalysissurface area wasdivided into128×128 pixels, withacountingtimeof1 msperpixel.Thenumberofcycleswas setat100. Threetotenminutesofpre-sputteringwas set-upbe- foreanalysisonalargerareathantheanalysedsurfacetoremove anycontamination fromthesurface andestablishsputterequilib- rium. An electron flood gun was used for charge compensation.
Secondarynegative ions of18O, 29Si, 35Cl,37Cl and27Al16Owere imaged by scanning ion imaging, with a MRP of 7000. Basaltic glassMD57-D9-1(Bonifacieet al.,2008)wasusedforrelativesen- sitivityfactorofClabundanceandSanCarlosolivinewasusedfor establishingbackground counts forCl.Data were processedwith theL’IMAGEsoftwaredevelopedbyLarryNittlerfromtheCarnegie
Institute ofWashington. The deadtimewas set at44 ns andcor- rectedwiththeL’IMAGEsoftware.Regionsofinterest(ROIs)were determinedoneachimagebasedonthe27Al16O/18Oand35Cl/18O imagesto locatetheMIandexcludeanycracks, voidsoranoma- lously Cl-rich hotspots associated withextraneous contamination (see section 3.2). Indeed, MI are enriched in 27Al16O compared to their nominally anhydrous host and contamination induced hotspotsof35Cl.Becausethebeamoverlapsatboundariesbetween phenocrysthostandMI,ROIsweredefinedon27Al16O/18Ocounts plateautomakesureonlyMImaterialistakenintoconsideration fortheir35Cl/18Oand37Cl/35Clratios.All27Al16O/18Oand35Cl/18O NanoSIMS images withdefinedROIs are provided insupplemen- taryFig. S1.
Melt inclusion Cl abundances were calibrated using the mea- sured35Cl/18Oratios andthe knownCl abundances ofterrestrial MORBstandardsSR2-DR04andMD57-D9-1(Bonifacieet al.,2008), as well as DR5, DR15 and DR32 (Clog, 2010). These standards were set inan indium amount.Average count rate of 35Cl fora
∼12 μm×12 μm ROI area in MD57-D9-1 was ∼3000 counts per second (50±10 ppm). A negligible Cl background was de- termined usingSanCarlos olivine (0.04count rate per second of 35ClforROIof∼12 μm×12 μm).Theslopeofthecalibrationline was estimatedusing R program to be 3.08×10−4±0.6×10−4 at 95% confidence level (Fig. S2), taking into accountsanalytical errors on glass standards. Uncertainties reported on Cl contents combinethe 2σ analytical uncertainties associated witheach in- dividual measurement and the uncertainty associated with the calibration line. Chlorineconcentrationsin MIwere corrected for post-entrapment crystallization(PEC), determined usingthe soft- warePetrolog3(DanyushevskyandPlechov,2011).
For Cl isotope measurements, the instrumental mass frac- tionation (IMF) factor, α, based on analyses of SR2-DR04 and MD57-D9-1 MORB glass standards is 1.008±0.003 (2SD). The measured37Cl/35ClratiosarecorrectedfortheIMFandexpressed in δ37Cl notationasdefined inequation (1), withstandard mean oceanchlorideδ37Cl SMOC=0h(Kaufmanet al.,1984).Errorses- timatedforδ37Cl valuestakeintoconsiderationtheerrorbasedon countingstatistics,aswell astheuncertaintyassociated withthe IMFcalculation.
δ37Cl(h)=37
Cl/35Clsample
37
Cl/35ClSMOC
−1
×1000 (1)
TheClisotopic compositionfromsome MIcouldnot beprecisely determined because of counting statistics (2SD on δ37Cl>10h) andare,therefore,notreportedhere.
GlassstandardsSR2-DR04andMD57-D9-1werebothmeasured on each ofthe five days.In order toverify the accuracyand re- producibility of our analyses, we treat one of the standards as unknownandrecalculateitsδ37Cl valuerelativetotheotherstan- dard for each ofthese fivedays using the other standard asthe knownone (cf.TableS2–Fig. S3). MD57-D9-1hasaClcontent of 50±10 ppm,whichisclosestinabundancetothatofApolloMIs.
The average for these five independent δ37Cl measurements for MD57-D9-1is−1.7±2.3h,whileitstruevalue is−1.4±0.7h, demonstrating good reproductivity and accuracy for these mea- surementsatlowClabundances,providingconfidenceinourresult at a levelof ∼ ±2h.Furthermore, the Clisotope measurements employed inthisstudywere baseduponarecentprotocoldevel- opedbyBarrettet al.(2019).Inparticular,usingastandardwithas littleas17 ppmCltheyreportednoisotopiceffectswithClabun- dancedowntolevelscomparabletothoseinsomeofthesamples reportedhere.
