Early degassing of lunar urKREEP by crust-breaching impact(s)

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Early degassing of lunar urKREEP by crust-breaching impact(s)

Jessica J. Barnes, Romain Tartese, Mahesh Anand, Francis M. Mccubbin, Clive R. Neal, Ian A. Franchi

To cite this version:

Jessica J. Barnes, Romain Tartese, Mahesh Anand, Francis M. Mccubbin, Clive R. Neal, et al.. Early

degassing of lunar urKREEP by crust-breaching impact(s). Earth and Planetary Science Letters,

Elsevier, 2016, 447, pp.84-94. �10.1016/j.epsl.2016.04.036�. �hal-01318456v2�

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Contents lists available atScienceDirect

Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Early degassing of lunar urKREEP by crust-breaching impact(s)

Jessica J. Barnesâ^,^∗, Romain Tartèseâ^,^b, Mahesh Anandâ^,^c, Francis M. McCubbin^d, Clive R. Nealê,Ian A. Franchiâ

aPlanetary&SpaceSciences,TheOpenUniversity,WaltonHall,MK76AA,UK

bInstitutdeMinéralogie,dePhysiquedesMatériauxetdeCosmochimie,MuséumNationald’HistoireNaturelle,SorbonneUniversités,CNRS,UPMC&IRD, 75005 Paris,France

cEarthSciencesdepartment,NaturalHistoryMuseum,London,SW75BD,UK

dNASAJohnsonSpaceCenter,MailcodeXI2,2101NASAParkway,Houston,TX 77058,USA

eDepartmentofCivil&EnvironmentalEngineering&EarthScience,UniversityofNotreDame,IN46556,USA

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received4December2015

Receivedinrevisedform18April2016 Accepted20April2016

Availableonline13May2016 Editor:B.Marty

Keywords:

Moon apatite volatiles NanoSIMS chlorine magmaocean

Current models for the Moon’s formation have yet tofully account for the thermal evolutionof the MooninthepresenceofH2Oandothervolatiles.Ofparticularimportanceischlorine,sincemostlunar samples are characterised by unique heavy δ³⁷Cl values, significantly deviating from those of other planetary materials,includingEarth, forwhichδ³⁷Clvalues clusteraround ∼⁰h^.Înôrder^to ûnravel the cause(s)oftheMoon’suniquechlorineisotopesignature,weperformedacomprehensive studyof high-precisioninsituClisotopemeasurementsofapatitefromasuiteofApollosampleswitharangeof geochemicalcharacteristicsandpetrologictypes.TheCl-isotopiccompositionsmeasuredinlunarapatite inthestudiedsamplesdisplayawiderangeofδ³⁷Clvalues(reachingamaximumvalueof+³⁶h^),^which are positivelycorrelatedwiththeamountofpotassium(K),RareEarthElement(REE)andphosphorous (P)(KREEP)component ineachsample.Usingthesenewdata,integratedwithexistingH-isotope data obtainedforthesamesamples,weareabletoplacethesefindingsinthecontextofthecanonicallunar magmaocean(LMO)model.TheresultsareconsistentwiththeurKREEPreservoirbeingcharacterisedby aδ³⁷Cl∼+³⁰h^.^Suchâ^heavy^Clîsotope^signature^requiresmetal-chloridedegassingfromaCl-enriched urKREEPLMOresidue,aprocesslikelytohavebeentriggeredbyatleastonelargecrust-breachingimpact eventthatfacilitatedthetransportandexposureofurKREEPliquidtothelunarsurface.

©²⁰¹⁶^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

ExistingmodelsfortheformationoftheMoon(´CukandStew- art, 2012; Canup, 2012) predict widespread melting and partial vaporisation of silicate material. Hydrodynamic escape (Pahlevan and Stevenson, 2007) may have facilitated the extensive loss of volatilestothevacuumofspaceduringtheproto-lunardiskphase.

PahlevanandStevenson (2007)alsopresentedanelegantmodelto explain the isotopic similarity of oxygen isotopes (e.g., Wiechert etal.,2001) betweentheEarthandtheMoon.Inaddition,Canup et al. (2015) have recentlyprovided possible mechanisms to ex- plainsomeisotopicdifferencesbetweenthesetwobodies(e.g.,Zn).

However,collectively thesemodelshaveyettofullyreconcilethe thermalevolutionoftheearlyMoonwiththepresenceofH₂Oand otherassociatedvolatilesignaturesinthelunarinterior,that have beenrecognisedthroughstudiesoflunarsamples.

* Correspondingauthor.

E-mailaddress:jessica.barnes@open.ac.uk(J.J. Barnes).

In lunar rocks, apatite [Ca5(PO4)3(F,Cl,OH)] is the most com- monvolatile-bearingmineral(McCubbinetal.,2015a).Amultitude of studies have investigated the abundances of volatiles (H2O, F, and Cl, e.g., McCubbin et al., 2010a, 2010b, 2011; Boyce et al., 2010) and the hydrogen isotopic compositions of lunar apatite (e.g., Greenwood et al., 2011; Barnes et al., 2013, 2014; Tartèse etal.,2013, 2014a,2014b;Boyceetal.,2015).Thesestudieshave shownthat(i) parentalmeltstothemarebasaltscontainedH2O>

F> Cl,whilsttheparentmeltstothelunarhighlandsrockswere characterisedby Cl> H2O≈ ^F^(McCubbinêtâl., ^2015a), ând⁽ⁱⁱ⁾ that apatite in mare basalts were generally characterised by el- evated H-isotope signatures (>+⁶⁰⁰h^, ^Greenwood êt âl., ^2011;

Tartèse et al., 2013). TheseH-isotope compositionsare similar to those measured in the lunar picritic glasses (Saal et al., 2013;

Füri et al., 2014), which have been interpreted, at least in part, as resulting from degassing of molecular H2 during ascent and emplacement of basaltic magmas on the lunar surface under re- ducing conditions.Currently, it has been argued that only a few samples (olivine-hosted melt inclusions trapped within picritic http://dx.doi.org/10.1016/j.epsl.2016.04.036

0012-821X/©²⁰¹⁶^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).

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glass beads, some KREEP basalts and a few plutonic highlands rocks) may have preserved primitive lunar H-isotopic compositions(Saaletal., 2013; Barnesetal., 2014; Tartèseetal., 2014a).

In the context of the lunar magma ocean (LMO) model, late- stage LMO residual melts are thought to have been enriched in incompatible elements such as K, REEs, and P, which are collectively called urKREEP or a KREEP-component, when referred toasa geochemicalreservoir orgeochemicalcomponent, respec- tively (Warren and Wasson, 1979). It is anticipated that chlorine (andother volatiles) shouldhavebeen concentratedin theresid- ualLMO melts givenits incompatibility in olivine and pyroxene, which were the dominant early phases to crystallise in the cu- mulate pile of the LMO (e.g., Snyder et al., 1992; Elkins-Tanton et al., 2002, 2011; Elkins-Tanton and Grove, 2011; Elardo et al., 2011). When compared to chondritic meteorites and terrestrial rocks (e.g., Sharp et al., 2013a), most lunar samples have exotic Clisotopiccompositions(Sharpetal.,2010; Treimanetal.,2014;

Tartèse et al., 2014b; Boyce et al., 2015), which are diﬃcult to explain in light of the abundance and isotopic composition of other volatilespecies, especially H,andthe currentestimatesfor Cland H2Oin the BulkSilicateMoon (BSM) (Hauri etal., 2015;

McCubbinetal.,2015a).

Inordertofullyunderstandthemeaningandsigniﬁcanceofthe uniqueCl-isotopecompositionsdisplayedbythemajorityoflunar rocksandtoplacetheseinthecontextofthedifferentiationofthe Moon,weinvestigatedtheCl-isotopiccompositionofapatitefrom a diverse set of lunar samples from Apollo missions: 11 (10044

&10058),14(14304),15(15386&15555),and17(70035,76535, 78235&79215).Oursamplesetcoversarangeoflunarlithological types,includingKREEPandveryhighpotassium(VHK)basalts,and selectedplutonichighlandsrocks(fulldetailscan befoundinthe SupplementaryInformation).

2. Analyticalprotocols

High-precisioninsitumeasurementsofCl-isotopesandvolatiles inapatite fromthefollowing nine Apollothin-sections were car- riedoutinthiswork:10044,645;10058,254;14304,177;15386,46;

15555,206;70035,195;76535,51; 78235,43and79215,50.Twenty eightapatite grainswere identiﬁedas beingsuitable foranalysis by ion probe (see below). During the analytical campaign, scan- ningelectronmicroscopy (SEM)andelectronprobe microanalysis (EPMA) were carriedout in order to characterise mineral chem- istryinthe thin-sectionsstudied, followingthe methodsdetailed inBarnesetal. (2014).

The abundance and isotopic composition of Cl in lunar apatite grains were measured using the Cameca NanoSIMS 50L at the Open University and a protocol modiﬁed after Tartèse et al.

(2014b) overthree analytical campaigns fromNovember 2014to August2015.AnalyseswerecarriedoutusingaCs⁺primarybeam of∼^{1 μm}^diameter,^with^an accelerating voltageof ∼^{16 kV.} ^Be- foreeach analysis, thearea ofinterest was pre-sputteredusing a

∼^{120 pA} ^probe^current^for³ ^minutesât ⁸×^{8 μm to} ênsure^the area was thoroughly cleaned of surface contamination. Analyses wereperformedusingaprimaryprobecurrentofbetween40and 60pA,for ∼^{5 minutes,} ôver^rastered âreasôf^between³×^{3 μm} and5×^{5 μm.}Êlectronic^gating^wasûsed^to^collect^secondaryîons emittedfromonlytheinner 25%oftherasteredareas.Secondary negativeionsof¹⁶O¹H,¹⁸O, ³⁵Cl,³⁷Cl,and⁴⁰Ca¹⁹Fwerecollected simultaneouslyonelectronmultipliers(EMs).⁴⁰Ca¹⁹Fwasusedto easily identify apatite on real-time secondary ionimages during pre-sputteringandto monitorFcontents, butit was notused to preciselyquantifyapatiteFcontentsduetothepoorionisationef- ficiencyof⁴⁰Ca¹⁹F.A massresolvingpowerof∼⁸⁰⁰⁰^(Cameca^def- inition)wasusedinordertoreadilyresolveisobaricinterferences, particularlybetweenpeaksof ¹⁷Oand¹⁶O¹H. Each1-inchround

Fig. 1.CalibrationsfortheapatitestandardsusedinthisstudyofknownClcontent (wt%)versusthemeasured³⁵Cl/¹⁸Oratio,forarepresentativeanalyticaldayduring eachofthethreeanalyticalsessions.

thin-section was coated with ∼^{30 nm} ôf ^gold ând ân êlectron flood gun was used for charge compensation.Secondary ion im- ages of¹⁶O¹H weremonitored inrealtime during pre-sputtering andafter analysisto ensurethat the analysed areaswere free of anysurficialcontamination,cracks,orhotspots.

ApatiteClcontentswerecalibratedusingthemeasured³⁵Cl/¹⁸O ratios and the known Clcontents ofterrestrial apatite standards (described inMcCubbin etal., 2010a,2012), whichwere pressed intoanindiummountalongwithaSanCarlosolivinecrystalthat was used to estimate background abundances of H2O (between

∼¹⁰ând^{90 ppm)}ând^Cl⁽<5 ppm).The slopesofthecalibration lines(Fig. 1)definedbyapatitestandardswithvaryingClcontents were used to calculate the Cl contents of apatite in the Apollo samples.Theuncertainties reportedontheClcontentsofthe‘un- known’ apatite combine the 2σ uncertainty associated with the calibrations andthe analytical uncertainties associated witheach individualmeasurement.

Finally, two of the reference apatite grains, Ap005 or Ap004 (McCubbin et al., 2012), were used to correct the measured 37Cl/³⁵Cl ratios for instrumental mass fractionation. The isotopic composition of chlorine is reported using the standard delta (δ) notation withrespectto the³⁷Cl/³⁵Clratioofthestandard mean oceanchloride(SMOC, ³⁷Cl/³⁵Clratio= ^0.31977),^and^is^reported with the associated 2σ uncertainties, which combine the reproducibility of³⁷Cl/³⁵Clmeasurements on thereferenceapatite(ei- therAp005orAp004)andtheinternaluncertaintyofeachanalysis.

Fig. 2 shows an example of the reproducibility of δ³⁷Cl values measured on a secondary apatite standard (Ap003) run during theFebruarysessionandreferencedtoAp004(Ap004hasaδ³⁷Cl value of ∼+^0.11h^, ^Zach ^Sharp, ^University ^of ^New ^Mexico, ^pers.

comm.).AllAp003analysesarewithinerroroftheterrestrialvalue of∼⁰±¹h^(Sharpêtâl.,^2013a),^withânâverageδ³⁷Clvaluefor theanalyticalweekof+⁰.17±¹.26h^.

3. Results

3.1. Petrographiccontextoflunarapatite

Inthin-section 10044,645,apatiteoccursincontactwithnon- mesostasis pyroxene,plagioclaseandsometimes withtroilite (ap- atite5, forexample,whichisone ofthe largestapatite grainsin this sample at ∼^{200 μm in} ^the^longest ^dimension; ^{Fig. 3A).} ^Ap- atitealsooccursenclosed,almostexclusively,withinclinopyroxene crystals.Forexampleapatite 6C,whichis<50 μminthe longest dimension and is euhedral(showing basal section habit) on one sidebutdisplays apartiallyresorbedtextureontheotherside of

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Fig. 2.Plotofδ³⁷Clvaluesforsecondaryapatitestandard(Ap003)foranalysesfrom 13thFebruaryto18thFebruary2015.Theerrorbarsrepresentthe2σ uncertainties andareacombinationofthereproducibilityontheprimaryreferenceapatiteand analyticalerrorassociatedwithindividualmeasurements.Thereddiamondatthe extremerighthandsideoftheplot,istheaverageδ³⁷Clvalueforthatsessionand theerrorbarsrepresentthestandarddeviationofthemeasuredvalues(n=^32).

(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderis referredtothewebversionofthisarticle.)

thecrystal(Fig. 3B).Apatiteanalysed inthisstudyisalsopresent indirectcontactwithmesostasisminerals,mostlyK-feldspar,silica, fayalite,andasymplectiteassemblageconsistingoffayalite,silica, andCa-richpyroxenethatisenclosedbyplagioclaselaths(e.g.,the

∼^{60 μm}âcicularâpatite^6Dⁱⁿ^{Fig. 3C).}^The^majorityôfâpatiteⁱⁿ sample10044containsmall<5 μm-sizedinclusionsofplagioclase, K-feldsparandoccasionally,ironsulphide(troilite).

In thin-section10058,254, apatite is commonlyfound inlate- stage mesostasisareascomprisedofK-richphases(K-feldsparand occasionallyK-richglass),troilite,baddeleyite,pyroxene,silica,Fe- pyroxene, tranquillityite, ilmenite, fayalite, and a symplectite assemblage. We observed apatite in contact with either late-stage K-feldspar,silicaandotherphases(e.g.,Fig. 3D)orincontactwith plagioclase,pyroxene,andasymplectiteassemblage (e.g.,Fig. 3E).

Wedidnotobservearelationshipbetweenapatitecrystalsizeand petrographicsetting,andapatiteoccursassubhedralhexagonalto tabularcrystalsofupto∼^{90 μm}ⁱⁿ^length.

Inthin-section15555,206,apatite occursenclosedinpyroxene (type 1) or, more commonly, in contact with plagioclase, pyroxene,silica,andK-feldspar(type2).Apatite1analysedinthiswork is inthetype 2location (Fig. 3F)where itformsacicular rodsof up to100 μm inlength.Apatite5alsoanalysedhereisfoundbe- tween type 1andtype 2petrographic settings (Fig. 3G) whereit

Fig. 3.Back-scatteredelectron(BSE)imagesofselectedapatitecrystalsinthemarebasaltsstudied:10044,645;10058,254;15555,206and70035,195.A=10044 apatite 5, B=10044 apatite6C,C=10044 apatite6D,D=10058 apatite 5,E=10058 apatite6,F=15555 apatite1,G=15555 apatite5,H=70035 apatite15,andI=⁷⁰⁰³⁵ apatite 17.Where:Ap=âpatite,^Bdy=baddeleyite,Cpx=clinopyroxene,Kfs=potassium-rich(oftenBa-rich)alkalifeldspar,Fa=^fayalite,Îlm=îlmenite,^Pl=plagioclase, Sym=symplectiteassemblage(Ca-richpyroxene,fayalite,silica),Tro=^troilite,ând^Trq=tranquillityite.

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Fig. 4.BSEimagesofapatitecrystalsandtheirsurroundingpetrographiccontextinKREEP-richbasalts:14304,177and15386,46.A=14304 apatite4,B=14304 apatite5, C=15386 apatite2,andD=15386 apatite5.MineralabbreviationsasincaptionforFig. 3,Mer=^merrillite^and^Zrn=^zircon,^and^Rgl=^residual^K-rich^glass.

Fig. 5.BSEimagesshowingthe petrographiccontextofapatiteinthehighlandsandthe impact-relatedsamplestudied:76535,51;78235,43and79215,50.A=⁷⁶⁵³⁵ apatite 3,B=78235 apatite 5,andC=79215 apatite12.MineralabbreviationsasincaptiontoFig. 3,Opx=orthopyroxene,Ol=ôlivine,^Fe=îron^nickel^blebs,ând^Sym= symplectiteassemblage(Mg–Alchromiteandtwo-pyroxenes,Elardoetal.,2012).

ismostcommonlyenclosed inclinopyroxeneaseuhedralto subhedral crystals of up to ∼^{30 μm} ⁱⁿ ^the ^longest ^dimension ^and typicallycontainingplagioclaseinclusions.

Inthin-section70035,195, apatite crystals15and17 bothoc- curasbasalsections(hexagonal)of∼^{20 μm}ⁱⁿ^diameter^{(Fig. 3H} and3I). Apatite inthis section isvery rare andoccursmostly in mesostasis areasincontactwithK-feldspar andsilica. Apatite17 isenclosedwithinatroilitegrain(Fig. 3I).

VHK basalt 14304,177 contains interstitial K-rich areas and vein-likestructuresthataresituatedbetweenpyroxeneandplagio- clasecrystals.InplacestheK-rich veinscontainK-feldspar,K-rich glass(appearingbrowninplanepolarisedlight),merrillite,baddeleyite,andapatite(anexampleisshowninFig. 4A).Inthissample, theapatite crystals are restricted to the K-rich regions and typi- callyrangefrom∼⁵⁰^to>300 μminthelongestdimension.Most oftheapatiteobservedaresubhedralandareusuallyheavilyfrac- tured(Fig. 4B).

Apatite in sample 15386 is almost exclusively found within mesostasis pockets surrounded by plagioclase and/or pyroxene crystals.Apatite2isfoundincontactwithK-feldspar,ilmenite,and pyroxene,anditsurroundsapocketofK-feldspar.Apatite2issub-

hedral and occurs asblades up to ∼100 μm in length (Fig. 4C).

Apatite 5 is an anhedral grain ∼^{250 μm} ⁱⁿ ^length ^that ^is ⁱⁿ ^di- rect contactwith merrillite, plagioclase and K-feldspar (Fig. 4D).

Apatite 10 is a subhedralcrystal that is ∼^{25 μm} ⁱⁿ ^the ^longest dimensionoccurringinter-grownwithmerrillite.Thecrystalisen- closedinmesostasisglassandtroilite.

Sample 76535,51contains threecrystalsofapatiteasreported in Barneset al. (2014). Eachcrystal co-existswithmerrillite and islocated betweenplagioclasecrystals.Thelargestapatite crystal is also found in contact with a symplectite assemblage (consist- ing of Mg–Al chromite and two pyroxenes, Elardo et al., 2012) and clinopyroxene (Fig. 5A, Barnes et al., 2014). Apatite crystals rangefromanhedraltosubhedralandrangeinsizefrom∼^{50 μm} to∼^{250 μm.}Âpatiteⁱⁿthin-section78235,43areeitherfound in smallpockets ofmesostasis containing Fe-metal, silica,merrillite, and clinopyroxene or associated with the main rock-forming orthopyroxene and plagioclase grains. Apatite occurs as subhedral grainsup to∼^{95 μm}ⁱⁿ^length ândâre ^generally^heavily^cracked (Fig. 5B).

Granuliticbreccia79215iscomposed ofbotha matrixportion dominated by an anorthositic troctolite composition and a troc-