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Cosmic ray effects on the isotope composition of
hydrogen and noble gases in lunar samples: Insights
from Apollo 12018
Evelyn Füri, Laurent Zimmermann, Etienne Deloule, Reto Trappitsch
To cite this version:
Evelyn Füri, Laurent Zimmermann, Etienne Deloule, Reto Trappitsch. Cosmic ray effects on the
isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018. Earth
and Planetary Science Letters, Elsevier, 2020, 550, pp.116550. �10.1016/j.epsl.2020.116550�.
�hal-02926751�
Contents lists available atScienceDirect
Earth
and
Planetary
Science
Letters
www.elsevier.com/locate/epsl
Cosmic
ray
effects
on
the
isotope
composition
of
hydrogen
and
noble
gases
in
lunar
samples:
Insights
from
Apollo
12018
Evelyn Füri
a,
∗
,
Laurent Zimmermann
a,
Etienne Deloule
a,
Reto Trappitsch
b aCentredeRecherchesPétrographiquesetGéochimiques,UniversitédeLorraine,CNRS,F-54000Nancy,FrancebLawrenceLivermoreNationalLaboratory,NuclearandChemicalSciencesDivision,7000EastAve,L-231,Livermore,CA94550,USA
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory:
Received19May2020
Receivedinrevisedform19August2020 Accepted21August2020 Availableonlinexxxx Editor:F.Moynier Keywords: hydrogenisotopes noblegases marebasalt cosmicrays cosmogenicnuclides exposureage
Exposure of rocks and regolithto solar (SCR)and galactic cosmic rays (GCR) at the Moon’s surface resultsinthe productionof‘cosmogenic’ deuteriumand noblegas nuclidesatarate thatdependson acomplex set of parameters,such as the energyspectrum and intensityof the cosmic rayflux,the chemical composition, size,and shape ofthe target as well as the shielding depth. Asthe effectsof cosmicraysontheDproductioninlunarsamplesremainpoorlyunderstood,wedetermineheretheD contentandnoblegas(He-Ne-Ar)characteristicsofnominallyanhydrousmineral(olivineandpyroxene) grainsandrockfragments,respectively,fromdifferentdocumenteddepths(0to≥4.8 cm)withinApollo olivinebasalt 12018.Deuterium concentrations,determined bysecondaryionmass spectrometry,and cosmogenic3He,21Ne,and38Arabundances,measuredbyCO
2laserextractionstaticmassspectrometry,
are constantover the depthrangeinvestigated. Neonisotope ratios (20Ne/22Ne ≈0.86and 21Ne/22Ne ≈0.85)ofthecosmogenicendmemberarecomparabletothetheoreticalsignatureofGCR-producedneon. TheseobservationsindicatethatthepresenceofsignificantamountsofSCRnuclidesinthestudied sub-samples canberuledout.Hence,Dwithintheolivines andpyroxenes musthavebeenpredominantly producedinsitu byGCR-inducedspallationreactionsduringexposureatthelunarsurface.Comparison ofthe amountofDwith the21Ne (184± 26 Ma) or38Ar(193± 25 Ma) exposureagesyields aD
productionratethatisingoodagreementwiththevalue of(2.17±0.11)×10−12 mol(g rock)−1Ma−1 fromFürietal.(2017).Theseresultsconfirmthatcosmicrayeffectscansubstantiallyalterthehydrogen isotope(D/H) ratioofindigenous‘water’inreturned extraterrestrialsamplesand meteoriteswithlong exposureages.
©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The hydrogen isotope ratio is the key indicator for plan-etary water origin(s) because different solar system reservoirs (solar, chondritic, cometary) have characteristic D/H signatures (e.g., Alexander, 2017; McCubbin and Barnes, 2019). Volcanic glassbeads andolivine-hosted meltinclusions therein,the phos-phate mineralapatite, andnominally anhydrousminerals in var-iousrock types returned from the Moon by the Apollo missions record a wide range of
δ
Dvalues (whereδ
D[]
= [(
D/
H)
sample/
[(
D/
H)
SMOW−
1]
×
1000,with(D/H)SMOW=
155.
76×
10−6;Hage-mannetal.,1970), between
≤ −
500and≥ +
1000 (see Mc-Cubbin et al., 2015 for a review), which have been interpreted toreflecthydrogen orwatercontributions frommultiplesources, suchasthesolarnebula,carbonaceouschondrites,and/or comets,*
Correspondingauthor.E-mailaddress:efueri@crpg.cnrs-nancy.fr(E. Füri).
tothe lunarinterior(e.g.,Anandetal., 2014; Barnesetal., 2016; Desch and Robinson, 2019; Füri et al., 2014; Greenwood et al.,
2011; Hui et al., 2017; Robinson et al., 2016; Saal et al., 2013; Sharp,2017;Singeretal., 2017;Tartèse andAnand, 2013). How-ever, the D/H ratio of mantle-derived samples does not always reflect the hydrogen isotope composition of the lunar mantle source.Inadditiontomagmaticprocesses(e.g.,degassing; Saalet al.,2013;TartèseandAnand,2013),solarwind(SW)implantation and cosmic ray induced spallation reactions – triggered by solar (SCR)andgalacticcosmicrays(GCR)thatcanpenetratelunar mat-tertodepths ofafew centimeters orseveralmeters,respectively (Reedy andArnold, 1972) –can modify the D/Hsignature of in-digenous‘water’(i.e.,H,H2,and/or H2O)inlunarrocks,minerals,
andvolcanicglasses.
SincetheMoon isanairless bodyandhasnoglobalmagnetic field, SWparticles, includingprotonsandnoble gasions,are im-plantedintothetopfewtensofnanometersofallrocksorregolith grainsexposedto thelunarsurface environment(e.g.,Hashizume etal., 2000). Nonetheless,a contributionofSW-implanted
hydro-https://doi.org/10.1016/j.epsl.2020.116550
0012-821X/©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2 E. Füri et al. / Earth and Planetary Science Letters 550 (2020) 116550
gen to themeasured waterabundances andD/H ratios can gen-erallybe ruledout because allrecent hydrogenisotope measure-mentsoflunarsampleshavebeencarriedoutinsitu bysecondary ion mass spectrometry (SIMS), in the interior of lunar volcanic glassbeads (Füri et al., 2014) or within mineralgrainsand melt inclusionsthatwereneverdirectlyexposedtoSWirradiation(e.g., Barnes et al., 2014, 2013; Boyce et al., 2010; Hui et al., 2017; Robinsonetal.,2016;Saaletal.,2013;Tartèseetal.,2013;Tartèse andAnand,2013).Incontrast,cosmicray produced(‘cosmogenic’ or‘spallogenic’) deuterium isexpected to contribute significantly totheD/Hratioofwater-poorlunarsamplesthatexperiencedlong exposure to cosmic rays at the surface of the Moon (Füri et al.,
2017).
Knowledgeof thecosmogenicDproductionrate( PD) andthe cosmicrayexposure(CRE)ageofthesampleofinterestiscritical forcorrecting measured D/Hratios for the cosmogenic contribu-tion,and,ultimately,fordeterminingthesource(s)oflunarwater. Tothisdate,a PD valueof0.92to1
×
10−12 mol(g rock)−1Ma−1, derivedbyMerlivatetal.(1976) andReedy (1981),hasbeenused inmoststudiesoflunarsamples(e.g.,volcanicglasses,melt inclu-sions,apatites,plagioclase),irrespectiveoftheirchemical composi-tion.However,the PD valuedependsontheabundanceofvarious targetelements(O,Mg,Si,Fe,Al),thesizeandshapeoftheobject, and its exposure history (Reedy, 1981). Furthermore, Greenwood etal.(2018) arguedthatmare basalt70215analyzed byMerlivat et al.(1976) contains indigenousOH (in addition to water from terrestrial contamination and cosmogenic D), rendering their PD estimate unreliable. Füri etal.(2017) recently obtaineda signifi-cantlyhigherPD valueof(
2.
17±
0.
11)
×
10−12mol(g rock)−1Ma−1 fortheMoon’ssurfacefromSIMSanalysesofnominallyanhydrous olivine grains with a wide range of CRE ages, either indicating thatprevious studieshadseverelyunderestimatedthe production ofcosmogenicdeuteriumbyGCRs,or,alternatively,hintingasthe presenceofadditionaldeuteriumproducedbySCRsinthestudied olivines.TheimportanceofSCReffectsinlunarsamplescanbeassessed through studies of particle tracks (e.g., Crozaz, 1980) or concen-trations ofradionuclides (14C, 10Be, 26Al, 53Mn, 81Kr) and stable noblegasnuclides(e.g.,Hohenbergetal.,1978;Leyaetal., 2001; Nishiizumi et al., 2009; Reedy, 1980; Reedy and Arnold, 1972; TrappitschandLeya, 2014). SCRnuclideproductionoccursalmost exclusively within the topmost 1–2 cm of exposed rock or soil surfaceson theMoon becauseofthe lower energyofSCRs com-paredtoGCRs(
∼
10–100 MeVvs.∼
1–10 GeV)(ReedyandArnold,1972).SincetheratesofneonisotopeproductionbySCRsare sub-stantially higher that those by GCRs within the uppermost few g/cm2 ofshielding(where‘shielding’depth[g/cm2]
=
depth[cm]×
density[g/cm3]),cosmogenicneonconcentrationsareexpected to decrease significantly with (shielding) depth in lunar samples that wereexposed to bothsolar andgalacticcosmicrays (Rao et al.,1994,1993;TrappitschandLeya, 2014).Importantly,themain targetelementsmagnesiumandsilicon,fromwhich21Neand22Neare predominantly produced in lunar rocks, show a lower cross section ratio for 21Ne/22Ne at lower energies, i.e., the region in whichproductionfromSCRsisimportant,thanathigherenergies, whereproductionfromGCRsdominates.Giventhat thedominant fluence of the SCR particle spectrum is at much lower energies than for the GCR particle spectrum, the 21Ne/22Ne ratio of SCR neonissignificantlylowerthanthatofGCRneon(Fürietal.,2017; Raoetal.,1994,1993).Therefore,thethreeisotopesofneon repre-senta powerfultool torecognize,andpossiblyquantify, different noble gascomponents (SW, SCR, GCR) in samples returned from theMoon.
To assess the importance of SCRs and GCRs for the produc-tion of cosmogenic deuterium in lunar samples, we determined thedeuteriumcontentofolivineandpyroxenegrainsfrom
differ-ent depths (0 to
≥
4.8 cm) within Apolloolivine basalt12018by SIMS. In parallel,we analyzed thenoble gas(He-Ne-Ar)contents andisotope ratiosofbulk rockfragmentsbyCO2 laser extractionstatic mass spectrometry to quantify the abundances of cosmo-genic noblegas nuclides (3He, 21Ne, 38Ar) at each depth and to constrain the irradiation conditions and duration. This combined data set permitsto determine ifdepth-dependent shielding vari-ations resultinsignificant inter-sample differencesinthe rateof cosmogenicDproductionwithinApollo12018.
2. Samplesandanalyticaltechniques
Apollosample12018isamedium-grained,low-Tiolivinebasalt (supplementary TableS1; Papike etal.,1976) –alsodescribed as olivine dolerite(Cuttittaetal.,1971;Kushiroetal., 1971) or gab-bro(Megrue,1971)–,composedofapproximately70%largeolivine andpyroxenecrystalssetinavarioliticmatrix(Walteretal.,1971). Ten chips were allocated for this study by NASA’s Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM); these chips were extractedfrom differentdocumented depths (0 to
≥
4.8 cm;supplementaryTableS2)alongaslabcutthroughthe middleoftherockthatwasoriginally8×
6×
6 cminsize(Fig.1). The chipsweregently crushedinan agatemortar toobtain indi-vidual olivine and pyroxenegrains forhydrogen isotopeanalyses bySIMSaswellassmallrockfragmentsfornoblegasanalysesby staticnoblegasmassspectrometry.Olivine andpyroxene grains, separated from eight out of the tendepthsamples,weremountedincrystalbondandpolished in-dividuallywithaluminapowderandethanoltominimize contami-nationbyterrestrialwater.Subsequently,thegrainsweremounted in indium together with two ‘dry’ standards: synthetic forsterite (4
.
5±
1 ppm H2O; Wetzel et al., 2015) and synthetic Suprasil3002 quartz glass(
≤
1.66ppm H2O; Haurietal., 2017). Thema-jor element composition of the olivine and pyroxene grainswas determined by electron microprobe analysis atthe Université de Lorraine(Service Communde MicroscopieElectroniqueetde Mi-croanalyses X,Nancy, France) witha Cameca SX100operating at 12 nA and15 kV. Countingtimesonpeaksandbackgroundwere 10 s for all elements. Results are reported insupplementary Ta-ble S3andshowthattheolivinegrainsdisplayarangeinforsterite contents (Fo
=
100×
Mg /[Mg+
Fe])betweenFo66and Fo77. The selected pyroxene grains can be classified as (high-Ca) pi-geonites(Kushiroetal.,1971).Insitu measurementsofH andDconcentrationsin theApollo 12018olivinesandpyroxenes–aswellasinApollo12004,12018, and15016olivinespreviouslystudiedbyFürietal.(2017) –were performed at the CRPG (Nancy, France) using the CAMECA 1280 HR2 ionmicroprobe. Samples were coated withgold and left in theinstrumentairlockat
∼
6×
10−9 mbarforseveraldaysinor-der toensurethorough removalofanyterrestrialadsorbedwater beforeintroductionintothesample chamber.Inaddition,aliquid nitrogen cold trapwas used to reduce the hydrogen background and maintain a pressure of
≤
7×
10−10 mbar during analyses.StHs6/80-Gandesiticglass(250
±
7 ppmH2Oandδ
D= −
95±
2;Jochumetal.,2006)andMON9 pyrope(56
±
6 ppmH2O;Belletal.,1995),onthesamplemountofFürietal.(2017),wereusedas additionalstandards. Spot analysesofH− andD− secondary ions were carried out in mono-collection mode on an electron multi-plier ata nominalmass resolution m
/
m=
1600 using a 10kV Cs+primaryionbeamandanormal-incidenceelectrongun (emis-sion=
0.21 mA) for charge compensation.The 18O− count rate wasmeasuredfor4sonaFaradaycupduringeachcycleto moni-torthestabilityofsecondaryionintensities.Standardsand miner-alswerepre-sputteredfor180soveranareaof30×
30 μmprior tosignalacquisition.Duringthepre-sputteringprocess,the inten-sity of the primary beam was measured withthe primaryFara-Fig. 1. Apollo12018fragmentsusedforthisstudyarederivedfroma)slab14;b)column17;c)slices49,51,55,and52(seesupplementaryTableS2fordetails).Sample 12018,14,17,52originatesfromthe‘top’exteriorsurfaceoftherock,wheretheangleofexposurewas∼45◦fromthezenith,asindicatedbycuttingdiagrams(Meyer,2011). (NASAimages#S70-19566,S70-19581,S70-19598).
daycup;it was foundto decreaseonlyslightly(i.e.,from11.2to 10.0 nAandfrom13.8to12.4 nA)overeach24-hourmeasurement period,whichincluded both standardsandsamples.Foranalysis, the
∼
15 μmbeamwasrasteredoveranareaof20×
20 μm,anda dynamicaltransferoperatingsystemwas usedto compensatethe primary rasteringandto refocusthe beaminthe secondarypart ofthe ionprobe. Toeliminate any hydrogencontamination from thecrateredges,a1800 μmfieldaperturelimitingtheanalysesto ionsfromthecentral∼
10 μmofthebeamandanelectronicgate of80%wereused.TheH−andD−ionintensitiesweredetermined for4and20 s, respectively,for 30cycles.Under theseanalytical conditions,typical count rates on StHs6/80-G were∼
99,000 cps forH− and16cpsforD−,whereascount ratesonthe‘dry’ stan-dardsaveraged1770±
667 cpsforH−and0.30±
0.14cpsforD− (supplementaryTable S4), in agreement with ourprevious study (Füri etal., 2017). Measured H− andD− count rates, aswell as estimateddeuteriumconcentrationsinApollo12018olivinesand pyroxenes,arereportedinsupplementaryTableS4.Noblegas (He-Ne-Ar)abundances andisotoperatios were de-terminedby CO2 laserextraction staticmassspectrometryatthe
CRPGnoblegasanalyticalfacility(Fürietal.,2018;Humbertetal.,
2000). For each depth sample, two separate fragments, between 1.54 and 5.88 mg in mass (supplementary Table S5), were ana-lyzed. The fragments were placed into different pits of the laser chamber connectedto thepurification lineof theHELIX-MC Plus
(ThermofisherScientific)multi-collectornoblegasmass spectrom-eter.Aftersample introduction,thesamplechamberwasbakedat 110◦C overnight under ultra-highvacuum to remove any terres-trialadsorbed gases. Eachrock fragment was heated individually withacontinuous-modeinfraredCO2 laser(
λ
=
10.
6 μm).Twotothreeheatingsteps wereappliedby modulatingthepowerofthe laser and monitoring the heating procedure on a TV screen us-ing a CCD camera. Alow-temperature step (
∼
600◦C)was aimed atremoving surface-sited(atmospheric,solar) gases, an ‘interme-diate’ heating step (∼
800◦C) was applied for samples from the near-surface(≤
0.6 cm)toremove anyremaining solargases,and fusion was achieved in the last heating step to release ‘volume-correlated’(cosmogenic,radiogenic)noblegascomponents(Curran etal.,2020).Re-heatingofthreesamplesfrom∼
1to1.5 cmdepth (supplementary Table S5)demonstrated that≥
97.7% of the total noblegascontentwas extractedupon melting.Theextracted no-ble gases were purified using five hot (500◦C) and cold (room temperature) Ti sponge getters (Zimmermann et al., 2015).Ar-gon was separated fromhelium and neonby adsorption onto a charcoal fingerat 77K,andhelium andneonwere subsequently trapped onto a He-cooled cryogenic trap at
∼
15 K. Helium was firstreleasedfromthistrapat34Kandanalyzedinpeak-jumping mode (4He on the H2 Faraday detector, 3He on the central (Ax)compactdiscretedynode(CDD)detector).Neonwasreleasedfrom the cryogenic trap by increasing the temperature to 110 K, and the amount of gas introduced into the mass spectrometer was adjustedthrough volumedilutioninthevolume-calibrated purifi-cationlinetomatchthe20Nesignalofairstandardmeasurements
with20Ne/22Ne
=
9.80 (Fig.2). The three isotopes ofneonwere analyzed inmulti-collection mode(22Ne onH1CDD, 21Ne onAx CDD,20NeonL2CDD).Neonisotopeanalysesconsistedof5blocksof30cycleseach, andpeakcentering wasperformedatthestart ofeachmeasurementblock.Acharcoalfingerat77KandaZr-Al getter at room temperature were used to minimize the contri-bution of doubly charged 40Ar and CO2 to the 20Ne and 22Ne
signals, respectively. Given the high mass resolution of the de-tector in the L2 position(
∼
1800), 40Ar++ was partially resolvedfromthepeakofinterest(Hondaetal.,2015;WielandtandStorey,
2019; Zhang et al., 2016); therefore, no correction was applied to the20Ne+ signal.The CO+
2 signal was measuredat the
begin-ning ofeach analysis, andthe 22Ne+ signal was corrected using a CO++2 /CO+2 ionization ratioof0.4%;notably,the contributionof CO++2 tothe22Ne+signalamountedtoonly
∼
1cps,andisthere-forenegligible.The21Ne+signalwasmeasuredatthepeakcenter; nohydride(20NeH+)correctionwasperformedbecausethegetter allowed maintaining a low hydrogen background. After releasing argonfromthecharcoalfinger,theargonisotopeswereanalyzedin peak-jumping mode(40Aron thecentralFaraday detector,38,36Ar onAxCDD).
StandardHESJ(HeStandardofJapan)ofMatsudaetal.(2002) witha 3He/4He ratioof20.63
±
0.10RA (where RA isthe
atmo-spheric3He/4Heratio)wasusedasaheliumstandard,whereasair aliquotswereused todeterminetheanalytical sensitivityand re-producibility for neon and argon (Zimmermann and Füri, 2015). The reproducibility (1
σ
s.d.) ofstandard measurements was 1.2% for4He,0.9–1.1%for20Ne,and1.3%for36Arabundances,and1.5%for3He/4He,0.2%for20Ne/22Ne,0.6%for21Ne/22Ne,and0.4%for
38Ar/36Ar.Reporteduncertainties (2
σ
s.d.)forsample isotopicra-tios take into account the standard reproducibility (in addition to the analytical precision), even though samples and standards havesignificantlydifferentisotoperatiosforhelium,neon,and
ar-4 E. Füri et al. / Earth and Planetary Science Letters 550 (2020) 116550
Fig. 2. a)20Ne/22Neandb)21Ne/22Neratios(correctedforinstrumentalmassfractionation)asafunctionofthe20Nesignal(incountspersecond,cps)forairstandards
(n=37)andApollo12018fragmentsanalyzedbystep-wiseheating.Thevariable20Necountratesfortheairstandardmeasurementswereobtainedbyvaryingtheamount
ofgasintroducedintothemassspectrometerthroughvolumedilutioninthevolume-calibratedpurificationline.Thehorizontaldashedlinesindicatethe20Ne/22Neand 21Ne/22Neratiosoftheterrestrialatmosphere(20Ne/22Ne=9.80and21Ne/22Ne=0.0288–0.0290;Eberhardtetal.,1965;Györeetal.,2019;Saxton,2020;Wielandtand
Storey,2019).Uncertainties(2σs.d.)aresmallerthansymbolsizes.
gon(Fig. 2).Procedural blanks,withthelaseroff,averaged 2.1
×
10−16 mol 20Neand 2.1×
10−17 mol 36Ar. Heliumblanks were below the detection limit. Blank-corrected He-Ne-Ar abundances andisotoperatiosforApollo12018rockfragmentsarereportedin supplementaryTableS5.3. Results
3.1. HydrogenanddeuteriumcontentofApollo12018olivinesand pyroxenes
ThemeasuredH−countrates,normalizedtoeithertheprimary ionbeamintensity(H−/Ip) orthe 18O− signal (H−/18O−), ofthe
‘dry’ standards(i.e., synthetic forsteriteand Suprasil3002 quartz glass) correspondto H2O concentrationsof4.1to 4.6ppm when
compared tothe glass andgarnet standards. Given that theH2O
content ofthe quartz glass (
≤
1.66ppm H2O;Hauri etal., 2017)cannotbeclearlyresolvedfromthatoftheforsteritestandard(4.5
±
1ppmH2O;Wetzeletal.,2015),theH2Odetectionlimitisin-ferred to beon the orderof 4.5ppm inthisstudy. The majority (n
=
33 out of43) of thelunar olivine andpyroxene grains ana-lyzedhereyieldH−countratesthatareslightlyhigherthanthose ofboth ‘dry’standards(supplementaryTable S4).TheirH2Ocon-centrationsareestimatedtovarybetween
∼
4.5and9.2ppm,i.e., measured watercontents are upto∼
5ppm above the detection limit. The elevated H− signals may indicate that Apollo olivines andpyroxenes can contain severalppm ofindigenous lunar wa-ter,althoughtwopreviousstudiessuggestedthatindigenouswater ispresentinvery lowabundancewithin lunarolivines(≤
2ppm) (Hui et al., 2013; Mosenfelder and Hirschmann, 2016). Alterna-tively,the samples might be variablycontaminated by terrestrial adsorbedwater.Distinguishingbetweenthesetwoscenariosisnot possible. This demonstrates the difficulty of accurately and pre-cisely quantifying trace amounts of indigenous water (hydrogen) inlunarsamplesusingSIMSbecauseterrestrialcontamination (in-troducedduring samplepreparationand/or analysis) mightresult in ppm-level variations in measured water contents. SW-derived orcosmogenichydrogenisnotexpectedtocontributetothe mea-suredH− signalsbecauseanalyses werecarriedout insitu withinmineral grainsthat were never directly exposed to the SW, and indistinguishableHcontentswere foundforsampleswithawide rangeofCREages(Section4.3;Fürietal.,2017).
The D− signals of 13 olivine and two pyroxene grains from the Apollo 12018 depth samples vary between 2.09
±
0.26and 2.93±
0.31 cps, and the Apollo 12018 olivines prepared by Füri et al.(2017) yieldsimilar signals of 2.22±
0.27 to 2.33±
0.28cps uponreanalysisforthisstudy (supplementaryTableS4). Apollo 12004 and 15016 olivinesyield different D− ion intensi-ties of 1.09–1.10 cps and 4.09–4.26 cps, respectively, because of distinct CRE ages(Section 4.3; Fürietal., 2017).All ofthese val-uesaresignificantlyhigherthantheD− signalsofthequartzglass (0.25±
0.09 cps) andforsterite(0.33±
0.11cps) standards, in-dicating that theD− measurements arenot substantially affected by anyterrestrialcontamination. Based on theknown D content (3.
9×
10−9 mol/g; Jochum et al., 2006) and the measured D− signal (16.
0±
1.
2 cps),normalized to the primary ionbeam in-tensity, of standard StHs6/80-G, the D− signals of Apollo 12018 olivinesandpyroxenes correspondto Dconcentrationsof0.50 to 0.61×
10−9 mol/g.Similar valuesof 0.48to0.62×
10−9 mol/gareobtainedwhentheDabundancesarecalculatedusingthe18O−
count ratefornormalization(supplementaryTableS4).Within er-ror,thesevaluesareconsistentwiththeDcontentofApollo12018 olivinesdeterminedpreviously(0.49to0.54
×
10−9 mol/g;Fürietal.,2017).Importantly,thenewresultsshowthattheD concentra-tion inolivine andpyroxene isindistinguishable, andthereis no Dconcentrationgradientbetweenthesurfaceand
∼
4.5 cmdepth (Fig.3).3.2. He-Ne-ArcharacteristicsofApollo12018rockfragments
Atthefirstheatingstep,roughlyhalfofthetotal 3Heand4He content isextracted, together withonly asmallfraction (
∼
1.5to 7%,inmostcases)ofthetotalneonandargonabundance (supple-mentaryTableS5).ThisindicatesthatnoblegasesinApollo12018 fragmentsarepredominantly‘volume-correlated’,i.e.,produced in situ by radioactive decay (4He and 40Ar) or spallation reactions,and are only efficiently released upon melting at high temper-atures. Indigenous (light) noble gases have never been found in lunarsamples(Fürietal.,2018;WielerandHeber,2003),andare, therefore,ruledoutasapossiblecomponent. Totalconcentrations of radiogenic 4He and 40Ar vary by
∼
25 to 30% in the twenty rockfragmentsanalyzedhere,whereas 3He,21Ne,and38ArFig. 3. DeuteriumconcentrationinApollo12018olivinesandpyroxenes,aswellas
3He,21Ne,and38Arconcentrationsinrockfragments(duplicatesateachdepth),
asafunctionofdepthbelowthesurface.Averagevalues(solidlines)andtheir1σ
standarddeviations(shadedareas)areindicatedforeachnuclide.Uncertainties(2σ
s.d.)ofnoblegasabundancesaresmallerthansymbolsizes.
notionthat small-scalechemical ormineralogical heterogeneities (e.g., olivine phenocrysts, up to 1–2 mm in size) can affect the concentrationofparent (ortarget)nuclides,and, correspondingly, theabundancesofradiogenicandcosmogenicisotopesinmg-sized sub-samples (Füri etal., 2017). Nonetheless, Fig.3 clearly shows that duplicates froma givendepth contain similar gas amounts, andthereisnonoblegasconcentrationgradientbetweentherock surfaceand4.8 cmdepth.
Sinceonlyasmallamountofneon(andargon)isreleasedfrom Apollo 12018rock fragments at low temperature, neon (and ar-gon)isotopecountratesaresignificantlyloweratthefirstheating stepcomparedto thesecond step(Fig. 2).However, Fig.2 shows thatairstandardsyieldconstantneonisotoperatios,i.e.,20Ne/22Ne and21Ne/22Ne,overtheentirerangeofcount ratesmeasuredfor
thesamples.Thisdemonstratesthatthereisnoanalyticalbiasfor thedifferentheatingsteps,anddifferencesinneon(and36Ar/38Ar) isotoperatioscanbeattributedtovariablecontributionsfrom dif-ferentcomponents(SW,SCR,GCR) intheanalyzednoblegas frac-tions.
Previous analyses by single-step heating revealed that frag-mentsfrom“theupperpittedsurface”ofApollo12018contained significant quantities of SW-derived noble gases, asindicated by
20Ne/22Neand36Ar/38Arratiosofup to11.4and5.4,respectively
(Megrue,1971), whereas three other chipsfromthe near-surface weredominatedbycosmogenicneonandargon,withmuchlower
20Ne/22Ne and 36Ar/38Ar ratios of 1.20–1.84 and 0.772–0.98,
re-spectively(Bogardetal.,1971;Hintenberger etal.,1971)(Fig. 4). Theresults fromthisstudy show thatseveralApollo 12018 frag-mentsfromvariousdepthscontainasmallfractionofSW-derived noble gases (with 20Ne/22Ne
=
2.135 to 9.708 and 36Ar/38Ar=
1.026 to 3.016), predominantly released during the first heat-ing step (Fig. 4; supplementary Table S5). Even fragments ex-tracted from 1.5 to
∼
3 cm depth yield elevated 20Ne/22Ne and 36Ar/38Ar ratios compared to the cosmogenic endmember; thisclearly demonstrates that some sub-samples belowthe rock sur-facewerecontaminatedbySW-loadeddustduringcutting,as pre-viously suggested for other Apollo samples by Füri et al. (2015,
2017).
In a three-isotope plot of neon, the data from Bogard et al. (1971), Hintenberger et al. (1971), and Megrue (1971) define a mixing line between implantation-fractionated SW-derived neon
(see Wieleret al.(2007) fordetails onthiscomponent) and cos-mogenic neon produced by nuclear interactions (Fig. 4a). Forall depth samples fromthisstudy, the isotopiccomposition of neon extractedathightemperatures (heatingsteps 2and3) fallsonto the same mixing line, irrespective of the analyzed neon amount (Fig.2).Furthermore,themeasuredneonisotoperatiosofthis cos-mogenic endmember (20Ne/22Ne
≈
0.86and 21Ne/22Ne≈
0.85)arecomparabletothetheoreticalsignatureofGCR-producedneon (20Ne/22Ne
≈
0.75and 21Ne/22Ne≈
0.89), which can be calcu-latedusingthe2π
exposuremodelfromLeyaetal.(2001) forlow shielding(0–15g/cm2),togetherwiththemajorelementcomposi-tionofolivinebasalt12018giveninsupplementaryTableS1. How-ever,formostsamples,neonextractedatthefirstheatingstephas alower 21Ne/22Ne ratioof
∼
0.72(ata 20Ne/22Neratioof∼
0.86;Figs.2and4a),comparable tothesignatureofneonproducedby SCRs,calculatedforarigidity R0
=
100 MVandanincidentparti-cleflux J0
=
100 protons/s/cm2 (Füri etal., 2017;TrappitschandLeya,2014). 4. Discussion
4.1. SCRnuclidesinApollo12018?
The low 21Ne/22Ne ratio of neon extracted at the first
heat-ing step appears to hint at the presence of SCR noble gases in Apolloolivine basalt 12018. Indeed,the fragmentsanalyzed here cover the entiredepth range in which SCR nuclides can be pro-duced. Models indicate that the abundance of SCR 21Ne should dropoffveryrapidlywithinthetopmostcm,whereasthe concen-trationofGCR21Neisexpectedtoincreaseslightlywithincreasing
shielding (i.e., withincreasing depth belowthe surface) because, for each incident primary particle, a cascadeof secondary parti-clesisproducedwhichcanthenundergonuclearinteractions(e.g., Reedy andArnold,1972;Fig.5).Giventhat theneonisotope pro-duction ratesbySCRs are significantlyhigherthat thoseby GCRs at theuppermost rock surface (i.e., by abouta factor offour for
21Ne; Leya etal., 2001; Trappitsch andLeya, 2014; Fig.5), ‘total’
(SCR
+
GCR)cosmogenicneonconcentrationsareexpectedto de-crease significantly with(shielding) depth in lunar samples that were exposedto bothsolarandgalacticcosmicrays;thesame is trueforcosmogenic3Heand38Ar.Indeed,Raoetal. (1993,1994) observed asystematicdecrease of3He, 21,22Ne,and38Arconcen-trationswithincreasingdepthinsub-samplesofApollo61016and 68815,particularlywithin thefirstcmbelowthesurface, indicat-ing a progression from a mixed noble gas component produced fromSCRs and GCRs toa pure GCR component. It is noteworthy thatApollosamples61016and68815bothhaveexposureagesof
∼
2Ma only(Rao etal., 1994, 1993), implyingthat anyshielding changesduringtheirresidenceonthelunarsurfaceareunlikelyto beimportant.Fig. 3 shows that D, 3He, 21Ne, and 38Ar concentrations in Apollo 12018 are constant over the studied depth interval, im-plying that the theoretical depth-dependent cosmogenic nuclide production rates are not appropriate for the sub-samples stud-iedhere.SCRproductionratesdependontherigidity(momentum per unit charge;R0)andflux ( J0) ofSCRparticles,aswell ason
thesample size, shape,andorientation,andits erosionrate(e.g., Rao et al., 1994). While there is some debate about the correct choiceofR0 and J0 formodelingcosmicrayinteractionswith
lu-nar samples (e.g., Rao et al., 1994; Reedy, 1980), the irradiation history of Apollo 12018 at the surface of the Moon is the key unknown.Erosionbycosmicraysputteringandbyimpactsof cos-micdustandmicrometeoritescouldhavemodifiedtheoutermost rocksurface,whereastumblingandintermittentburialcouldhave resulted in changes in sample orientation and variable shielding conditions (Reedy, 1980; Reedy and Arnold, 1972). Furthermore,
6 E. Füri et al. / Earth and Planetary Science Letters 550 (2020) 116550
Fig. 4. a)Three-isotopeplotofneonandb)20Ne/22Neversus36Ar/38ArforApollo12018fragments.Step-heatingdatafromthisstudyareshowntogetherwithprevious
resultsobtainedbysingle-stepheating(Bogardetal.,1971;Hintenbergeretal.,1971;Megrue,1971).Theneonandargonisotopecompositionsofmodernsolarwind(SW; Heberetal.,2009)andtheterrestrialatmosphere(Air;Eberhardtetal.,1965;Györeetal.,2019;Saxton,2020;WielandtandStorey,2019),aswellasthecalculatedisotope ratiosofcosmogenicneonproducedbysolarcosmicrays(SCR;TrappitschandLeya,2014)andgalacticcosmicrays(GCR;Leyaetal.,2001)forshieldingbetween0and15 g/cm2,areshownforcomparison.Thesolidlineina)representsamixinglinebetweentheGCRendmemberandimplantation-fractionatedSW-derivedneon.Uncertainties
(2σ s.d.)ofisotoperatiosfromthisstudyaresmallerthansymbolsizes(uncertaintiesforpreviousresultswerenotreported).
Fig. 5. Theoreticaldepth-dependentproductionratesofcosmogenic21Nebysolar
(SCR;TrappitschandLeya,2014)andgalacticcosmicrays(GCR;Leyaetal.,2001) asafunctionofdepthbelowthesurfaceofApolloolivinebasalt12018.
surfacedocumentationandcuttingdiagramsindicatethatrock col-umn 12018,14,17, from which the fragments for thisstudy were extracted, was orientedatanangle of
∼
45◦ relativeto thelunar zenith(Fig. 1; Meyer, 2011). Therefore,even though our samples arethoughttooriginatefromthe‘top’exteriorsurfaceoftherock, itispossiblethattheyhavenotbeenmeasurablyaffectedbySCRs, asaresultof thecomplexandprolonged (seeSection 4.2) expo-surehistoryofApollo12018.Overall,basedontheflatD,3He,21Ne,and38Arconcentration
profiles(Fig.3)andtheneonisotopecompositionofthe ‘volume-correlated’cosmogeniccomponent(Fig.4a),thepresenceof signif-icant amountsofSCR nuclidesin thestudied rockfragmentscan beruledout.Consequently,cosmogenicdeuteriumandnoblegases inApollo12018musthavepredominantlybeenproducedbyGCRs. Theoriginandnatureofthelow-21Ne/22Neneonendmember re-mains enigmatic; given that this component is detected at low temperaturesandrepresentsonlyasmallfractionofthetotalgas amount,itmightrepresentsomeunidentifiedformofsurface
con-tamination,isotopicallydistinctfromimplantedSWandadsorbed terrestrialatmosphericgases(Fig.4).
4.2. GCRnoblegasesandcosmicrayexposure(CRE)ages
Assuming that olivine basalt12018 contains a binary mixture ofSW-derivedandcosmogenic(GCR-derived)20,21,22Neand36,38Ar,
theamountofcosmogenic21Neand38Arcanbederived numeri-cally foreach rockfragment, basedon theisotopecomposition of the two endmembers (see Füri et al., 2014, 2018,for details on the component deconvolution). We use here 21Ne/22Ne ratios of 0.035 (measuredby Megrue,1971) and0.89(calculated fromthe modelofLeyaetal.,2001) forSW-derived andcosmogenicneon, respectively, and 36Ar/38Ar ratios of 5.4 (measured by Megrue,
1971) and0.65(i.e., thelowest ratiomeasured inthis study)for SW-derived and cosmogenic argon (Fig. 4). Based on this two-componentmodel,weestimatethat
≥
99.4%ofthemeasured21Neand 38Ar in mostApollo 12018 depth samples was produced in situ by GCR-induced spallation reactions during exposure at the lunarsurface.Onlythethreefragmentswith36Ar/38Arratios
≥
1.5 atthefirstheating stepcontain∼
2–4%SW-derivedargon. Impor-tantly, theseresultsare insensitive to the preciseratios assumed forthetwoendmembers.Themeasured3Hecanbeinferredtobe entirely of cosmogenic origin;however, since cosmogenic 3He isreadilylostbydiffusion(e.g.,Rao etal., 1994),3Heexposureages arenotdiscussedhere.
ACREagecanbederivedbycomparingtheaccumulated abun-danceofcosmogenicnoblegasnuclides(21Necosm,38Arcosm)with
empiricalortheoreticalnoblegasproductionrates(e.g.,Hohenberg et al., 1978; Leya et al., 2001; Reedy, 1981). However, it should againbeemphasizedthatnuclideproductionratesarefunctionof theconcentrationofvarioustargetelements,aswellasofthe sam-pleorientationandshieldingduringtheentireexposurehistoryat the lunar (near-)surface.Intra- andinter-sample variations in ef-fective nuclideproductioncan,therefore,besubstantial(Drozdet al., 1974; Füri et al., 2017). Nonetheless, since the 2
π
exposure model from Leya et al. (2001) closely reproduces the neon iso-toperatiosofApollo12018fragments, andbecausetheseauthors argued that theirmodeled noblegas dataareconsistent with ra-dionuclide results,we use their physical modelto calculate21NeFig. 6. Cosmicrayexposureagesderivedfrom21Ne
cosm(T21)and38Arcosm(T38)
concentrations,assuminganuncertaintyof10%fortherespectiveproductionrates. ResultsforthetwentyApollo12018rockfragments(blackcircles)andtheiraverage value(redcircle)arecomparedtothe81Kr-KrexposureagefromMartiandLugmair
(1971) (greydiamond).(Forinterpretationofthecolorsinthefigure(s),thereader isreferredtothewebversionofthisarticle.)
variesbetween4.8and6.1
×
10−14mol(grock)−1Ma−1 forshield-ing between0and15 g/cm2 in olivine basalt12018(Fig. 5), we
obtain an average 21Ne CRE age (T21) of 184
±
26 Ma for thetwentyrockfragments (Fig. 6). As discussed byFüri etal. (2017,
2018), theproductionrateofcosmogenic 38Arisdebated,inpart
because the ‘total’ abundance of the major target element Ca is highly sensitive to mineralogical heterogeneities (e.g., the distri-bution of high-Ca pigeonites and plagioclase). We use here the empirically-derivedvalueof4.6
×
10−14mol(grock)−1Ma−1 fromBogardet al.(1971) andHintenberger etal.(1971);thus, the re-sulting38ArCREages(T38)agree,inmostcases,within
uncertain-tieswiththosederivedfrom21Ne(T
21),andyieldanaveragevalue
of193
±
25Ma (Fig.6).Hintenbergeretal.(1971) reported com-parableexposureagesbasedon3He(180Ma),21Ne(210Ma),and38Ar (200Ma) concentrations,andStettler et al.(1973) obtained
asimilar38ArCREageof170–180 MaforApollo12018.
Further-more,MartiandLugmair (1971) determined aCRE ageof 195
±
16 Mabyusingthe81Kr-Krtechnique;thismethodisassumedto be largelyindependent of chemistryand shielding. We note that Fürietal.(2017) argued fora significantlylonger exposure dura-tion of242±
42Ma becausetheir Apollo 12018fragment con-tainedalarger amountofcosmogenic21Ne(21Necosm
=
13.86×
10−12 mol/g)comparedto the21Ne
cosm concentrationsmeasured
here(21Necosm
=
10.0±
1.3×
10−12mol/g;Fig.3),possiblyasaresultofahigherproportionofforsteriticolivine(richinthetarget elementMg);however,theamountofcosmogenic38Arwas
com-parablebetweenthetwostudies(38Arcosm
=
10.29×
10−12mol/gvs. 9.2
±
1.5×
10−12 mol/g;Fig. 3).Theseobservations demon-stratethat,eventhough noblegascontentsandisotoperatioscan bedeterminedatveryhighprecisionformg-sizedsamples, small-scale chemical or mineralogical heterogeneities can result in a rangeofCREagesobtainedfromthenuclideaccumulationmethod, particularlyformedium- or coarse-grainedlunarsamplessuch as olivinebasalt12018(Fig.6).4.3.Cosmogenicdeuteriuminlunarolivinesandpyroxenes
Fig. 7 shows the deuterium content of Apollo 12018olivines and pyroxenes – together with the deuterium concentration in Apollo12004and15016olivines–asafunctionoftheir21NeCRE
Fig. 7. Deuterium contentofolivinesandpyroxenesasafunctionofthecosmic rayexposureagederivedfromthe21Ne
cosmconcentrationinmarebasaltfragments
(T21).ResultsfromthisstudyforApollo12004,12018,andApollo15016areshown
togetherwiththedatafromFürietal.(2017).ThenewresultsagreewiththeD productionrateof(2.17±0.11)×10−12mol(grock)−1Ma−1(solidline)butare
inconsistentwiththe PD valueof∼1×10−12mol(grock)−1Ma−1 fromMerlivat
etal.(1976) andReedy(1981) (dashedline).
ages (T21). The resultsfrom thisstudy are consistent withthose
fromFürietal.(2017),andcanbeexplainedbyinsitu production
ofcosmogenicDbyGCR-inducedspallationreactionsduring expo-sureatthelunarsurfaceatatime-averagedrateof
(
2.
17±
0.
11)
×
10−12mol(g rock)−1Ma−1.Itisworthnotingthataslightlylonger exposure duration of
∼
215Ma for Apollo12018would resultin an even better agreement with the other coupled D-21Ne data;thisvalueisentirelycompatiblewiththeexposureagesfoundhere whenthescatterinthedataandtheuncertaintiesareconsidered (Fig.6).Theinterceptofthebest-fitlinethroughthedataindicates that olivineandpyroxenegrainsinApollomarebasaltswithCRE ages
=
0Ma contain∼
0.
1×
10−9 mol D (g rock)−1.This iscon-sistent withthe observationthat themineralgrainsstudied here contain a few ppm indigenous and/or terrestrial adsorbed water withaterrestrial-likeD/Hratio(seeSection3.1).
Although Reedy (1981) argued for a significantly lower PD value of
∼
1×
10−12 mol(g rock)−1Ma−1 for lunar mare basalts exposed to GCRs, his theoretical calculationsclearly demonstrate the importance of chemistry andshielding forthe production of cosmogenicnuclides.AssumingthatDisproducedfromthetarget elements O, Mg, Al, Si,and Fe ata ratioof∼
2:1:1:1:0.5,olivine and pyroxene (pigeonite) in Apollo 12018 (supplementary Table S3)areexpectedtocontainasimilaramountofcosmogenicDafter∼
200Maofexposure,butthesamewouldnotbetruefor chemi-callydistinctminerals.Furthermore,SCReffectscouldsignificantly increasetheeffectivecosmogenicDproductionrateatthe upper-mostrocksurface, unlessa highsurface erosionratemodifiesthe nuclideproductionprofile,asexpectedforsamplesthatexperience long(≥
10Ma)exposureatthesurfaceoftheMoon(Reedy,1981). Consequently,the chemicalcomposition ofthesample ofinterest andtheirradiation conditionsmustbe knownandtakeninto ac-count when correcting measured D/Hratios ofwaterin lunar or otherextraterrestrialmaterialsforthecosmogenicDcontribution. TheproductionrateofcosmogenicDinother mineralshasyetto be determined. Thus, care should be taken when applying pub-lished PD valuesforderiving theisotope signature ofindigenous waterinapatiteorplagioclase,particularlyforwater-poorsamples thatexperiencedprolongedexposuretocosmicraysatthesurface oftheMoon.8 E. Füri et al. / Earth and Planetary Science Letters 550 (2020) 116550
5. Conclusions
Ten chips from different documented depths (0 to
≥
4.8 cm) within Apollo olivine basalt 12018 were targeted for coupled deuterium–noblegasanalyses.Giventhatthesesamplescoverthe entire depth range (i.e., the topmost 1–2 cm) in which SCR nu-clidescan be producedduring exposureto cosmic rays, a depth-dependent concentration profile was expected to be observed for cosmogenic D, 3He, 21Ne, and 38Ar. However, abundances ofthese nuclides are constant with depth, and neon isotope ratios (20Ne/22Neand21Ne/22Ne)aredistinctfromtheisotopesignature of neon produced by SCRs. Consequently, cosmogenic deuterium and noble gas nuclides in the Apollo 12018 rock fragments and mineralsmusthavepredominantlybeenproducedbyGCRsduring prolonged(184
±
26Mato193±
25Ma),andpossiblycomplex, exposureatthelunar(near-)surface.AlthoughtheeffectsofSCRs ontheproductionrateofDcannot beevaluated,thenewdataset confirmsthe PD valueof(
2.
17±
0.
11)
×
10−12mol(g rock)−1Ma−1 from Füri et al. (2017). This value can be used to correct the D/Hratioof‘water’(i.e.,hydrogen)inolivineandpyroxenegrains that were irradiated by GCRs on theMoon, provided their expo-sureages are well established; however, applying the same pro-ductionrateto water-poor mineralswitha differentcomposition (e.g.,thephosphatemineralapatite),andwhoseexposure history is unknown, willlikely lead to misinterpretation of the resulting hydrogenisotoperatios.CRediTauthorshipcontributionstatement
EvelynFüri: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Writing, Project adminis-tration,Fundingacquisition. LaurentZimmermann: Methodology, Investigation, Writing. Etienne Deloule: Methodology, Investiga-tion,Writing. RetoTrappitsch: Conceptualization,Software,Formal analysis,Writing.
Declarationofcompetinginterest
Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.
Acknowledgements
We thank Apollo Sample Curator Ryan Zeigler and NASA
CAPTEMforallocationofApollo12018sub-samplesforthisstudy, aswell asCharis Krysherfor tedioussample processing atNASA
Johnson Space Center. We also thank Romain Tartèse and an
anonymousreviewerfortheircomments,andFredericMoynierfor efficient editorial handling. E.F., L.Z., andE.D. were supported by theEuropean ResearchCouncil(ERC)underthe EuropeanUnion’s Horizon 2020research andinnovationprogram (grantagreement no.715028).WorkbyR.T.wasperformedundertheauspicesofthe U.S. Department of Energy by Lawrence Livermore National Lab-oratory under Contract DE-AC52-07NA27344. LLNL-JRNL-813331. CRPG-CNRScontribution2739.
Appendix A. Supplementarymaterial
Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.epsl.2020.116550.
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