Evelyn Füri

Laurent Zimmermann

Alberto E Saal

Apollo 15 green glass He-Ne-Ar signatures -In search for indigenous lunar noble gases

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The recent discovery of indigenous water and other highly volatile elements (C, F, S, Cl;[START_REF] Lo Cascio | Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior[END_REF][START_REF] Sharp | The chlorine isotope composition of the moon and implications for an anhydrous mantle[END_REF][START_REF] Wetzel | Carbon content and degassing history of the lunar volcanic glasses[END_REF] in samples returned by the Apollo missions demonstrates that the formation and evolution of the Moon involved processes that allowed for the accretion and retention of the most volatile elements in the Solar System. Although the source and timing of volatile accretion is still debated (Hauri et al., 2015;[START_REF] Barnes | An asteroidal origin for water in the Moon[END_REF], these findings raise the question of whether the lunar mantle contains indigenous (i.e. primordial) noble gases that were supplied to the growing Moon by the delivery of volatile-rich chondritic matter or were inherited from the proto-Earth.

The isotopic and/or elemental abundance signature of indigenous lunar noble gases could provide crucial constraints on the origin and evolution of the Moon. However, indigenous lunar noble gases have never been unambiguously found despite extensive searches over the past five decades [START_REF] Wieler | Noble gas isotopes on the Moon[END_REF], although recent high precision analyses hint at the presence of indigenous xenon in lunar anorthosites [START_REF] Bekaert | Stepwise heating of lunar anorthosites 60025, 60215, 65315 possibly reveals an indigenous noble gas component on the Moon[END_REF]. The key caveat is that noble gases of primordial origin can easily be masked by additional noble gas components. All rock and soil samples collected at the Moon's surface contain 'trapped' surface-correlated noble gases implanted by SW irradiation. Volume-correlated noble gases comprise cosmogenic nuclides ( 3 He, 21 Ne, 38 Ar) produced by spallation reactions, and radiogenic noble gases ( 4 He, 40 Ar) produced by the decay of long-lived radionuclides. Due to the modifications that occur during space exposure, discerning indigenous noble gases in currently available lunar samples is highly challenging. [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF] proposed that green glass spherules from Apollo sample 15426 contain solar-like noble gases that may be of primordial origin. 15426 is a friable greenish clod that was collected by the Apollo 15 crew from the north rim of Spur Crater on the Apennine front. The green material is the common Apollo 15 very-low-Ti glass that was formed by volcanic fire-fountaining of primitive melts ~3.4 Gyr ago (Table S-1;[START_REF]Argon in Apollo 15 green glass spherules (15426): 40 Ar-39 Ar age and trapped argon[END_REF][START_REF] Delano | Apollo 15 green glass: Chemistry and possible origin[END_REF][START_REF] Delano | Pristine lunar glasses: criteria, data, and implications[END_REF][START_REF] Spangler | 39 Ar-40 Ar ages for the Apollo 15 green and yellow glasses[END_REF]. Importantly, the presence of water, C, F, S, and Cl in the interior of the glass beads [START_REF] Lo Cascio | Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior[END_REF][START_REF] Wetzel | Carbon content and degassing history of the lunar volcanic glasses[END_REF], together with the condensation and enrichment of volatile elements on the glass surface [START_REF] Delano | Apollo 15 green glass: Chemistry and possible origin[END_REF], requires the existence of a volatile-rich reservoir in the lunar mantle source.

Step-wise gas extraction, combined with multi-element isotope analysis, provides the only means for resolving noble gases into constituent components, and for detecting indigenous gas in lunar samples. Therefore, in this study, we re-assess the noble gas signature of 15426 green glasses by step-wise CO 2 laser extraction static mass spectrometry analysis (see Supplementary Information for details). Noble gas (He, Ne, Ar) concentrations and isotope ratios of twelve single vitreous glass beads, between 13 and 25 (±2) micrograms in mass, Letter Geochem. Persp. Let. (2018) 8, 1-5 | doi: 10.7185/geochemlet.1819 2 were measured using the Helix MC Plus noble gas mass spectrometer at CRPG in Nancy, France. Two heating steps were applied: a low temperature step (~600 °C) allowed extracting surface-sited, thermally labile gases, whereas the fusion step (~1500 °C) was aimed at releasing volume-correlated, more refractory noble gas components. Since neon isotopes were measured in multi-collection mode on three electron multipliers, the analytical precision on neon isotope ratios has improved by a factor of two compared to our previous study of orange lunar volcanic glasses [START_REF] Füri | New evidence for chondritic lunar water from combined D/H and noble gas analyses of single Apollo 17 volcanic glasses[END_REF]. Such high precision measurements are key for distinguishing different noble gas components in lunar samples. The 4 He content of several 15426 spherules analysed here is below the detection limit, implying that the glasses experienced severe diffusive helium losses. Any remaining helium is predominantly released at the first heating step, and, in most cases, shows an isotope ratio comparable to that of modern SW (Table S-2). These results indicate that the glass beads contain surface-sited noble gases implanted by SW irradiation, with only minor remnants of radiogenic 4 He and cosmogenic 3 He in the grain interior. The neon and argon abundances vary by two orders of magnitude (Figs. 1 and2d), and the concentration range recorded by the small 15426 green glass beads analysed here is significantly greater than that reported by [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF] for five grain size fractions and two large single spherules. 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios vary between 12.50 and 1.18 and between 0.0350 and 0.815, respectively, and in a three-isotope plot of neon, most isotope signatures fall onto a mixing line between a high-20 Ne/ 22 Ne component (with 20 Ne/ 22 Ne = 12.42 ± 0.05 for 21 Ne/ 22 Ne = 0.03), interpreted to represent isotopically fractionated SW-derived neon, and cosmogenic neon (Fig. 2a). Similarly, 36 Ar/ 38 Ar ratios range from 5.51 to 0.91, also reflecting varying mixing proportions between SW-derived argon and a low-36 Ar/ 38 Ar cosmogenic component (Fig. 2b). The 'trapped' neon and argon component, implanted by SW irradiation, is predominantly released during the first heating step, whereas the cosmogenic isotopes are extracted during sample melting (Fig. 2). The solar gas component is isotopically depleted in the light neon isotopes compared to modern SW due to depth-dependent isotope fractionation upon implantation of SW and removal of surface-sited solar gas by ion sputtering [START_REF] Wieler | Consequences of the non-existence of the "SEP" component for noble gas geo-and cosmochemistry[END_REF]. Importantly, four glass beads stand out by having the highest noble gas contents and by showing a distinct gas release pattern; i.e. four spherules release large amounts of SW-derived helium, neon, and argon at low temperatures, and a volume-correlated component with high 20 Ne/ 22 Ne, 36 Ar/ 38 Ar, and 36 Ar/ 22 Ne ratios is extracted at the second heating step (Fig. 2). In the three-isotope plot of neon, this end member falls below the mixing line defined by the other glass beads, and points to a 20 Ne/ 22 Ne ratio of 11.41 ± 0.04 (for 21 Ne/ 22 Ne = 0.03).

Noble Gas Signature of 15426 Green Glasses

The lack of correlation between the noble gas abundance and the grain size (i.e. the surface/volume ratio) lead [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF] to propose that solar-like neon and argon in Apollo 15 green glasses may represent primordial lunar gas. However, the concentrations of 'trapped' solar neon and argon (Supplementary Information) found in 15426 glasses are several orders of magnitude greater than the Ne-Ar abundances in the 2PD43 popping rock, which is the least degassed terrestrial mid-ocean ridge basalt [START_REF] Moreira | Rare gas systematics in Popping rock: Isotopic and elemental compositions in the upper mantle[END_REF]. Although an indigenous origin for noble gases in lunar volcanic glasses cannot be ruled out, it appears unlikely that i) lunar magmas escaped degassing, and ii) the lunar mantle contains a larger amount of neon and argon than the terrestrial upper mantle, given the Moon's general volatile element depletion compared to Earth [START_REF] Rutherford | Water in the Moon's interior: Truth and consequences[END_REF]. As an alternative to an indigenous origin, the melt that formed the 15426 green glasses may have assimilated SW-irradiated regolith upon eruption, resulting in dissolution of solar noble gases into the interior of the melt droplets before quenching [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF]. Since lunar regolith contains 1.5 to 2 wt. % carbonaceous chondrite-like material (e.g., [START_REF] Keays | Trace elements and radioactivity in lunar rocks. Implications for meteorites infall, solar-wind flux, and formation conditions of Moon[END_REF], regolith assimilation should be detectable through analyses of highly siderophile elements (HSEs). [START_REF] Walker | Low abundances of highly siderophile elements in the lunar mantle: evidence for prolonged late accretion[END_REF] argued that a significant proportion of chondritic HSEs is only present in the "etchate" of the surface of the green glasses, whereas concentrations are low in the residual glass. Another possibility is that the 15426 melt droplets trapped a gas phase that was released by heating and outgassing of vast quantities of solar wind-impregnated regolith [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF]. However, it is difficult to envision how high noble gas partial pressures, allowing for the dissolution of inert gases into the erupting melt, could have been maintained during fire-fountain style eruptions in near vacuum at the lunar surface. In addition, rapid quenching of the erupting melt droplets must have prevented ingassing of volatiles released from the regolith.

In light of these caveats, the noble gas characteristics of the He-Ne-Ar-rich 15426 green glasses are best explained by inward diffusion of implanted SW-derived gas (with 20 Ne/ 22 Ne = 12.42 ± 0.05) -from the surface of the spherules into their interiors -during prolonged space exposure. When equilibrium conditions were established, the solar gases became volume-as opposed to surface-correlated [START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF]. Upon CO 2 laser heating, the diffusive gas release fractionates the 20 Ne/ 22 Ne ratio according to:

R residual = R initial × f (1-a)
Eq. 1

where R initial is the initial (solar-like) 20 Ne/ 22 Ne ratio, f is the fraction of neon remaining in the glass, and a is the isotope fractionation factor, which can be approximated by √ m 20 -/m -22 for mass dependent fractionation. Since ~90 % of the total amount of neon was extracted from the two gas-richest spherules during the first heating step (i.e. f = 0.1; Table S-2), the cumulative fraction of neon released at low temperature is expected to record an integrated 20 Ne/ 22 Ne ratio of 12.05, whereas the residual 'trapped' neon is estimated to yield a 20 Ne/ 22 Ne ratio of 11.2; both estimates are in excellent agreement with the values observed here (Fig. 2a and Table S-2). et al., 1973) and by laser ablation of individual spherules [START_REF] Megrue | Distribution of gases within Apollo 15 samples: implications for the incorporation of gases within solid bodies of the Solar System[END_REF]. The neon and argon isotope compositions of the cosmogenic endmember (Cosm), modern solar wind (SW; [START_REF] Heber | Noble gas composition of the solar wind as collected by the Genesis mission[END_REF], and the meteoritic components A and Q [START_REF] Ott | Planetary and pre-solar noble gases in meteorites[END_REF] are shown for comparison. Error bars for the results from this study represent 1s uncertainties and are, in some cases, smaller than symbol sizes. et al., 1973;[START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF][START_REF] Megrue | Distribution of gases within Apollo 15 samples: implications for the incorporation of gases within solid bodies of the Solar System[END_REF][START_REF] Spangler | 39 Ar-40 Ar ages for the Apollo 15 green and yellow glasses[END_REF].

Irradiation History of 15426 Green Glasses

Since the 15426 glass beads contain a binary mixture of SW-derived and cosmogenic 20,21,22 Ne and 36,38 Ar, the amount of 21 Ne cosm and 38 Ar cosm can be derived numerically for each sample, based on the isotopic signature of the two end members (see Supplementary Information for details on the component deconvolution). The concentrations of cosmogenic 21 Ne and 38 Ar correspond to cosmic ray exposure (CRE) ages (T 21 and T 38 , respectively) that agree within uncertainties, and vary between 87 ± 9 and 347 ± 73 Myr in most cases (Fig. 3 and Table 1). These CRE ages are comparable to the values of 139 to 400 Myr obtained by previous noble gas analyses of 15426 green glasses [START_REF] Wasserburg | An argon bouillabaisse including ages from the Luna 20 site[END_REF][START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF][START_REF] Megrue | Distribution of gases within Apollo 15 samples: implications for the incorporation of gases within solid bodies of the Solar System[END_REF][START_REF] Spangler | 39 Ar-40 Ar ages for the Apollo 15 green and yellow glasses[END_REF]. However, three out of the four gas-rich glass beads clearly stand out by having high concentrations of cosmogenic 21 Ne and 38 Ar, reflecting extremely high exposure ages (T 38 ) between 542 ± 87 and 2139 ± 303 Myr (Fig. 3 and Table 1). This indicates that these spherules were exposed to cosmic, and possibly solar wind, irradiation at the (sub-) surface of the Moon for a much longer duration than the majority of the Apollo 15 green glasses which could have resulted in inward diffusion of SW-derived gases, facilitated by radiation damage. At the same time, diffusive loss of 21 Ne cosm likely occurred, thus severely affecting the T 21 exposure ages. These observations demonstrate that, due to intense stirring of the regolith by impact gardening, each individual lunar volcanic glass bead records its own unique irradiation history, which is accessible only through single grain analyses. The four glass beads with the highest He-Ne-Ar abundances (Table S-2) are identified by asterisks.

Uncertainties for the 21 Ne cosm and 38 Ar cosm concentrations are on the order of 10 to 15 %.

In Search of Indigenous Lunar Noble Gases

The hydrogen (and nitrogen) isotopic signature of lunar samples is currently best explained by late accretion of volatile-rich chondritic material to the Moon, although the possibility that some volatiles were inherited from the proto-Earth cannot be ruled out (e.g., [START_REF] Füri | New evidence for chondritic lunar water from combined D/H and noble gas analyses of single Apollo 17 volcanic glasses[END_REF][START_REF] Rutherford | Water in the Moon's interior: Truth and consequences[END_REF][START_REF] Barnes | An asteroidal origin for water in the Moon[END_REF]. Could indigenous lunar noble gases provide further constraints on the origin of volatiles in the lunar interior, and, thus, on the formation and evolution of the Moon? Noble gas elemental ratios must have been fractionated throughout the volatile accretion process(es) by the Moon and/or upon partial melting and degassing during magmatic eruption; thus they are unlikely to preserve the original source characteristics. Noble gas isotope ratios represent excellent volatile source tracers; nonetheless identifying a chondritic or terrestrial provenance may be highly challenging because secondary contributions from the SW and/or cosmogenic nuclides hamper the identification of the indigenous component.

The terrestrial mantle is characterised by 20 Ne/ 22 Ne and 36 Ar/ 38 Ar ratios of 12. 65 and 5.3, respectively (Péron et al., 2017); thus, transfer of noble gases from Earth to the Moonforming material would have resulted in isotope compositions that are very similar to those of the 'trapped' component in Apollo 15426 green glasses. SW-irradiated chondritic dustwith a high surface/volume ratio -would be characterised by comparable noble gas isotope signatures; however small dust grains can be ruled out as the dominant contributors to the lunar volatile inventory on the basis of predominantly non-solar hydrogen and nitrogen isotope signatures of Apollo samples. For large chondritic bodies, the neon inventory is dominated by neon in presolar diamonds (Ne-A), whereas the argon budget is dominated by the Q component. These end members are characterised by 20 Ne/ 22 Ne and 36 Ar/ 38 Ar ratios of ~8.2 and 5.34, respectively (Fig. 2a,b;[START_REF] Ott | Planetary and pre-solar noble gases in meteorites[END_REF]. Therefore, step-heating neon extraction, combined with precise isotope analysis by state-of-the-art static vacuum mass spectrometry, would allow a distinction between a chondritic and a terrestrial provenance of lunar volatiles. Although high-precision (He-)Ne-Ar data can now be obtained for very small (≤25 mg) lunar volcanic glasses, new lunar samples are needed to detect a possible indigenous noble gas component, and to identify unambiguously its provenance. Pristine samples that have never been exposed to space at the lunar surface, i.e. rock samples retrieved by drilling from depths of several metres or fresh crater ejecta, are key for assessing whether the lunar mantle retains indigenous noble gases that were supplied to the growing Moon by the delivery of volatile-rich chondritic matter were inherited from the proto-Earth.

Figure 1

 1 Figure 1 Total 21 Ne abundances of twelve single 15426 green glass beads. The uncertainty of 8 to 15 % is controlled by the precision in determining the sample mass. The 21 Ne concentration range reported by Lakatos et al. (1973) for five grain size fractions and two large single spherules is indicated for comparison.

Figure 2 (

 2 Figure 2 (a) Three-isotope plot of neon, (b) 20 Ne/ 22 Ne versus 36 Ar/ 38 Ar, (c) 36 Ar/ 22 Ne versus 21 Ne/ 22 Ne, and (d) 36 Ar/ 22 Ne versus the inverse of the 22 Ne concentration measured in 15426 green glasses. Step-heating data for single glass beads from this study are shown together with results obtained by single-step heating of different grain size fractions and two large single spherules (Lakatos et al., 1973) and by laser ablation of individual spherules[START_REF] Megrue | Distribution of gases within Apollo 15 samples: implications for the incorporation of gases within solid bodies of the Solar System[END_REF]. The neon and argon isotope compositions of the cosmogenic endmember (Cosm), modern solar wind (SW;[START_REF] Heber | Noble gas composition of the solar wind as collected by the Genesis mission[END_REF], and the meteoritic components A and Q[START_REF] Ott | Planetary and pre-solar noble gases in meteorites[END_REF] are shown for comparison. Error bars for the results from this study represent 1s uncertainties and are, in some cases, smaller than symbol sizes.

Figure 3

 3 Figure 3 Cosmic ray exposure ages derived from the 21 Ne cosm (T 21 ) and 38 Ar cosm (T 38 ) concentrations. Data for the four He-Ne-Ar-rich 15426 glass beads (open diamonds) and eight other spherules (filled diamonds) are shown together with the CRE ages determined previously (grey rectangle; Hunekeet al., 1973;[START_REF] Heymann | Green spherules from Apollo 15: Inferences about their origin from inert gas measurements[END_REF][START_REF] Megrue | Distribution of gases within Apollo 15 samples: implications for the incorporation of gases within solid bodies of the Solar System[END_REF][START_REF] Spangler | 39 Ar-40 Ar ages for the Apollo 15 green and yellow glasses[END_REF].

Letter

  Geochem. Persp. Let. (2018) 8, 1-5 | doi: 10.7185/geochemlet.1819 4

Table 1

 1 Abundances of cosmogenic 21 Ne and 38 Ar and corresponding cosmic ray exposure ages of twelve single 15426 green glass beads.

	Sample ID	21 Ne cosm (pmol/g)	% total	T 21 (Myr)	38 Ar cosm (pmol/g)	% total	T 38 (Myr)
	15426-4	7.1	97	105 ± 21	3.7	59	87 ± 9
	15426-11	7.8	96	116 ± 26	4.2	52	98 ± 14
	15426-2	8.3	98	125 ± 29	4.2	64	97 ± 15
	15426-5	10.7	92	160 ± 34	5.9	41	137 ± 17
	15426-I	12.4	97	185 ± 36	7.0	62	163 ± 15
	15426-13* 13.1	36	196 ± 43	13.0	9	303 ± 41
	15426-II	17.9	99	267 ± 56	9.8	87	228 ± 26
	15426-10	19.4	98	289 ± 68	11.0	51	255 ± 41
	15426-6	23.2	92	347 ± 73	13.5	44	315 ± 38
	15426-12* 35.3	84	527 ± 124 23.3	25	542 ± 87
	15426-III* 47.8	34	714 ± 164 64.6	10	1503 ±
							227
	15426-3*	75.4	33	1125 ±	92.0	10	2139 ±
				251			303

Acknowledgements

We thank two anonymous reviewers for their constructive comments. This work was supported by the European Research Council (grant no. 715028 to E.F.). This is CRPG-CNRS contribution 2601.

Editor: Cin-Ty Lee

Additional Information

Supplementary Information accompanies this letter at http:// www.geochemicalperspectivesletters.org/article1819. This work is distributed under the Creative Commons Attribution Non-Commercial No-Derivatives 4.0 License, which permits unrestricted distribution provided the original author and source are credited. The material may not be adapted (remixed, transformed or built upon) or used for commercial purposes without written permission from the author. Additional information is available at http://www.geochemicalperspectivesletters.org/ copyright-and-permissions.

Cite this letter as: Füri, E., Zimmermann, L., Saal, A.E. (2018) Apollo 15 green glass He-Ne-Ar signatures -In search for indigenous lunar noble gases. Geochem. Persp. Let. 8,[1][2][3][4][5]