In the last decade, several studies have reported enrichments of the heavy isotopes of moderately volatile elements in lunar mare basalts. However, the mechanisms controlling the isotope fractionation are still debated and may differ for elements with variable geochemical behaviour. Here, we present a new comprehensive dataset of mass-dependent copper isotope compositions (delta 65Cu) of 30 mare basalts sampled during the Apollo missions. The new delta 65Cu data range from +0.14 %o to +1.28 %o (with the exception of two samples at 0.01 %o and -1.42 %o), significantly heavier than chondrites and the bulk silicate Earth. A comparison with mass fractions of major and trace elements and thermodynamic constraints reveals that Cu isotopic variations within different mare basalt suites are mostly unrelated to fractional crystallisation of silicates or oxides and late-stage magmatic degassing. Instead, we propose that the delta 65Cu average of each suite is representative of the composition of its respective mantle source. The observed differences across geographically and temporally distinct mare basalt suites, suggest that this variation relates to large-scale processes that formed isotopically distinct mantle sources. Based on a Cu isotope fractionation model during metal melt saturation in crystal mush zones of the lunar magma ocean, we propose that distinct delta 65Cu compositions and Cu abundances of mare basalt mantle sources reflect local metal melt-silicate melt equilibration and trapping of metal in mantle cumulates during lunar magma ocean solidification. Differences in delta 65Cu and mass fractions and ratios of siderophile elements between low- and high-Ti mare basalt sources reflect the evolving compositions of both metal and silicate melt during the late cooling stages of the lunar magma ocean.

The Moon can have elevated chlorine (Cl) isotope ratios, much higher than any other Solar System objects. Deciphering the Cl isotope compositions of volcanic lunar samples is critical for unraveling the volcanic processes and volatile inventory of the Moon's interior. However, the processes and mechanisms of Cl isotope fractionation are not yet fully understood through previous studies on lunar samples. The China's Chang'e-5 (CE5) basalt samples were collected far from the Apollo and Luna landing sites, and dated at about 2.0 billion years ago (Ga), approximately 1 Ga younger than previously reported lunar basalts. The CE5 samples, therefore, provide an opportunity to investigate Cl isotope characteristics and fractionation mechanisms during a younger lunar volcanism. In this study, we performed systematic petrography, mineral chemistry, volatile abundances and distribution, and Cl isotopic studies on the CE5 apatite via a combination of scanning electron microscopy, electron probe microanalyser, and nanoscale secondary ion mass spectrometry. The CE5 apatite grains from basalt clasts and fragments have subhedral to euhedral shapes with grains sizes mostly less than 10 mu m, mainly coexisting with the mesostasis, fayalite olivine, and the margins of pyroxene. These apatites are F-dominated (0.91-3.93 wt%) with a Cl abundance range of 820 to 11989 mu g.g(-1) and a water abundance range of 134 to 6564 mu g.g-1, similar to those in the mare samples previously reported. Chlorine displays notable zoning distributions in some CE5 apatite grains with higher abundance at the rims gradually decrease towards the cores. Chlorine isotopic compositions of CE5 apatite vary from 4.5 to 18.9%o, positively correlated with the Cl abundances. These lines of evidence suggest that magmatic degassing of Cl-bearing species during the crystallisation of apatite at or near the lunar surface could have resulted in a large Cl isotope fractionation. Our new findings highlight a significant role of magmatic fractionation of Cl isotopes during crystallisation of mare lavas and provide clues for determining the primordial Cl isotopic signature of the Moon.(c) 2022 Elsevier B.V. All rights reserved.

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The delta Ga-71 values of these samples range from -0.10 to +0.66 parts per thousand (where delta Ga-71 is the relative difference between the Ga-71/Ga-69 ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than delta Ga-71 values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk delta Ga-71 value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have delta Ga-71 values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher delta Ga-71 values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution. Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Delta Ga-71(Plagioclase-melt) of -0.3 parts per thousand, (where Delta Ga-71(plagioclase-melt) is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have delta Ga-71 values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP. We estimate that the delta Ga-71 value of the LMO was similar to 0.14 parts per thousand prior to the onset of plagioclase crystallization and extraction. Whether this delta Ga-71 value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 +/- 0.06 parts per thousand, Kato et al., 2017) and volatile loss drove the LMO to higher delta Ga-71 values prior to formation of the lunar crust. (C) 2021 Elsevier B.V. All rights reserved.

The Moon and Earth share similar relative abundances and isotope compositions of refractory lithophile elements, indicating that the Moon formed from a silicate reservoir that is chemically indistinguishable from the Earth's primitive silicate mantle. In contrast, most volatile elements are depleted in lunar mare basalts compared to Earth's mantle and differ in their isotope composition. However, the depletion of volatile elements is not a simple function of their condensation temperature, indicating multiple mechanisms that established the lunar volatile element budget. Specifically, the chalcophile elements S, Se and Te are not depleted in lunar basalts compared to their terrestrial counterparts. In this study, the abundances and stable isotope compositions of the volatile and chalcophile element Se measured in three lunar mare basalts and seven soils are used to refine the processes that caused volatile element depletion on the Moon. The Se isotope composition of two lunar mare basalts (delta(82)/Se-78 = 1.08 and 0.8 parts per thousand) is significantly heavier compared to chondrites (-0.20 +/- 0.26 parts per thousand; 2 s.d.) and terrestrial basalts (0.29 +/- 0.24 parts per thousand; 2 s.d.). The offset in the Se isotope composition is attributed to a volatility controlled loss of Se from the Moon. The lack of chalcophile element depletion in lunar mare basalts is then explained by sulphide segregation in the Earth's mantle after the Moon forming impact followed by a late veneer of chondritic material to the Earth. Seven lunar soils were found to have chondritic S/Se ratios, but have delta(82)/Se-78 values that are 6 to 13 parts per thousand heavier compared to mare basalts. This fractionation is likely the result of coupled and repeating processes of meteoritic material addition and concomitant partial evaporation. Results from numerical modelling indicate that isotope fractionation in lunar soils is due to partial evaporation of FeSe and FeS with evaporative loss of about 20% for both Se and S. (c) 2020 Elsevier B.V. All rights reserved.

Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar magmatic differentiation. Mare basalts sample the largest lunar mantle volume, from olivine- and pyroxene-rich cumulates, whereas ferroan anorthosites and magnesian-suite rocks represent early crustal materials. Incompatible element enriched rocks, known as 'KREEP,' probably preserve evidence for the last highly differentiated melts. Here we show that mare basalts, including Apollo samples and meteorites, have remarkably consistent delta Zn-66 values (+1.4 +/- 0.2 parts per thousand) and Zn abundances (1.5 +/- 0.4 ppm). Analyses of magnesian-suite rocks show them to be characterized by even heavier delta Zn-66 values (2.5 to 9.3 parts per thousand) and low Zn concentrations. KREEP-rich impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts (delta Zn-66 =1.3 parts per thousand) and a low Zn abundance (0.5 ppm). Much of this variation can be explained through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light Zn can be explained by either direct condensation or mixing and contamination processes at the lunar surface. The delta Zn-66 of Sayh al Uhaymir 169 is probably compromised by mixing processes of KREEP with mafic components. Correlations of Zn with Cl isotopes suggest that the urKREEP reservoir should be isotopically heavy with respect to Zn, like magnesian-suite rocks. Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to lunar formation, and planetary processes, either during giant impact, or magmatic differentiation. Our results provide unambiguous evidence for the latter process. Notwithstanding, with the currently available volatile stable isotope datasets, it is difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or exclusively from later magmatic differentiation. If the Moon did begin initially volatile-depleted, then the mare basalt delta Zn-66 value likely preserves the signature, and the Moon lost 96% of its Zn inventory relative to Earth and was also characterized by isotopically heavy Cl (delta Cl-37 = >= 8 parts per thousand). Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories. (C) 2019 Elsevier B.V. All rights reserved.

Studies of the lunar atmosphere have shown it to be a stable, low-density surface boundary exosphere for the last 3 billion years. However, substantial Volcanic activity on the Moon prior to 3 Ga may have released sufficient volatiles to form a transient, more prominent atmosphere. Here, we calculate the volume of mare basalt emplaced as a function of time, then estimate the corresponding production of volatiles released during the mare basalt-forming eruptions. Results indicate that during peak mare emplacement and volatile release similar to 3.5 Ga, the maximum atmospheric pressure at the lunar surface could have reached- similar to 1 kPa, or similar to 1.5 times higher than Mars' current atmospheric surface pressure. This lunar atmosphere may have taken similar to 70 million years to fully dissipate. Most of the volatiles released by mare basalts would have been lost to space, but some may have been sequestered in permanently shadowed regions on the lunar surface. If only 0.1% of the mare water vented during these eruptions remains in the polar regions of the Moon, volcanically-derived volatiles could account for all hydrogen deposits - suspected to be water - currently observed in the Moon's permanently shadowed regions. Future missions to such locations may encounter evidence of not only asteroidal, cometary, and solar wind-derived volatiles, but also volatiles vented from the interior of the Moon. Published by Elsevier B.V.

While it is now recognized that the Moon has indigenous water and volatiles, their total abundances are unclear, with current literature estimates ranging from nearly absent to Earth-like levels. Similarly unconstrained is the source of the Moon's water, which could be cometary, chondritic, or the primordial nebula. Here we measure H2O and D/H in olivine-hosted melt inclusions in lunar mare basalts 12018, 12035, and 12040, part of the consanguineous suite of Apollo 12 olivine basalts that differ primarily because of cooling rate (Walker et al., 1976). We find that the water contents are higher in the more rapidly cooled 12018 (62-740 ppm H2O) compared to the more slowly cooled basalts 12035 (28-156 ppm H2O) and 12040 (27-90 ppm H2O), suggesting that lunar basalts may have been dehydrating during slow cooling. D/H is similar in the olivine-hosted melt inclusions in all three samples, and indistinguishable from terrestrial water (dD = -183 +/- 212% to + 138 +/- 61%). When we compare the D/H of olivine-hosted melt inclusions to D/H of apatite in the same samples, the evolution of dD and water content can be better constrained. We propose that lunar magmas first exchange hydrogen with a low D/H reservoir during cooling, and then ultimately lose their water during extended subsolidus cooling. Due to high diffusion rates of hydrogen in olivine, it is likely that all basaltic olivine-hosted melt inclusions from the Moon exchanged hydrogen with a low D/H reservoir in near-surface magma chambers or lava flows. The most likely source of the low D/H reservoir on the Moon is the lunar regolith, which is known to have a significant solar wind hydrogen component.

Recent analytical advances have enabled first successful in-situ detection of water (measured as OH) in lunar volcanic glasses, and, melt inclusions and minerals from mare basalts. These in-situ measurements in lunar materials, coupled with observations made by orbiting spacecraft missions have challenged the traditional view of the Moon as an anhydrous body. By synthesizing and modeling of previously published data on OH contents and H isotope compositions of apatite from mare basalts, we demonstrate that a model of hydrogen delivery into the lunar interior by late accretion of chondritic materials adequately accounts for the measured water content and its hydrogen isotopic composition in mare basalts. In our proposed model, water in the lunar interior was mostly constituted by hydrogen, delivered by the late accretion of chondrite-type materials. Our model is also consistent with previously proposed models to account for other geochemical characteristics of the lunar samples. (C) 2012 Elsevier B.V. All rights reserved.