We compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited ranges in delta Zn-66, delta S-34, and delta Cl-37(SBC+WSC) values of 1.27 +/- 0.71, 0.55 +/- 0.18, and 4.1 +/- 4.0 parts per thousand, respectively, compared to the silicate Earth at 0.15, -1.28, and 0 parts per thousand, respectively. We find that the Zn, S, and Cl isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of a geochemical signature in the mare basalt source region that is inherited from lunar formation and magma ocean crystallization. The uniformity of these compositions implies mixing following mantle overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15 parts per thousand in delta Zn-66, 60 parts per thousand in delta S-34, and 30 parts per thousand in delta Cl-37). This reflects magma sources that experienced minimal volatile loss due to high confining pressures that generally exceeded their equilibrium saturation pressures. Alternatively, these data indicate effective isotopic fractionation factors were near unity. Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses (Saal and Hauri 2021), which vary widely in S isotope compositions from -14.0 to 1.3 parts per thousand, explained by extensive degassing of picritic magmas under high-P/P-Sat values (>0.9) during pyroclastic eruptions. The difference in the isotope compositions of picritic glass beads and mare basalts may result from differences in effusive (mare) and explosive (picritic) eruption styles, wherein the high-gas contents necessary for magma fragmentation would result in large effective isotopic fractionation factors during degassing of picritic magmas. Additionally, in highly vesiculated basalts, the delta S-34 and delta Cl-37 values of apatite grains are higher and more variable than the corresponding bulk-rock values. The large isotopic range in the vesiculated samples is explained by late-stage low-pressure vacuum degassing (P/P-Sat similar to 0) of mare lavas wherein vesicle formation and apatite crystallization took place post-eruption. Bulk-rock mare basalts were seemingly unaffected by vacuum degassing. Degassing of mare lavas only became important in the final stages of crystallization recorded in apatite-potentially facilitated by cracks/fractures in the crystallizing flow. We conclude that samples with wide-ranging volatile element isotope compositions are likely explained by localized processes, which do not represent the bulk Moon.

Elevated water contents in various lunar materials have invigorated the discussion on the volatile content of the lunar interior and on the extent to which the volatile element inventory of lunar magmatic rocks is controlled by volatility and degassing. Abundances of moderately volatile and siderophile elements can reveal insights into lunar processes such as core formation, late accretion and volatile depletion. However, previous assessments relied on incomplete data sets and data of variable quality. Here we report mass fractions of the siderophile volatile elements Cu, Se, Ag, S, Te, Cd, In, and Tl in lunar magmatic rocks, analyzed by state-of-the-art isotope dilution-inductively coupled plasma mass spectrometry. The new data enable us to disentangle distribution processes during the formation of different magmatic rock suites and to constrain mantle source compositions. Mass fractions of Cu, S, and Se in mare basalts and magnesian suite norites clearly correlate with indicators of fractional crystallization. Similar mass fractions and fractional crystallization trends in mafic volcanic and plutonic rocks indicate that the latter elements are less prone to degassing during magma ascent and effusion than proposed previously. The latter processes predominate only for specific elements (e.g., Tl, Cd) and complementary enrichments of these elements also occur in some brecciated highland rocks. A detailed comparison of elements with different affinities to metal or sulfide and gas phase reveals systematic differences between lunar magmatic rock suites. The latter observation suggests a predominant control of the variations of S, Se, Cu, and Ag by mantle source composition instead of late-stage magmatic degassing. New estimates of mantle source compositions of two low-Ti mare basalt suites support the notion of a lunar mantle that is strongly depleted in siderophile volatile elements compared to the terrestrial mantle.(C) 2022 Elsevier B.V. All rights reserved.

Lunar volcanic glasses associated with mare basalt magmatism experienced significant degree of degassing, and to retrieve their initial water contents requires data of water diffusivity. We carried out diffusion experiments at 0.5 GPa and 1703-1903 K in a piston cylinder apparatus for two synthesized lunar basaltic melts with compositions corresponding to Apollo green glass and yellow glass. The water diffusion profiles measured by FTIR spectroscopy yield water diffusivities 0.5-1 order of magnitude greater than those of terrestrial basaltic melts, which is attributed to the difference in melt polymerization and modest contribution from H-2 diffusion. However, hydroxyl (OH) is not only the dominant H species but is also inferred to be the major carrier of H in our experiments at oxygen fugacity estimated IW +/- 1 (IW: iron-wustite buffer). Modeling of previously reported profiles of volatile loss in an Apollo green glass bead using the new water diffusivity indicates an average cooling rate of 1-2 degrees C/s and an initial water content of 120-260 mu g/g. With the assumption of limited degassing before magma fragmentation, the lunar mantle source is inferred to contain 6-22 mu g/g H2O. The lunar interior appears to be less hydrous the Earth's interior but still contains a considerable amount of water. (C) 2019 Elsevier B.V. All rights reserved.

Experimental degassing of H-, F-, Cl-, C-, and S-bearing species from volatile-bearing magma of lunar composition at low pressure and fo(2), close to the quartz-iron-fayalite buffer (QIF) indicates that the composition of the fluid/vapor phase that is lost changes overtime. A highly H-rich vapor phase is exsolved within the first 10 min of degassing leaving behind a melt that is effectively dehydrated. Some Cl, F, and S is also lost during this time, presumably as HCI, HF, and H2S gaseous species; however much of the original inventory of Cl, F, and S components are retained in the melt. After 10 min, the exsolved vapor is dry and dominated by S- and halogen-bearing phases, presumably consisting of metal halides and sulfides, which evolves over time toward F enrichment. This vapor evolution provides important constraints on the geochemistry of volatile-bearing lunar phases that form subsequent to or during degassing. The rapidity of H loss suggests that little if any OH-bearing apatite will crystallize from surface or near surface (similar to 7m) melts and that degassing of lunar magmas will cause the compositions of apatites to evolve first toward the F-CI apatite binary and eventually toward end-member fluorapatite during crystallization. During the stage of loss of primarily H component from the melt, Cl would have been lost primarily as HCI, which is reported not to fractionate Cl isotopes at magmatic temperatures (Sharp et al. 2010). After the loss of H-bearing species, continued loss of CI would result in the degassing of metal chlorides, which have been proposed as a mechanism to fractionate Cl isotopes (Sharp et al. 2010). After the onset of metal chloride degassing, the delta Cl-37 of the melt would necessarily increase to +6 (82% Cl loss), +8 (85% Cl loss), and +20 parts per thousand (95% Cl loss) at 1, 4, and 6 h, respectively, which was approximated using a computed trajectory of delta Cl-37 values in basalt during degassing of FeCl2. This strong enrichment of Cl-37 in the melt after metal chloride volatilization is fully consistent with values measured for the non-leachates of a variety of lunar samples and would be reflected in apatites crystallized from a degassing melt. Our results suggest that a range in delta Cl-37 from 0 to >20 parts per thousand is expected in lunar apatite, with heavy enrichment being the norm. While 95% loss in the initial Cl content of the melt (280 ppm Cl left in the melt) would cause an increase to +20 parts per thousand in delta Cl-37, the ability to measure this increase in a lunar sample is ultimately dependent upon the starting Cl abundances and whether or not a mechanism exists to concentrate the remaining CI such that it can be subsequently analyzed with sufficient accuracy. Therefore, the higher the starting Cl abundances in the initial melts, the heavier delta Cl-37 values that can be measurably preserved. Importantly, such enrichments can occur in spite of high initial hydrogen contents, and therefore, our experiments demonstrate that elevated values of delta Cl-37 cannot be used as supporting evidence for an anhydrous Moon. Furthermore, if the H-bearing vapor has a significant H-2 component, this process should also result in strong enrichment of delta D in the residual magmas that reach the lunar surface or near-surface environment. Apatites within some mare basalts exhibit elevated delta D of 1000 parts per thousand depending on the initial value (Tartese and Anand 2013) in addition to the delta Cl-37 values, but elevated delta Cl-37 values are accompanied by only modest enrichments in 51) in apatites from samples of the highlands crust (McCubbin et al. 2015a).

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.

Recent SIMS analysis of water, F, and Cl in lunar apatite suggests significantly higher volatile abundances in lunar magmas than previously considered. However, apatite is commonly a late-crystallizing mineral and its volatile content may reflect late-stage open-system processes that have perturbed the magmatic volatile content and obscured direct information regarding the volatile contents of the parental magmas and magmatic source region. Degassing during magma ascent has the potential to perturb not only the absolute but also the relative magmatic volatile abundances. A set of evacuated silica tube degassing experiments were conducted that simulate ascent of high-Al basalt 14053 with 0.5 wt% Cl, 0.5 wt% F, 0.3 wt% S, and 2.2 wt% (and 2.5 wt%) water (in addition to dissolved C-O-H species) from 100 km to within similar to 20 m of the surface followed by degassing (at an f(O2) of similar to QIF). Extensive degassing occurred within 6 h during which 99-100% of the initial water, 89-84% of the initial Cl, 60-61% of the initial F, and 94-92% of the initial S was lost. During degassing, the relative volatile contents showed a strong decrease in water content and an increase in F:Cl ratio. In reflection of the changes in melt volatile contents, apatites crystallizing from the degassed melt would have much lower OH contents and higher F:Cl ratio than apatites crystallized from the non-degassed melt. These results confirm the possibility of significant underestimation of primary magma volatile contents, especially water and Cl, through use of apatite volatile contents and the assumption of simple increases in volatile abundance during magma differentiation.