The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon

Abstract

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 δ66Zn, δ34S, and δ37ClSBC+WSC values of 1.27 ± 0.71, 0.55 ± 0.18, and 4.1 ± 4.0‰, respectively, compared to the silicate Earth at 0.15, –1.28, and 0‰, 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‰ in δ66Zn, 60‰ in δ34S, and 30‰ in δ37Cl). 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‰, explained by extensive degassing of picritic magmas under high-P/PSat 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 δ34S and δ37Cl 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/PSat ~ 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.

Anomalous 33S in the Lunar Mantle

“…What remains to be reconciled is the apparent homogeneity of other genetic tracers like oxygen isotopes (Hallis et al., 2010; Spicuzza et al., 2007; Wiechert et al., 2001; Young et al., 2016). Although oxygen is chemically similar to sulfur and some heterogeneity in Δ 17 O values have been observed in lunar materials (Cano et al., 2020), similar isotope behavior is not shared between the oxygen and sulfur isotope compositions of lunar basalts (e.g., Gargano et al., 2022; Wing & Farquhar, 2015). From our S isotope data, it is possible that the isotopic composition of lunar glasses are windows into previously unrecognized heterogeneity associated with the building blocks of the Earth and Moon that should be assessed using other genetic tracers.…”

“…Figure 1. (a) Δ 33 S versus δ 34 S and (b) Δ 36 S versus Δ 33 S for lunar glasses (this study) and basalts(Gargano et al, 2022;Wing & Farquhar, 2015).…”

“…Incorporation of an isotopically fractionated vapor component that was produced either during the early solar nebula (Antonelli et al., 2014) or from the moon‐forming impact (either through photochemistry of gas released during the impact or the introduction of anomalous S from the moon forming impactor), into a poorly mixed lunar mantle, could generate our observations in the orange glasses. Although there is isotopic similarity in sulfur isotopes among the bulk silicate Earth (Labidi et al., 2013) and lunar basalts (Gargano et al., 2022; Wing & Farquhar, 2015), these estimates may not represent primitive unaltered signatures from accretion of sulfur during Earth and Moon formation as the Earth and Moon's primitive mantle have been altered. On Earth, processes, such as plate tectonics, differentiation, and late accretion have altered the original sulfur isotopic composition (Dottin, Labidi, Jackson, et al., 2020; Dottin, Labidi, Lekic, et al., 2020; Dottin et al., 2021; Labidi & Cartigny, 2016; Labidi et al., 2013, 2022).…”

“…Carbonaceous chondrites also often preserve negative Δ 33 S values (e.g., LEW 85312 bulk Δ 33 S of −0.058‰). Given the high S concentration of these materials, delivery of carbonaceous (Gargano et al, 2022;Wing & Farquhar, 2015).…”

“…To date, quadruple S‐isotope measurements on materials from the peak of volcanism on the moon only exist on lunar basalts and show no clear evidence of MIF‐S. The sulfur isotope composition of lunar basalts is δ 34 S = 0.62‰ ± 0.20‰, Δ 33 S = 0.002‰ ± 0.010‰, and Δ 36 S = 0.030‰ ± 0.13‰ (Gargano et al., 2022; Kaplan & Petrowski, 1971; Kaplan et al., 1970; Rees & Thode, 1972, 1974; Thode & Rees, 1971, 1972; Wing & Farquhar, 2015). These data have been used to argue that the lunar (upper) mantle is well‐mixed.…”