Elevated sulfur contents in lunar apatite: A result of late-stage oxidation?

Published analyses of lunar mare basalt samples yield contradictory results regarding water and other volatile abundances on Earth's moon [e.g., 1-3]. The mineral apatite has been used as a geochemical tool to estimate the volatile contents of lunar magmas, owing to its ability to incorporate all relevant volatiles as major (F-Cl-H) or trace elements (S-C). Recently, Konecke et al. [4-5] improved our understanding of sulfur incorporation into apatite as a function of redox conditions. Their [5] experimental work shows that (a) cryptic metasomatism, (b) hydrous conditions and/or (c) significant oxidation is required to explain the elevated sulfur (S) contents (>400 µg/g) observed in some lunar apatites. In this study, we test hypothesis (c).

We analyzed the major and trace element composition of apatite from several lunar mare basalts (e.g., 12040-232, 10044-52) by using electron probe micro-analyses. Consistent with previous studies [1,3], the data show significant variations in F (0.83-4.1 wt%), Cl (0.01-1.5 wt%) and by-difference calculated OH (0-3.3 wt%) contents. Sulfur contents are often low ( 50% <100 µg/g for 50% of the analyses), but many spot analyses ( 30%) reveal elevated S concentrations (>200 µg/g) reaching >500 µg/g S. The S oxidation state in selected apatite grains was constrained by micro X-ray absorption near edge structure (µ-XANES) spectroscopy [3]. The µ-XANES data reveal that S in lunar apatites is mainly present as sulfide (S^2-); however, a significant fraction of the spectra ( 50%) reveals the presence of sulfate (S⁶⁺), where the S⁶⁺/ΣS ratios ranges from zero to almost unity. Elevated S contents (>200 µg/g) and S⁶⁺/ΣS ratios (>0) are only feasible at redox conditions far above those typically suggested for lunar magmas [5-6]. Hence, the data may implicate a late-stage oxidation event. Considering the positive correlation between S, Cl, and H in lunar apatite [1,3], such an event can have broader consequences for our understanding of the volatile contents and evolution of lunar magmas.

[1] Boyce et al. (2014) Science, 344, 400-402; [2] Mills et al. (2017) Geochem perspect lett, 3, 115-123; [3] Greenwood et al. (2018) Space Sci Rev, 212, p.92; [4] Konecke et al. (2017) Am Mineral, 102, 548-557; [5] Konecke et al. (2017) Geology, 45, 739-742; [6] Sato et al. (1973) Proc Lunar Planet Sci Conf, 4, 1-30.