Maryjo Brounce: The oxidation state of sulfur in Apollo-era lunar rocks and curation best practices for future sample return missions

Lunar apatites contain hundreds to thousands of parts per million of sulfur. This is puzzling because lunar basalts are thought to form in low oxygen fugacity (fO2) conditions where sulfur can only exist in its reduced form (S2–), a substitution not previously observed in natural apatite. We present measurements of the oxidation state of S in lunar apatites and associated mesostasis glass that show that lunar apatites and glass contain dominantly S2–, whereas natural apatites from Earth are only known to contain S6+. It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S6+/S2– ratios of such apatites could be used to quantify the fO2 values at which they crystallized, given information on the portioning of S6+ and S2– between apatite and melt and on the S6+/S2– ratios of melts as functions of fO2 and melt composition. Such a well-calibrated oxybarometer based on this the oxidation state of S in apatite would have wide application.

References:

1. Brounce, M., Boyce, J., McCubbin, F.M., Humphreys, J*., Reppart, J., Stolper, E., and Eiler, J. (2019) The oxidation state of sulfur in lunar apatite. American Mineralogist, 104, 307-312. doi: https://doi.org/10.2138/am-2019-680