The oxidation state of sulfur in apatite of Martian meteorite Shergotty

Proteek Chowdhury, Maryjo Brounce, Jeremy W. Boyce, Francis M. McCubbin

JGR: Planets
First Published: 31 May 2023


“Apatite can incorporate sulfur in its reduced form (S2-) when apatite equilibrates with a silicate melt at reducing conditions. Incorporation of sulfate (S6+) has been observed in terrestrial apatite under oxidizing conditions. Thus, it has been suggested that the proportions of S6+/S2- in apatite may record the oxygen fugacity (fO2) during the formation and/or equilibration of apatite grains with a silicate melt in a wide variety of igneous and metamorphic rocks, including from Earth, Mars, the Moon, and in materials from the asteroid belt. Martian rocks, which record fO2 values intermediate between those recorded by rocks from the Moon and Earth, may have apatite that contains only S2- or mixtures of S6+ and S2-. Here, we present new measurements of the oxidation state of sulfur in apatite grains in the basaltic shergottite, Shergotty, which exhibits spectral features consistent with the presence of sulfide (S2-) structurally bound in apatite, and no evidence for the presence of sulfite (S4+) or sulfate (S6+). The presence of sulfide-only apatite in Shergotty is consistent with other mineralogical records of fO2 in this meteorite, which are calculated from other late-stage crystallizing phases like Fe-Ti oxides as well as from early crystallizing phases like clinopyroxene (DEuCpx/melt) of ΔIW+1.9 to ΔIW+3.5. At these fO2 values, S is present in silicate melts as only S2-, and this suggests that the oxidation state of sulfur records and preserves the fO2 during the igneous crystallization of apatite reinforcing the idea that sulfur in apatite can be used as an igneous oxybarometer.”