Determination of sulfur speciation in apatite from Martian meteorites (shergottites) using -XANES

Oxygen fugacity (fO2) is a thermodynamic property that controls igneous processes and speciation of multi-valent elements like sulfur (S). S can be incorporated into the structure of the accessory mineral apatite, which is ubiquitous in a wide variety of planetary materials, including Martian meteorites. S in terrestrial apatite is mostly S6+, as it formed in relatively high fO2 conditions where S6+ is abundant in the fluids and melts from which these apatite precipitate. S2--only bearing apatite have been documented in low fO2 lunar, terrestrial, and experimental conditions; both S6+ and S2- bearing apatite are reported in experiments equilibrated at intermediate fO2. Thus, it has been suggested that the S6+/S2- of apatite, together with major element compositions, T and P, may record the fO2 at the time of formation of apatite. If true, by the virtue of recording fO2 broadly intermediate to Moon and Earth, Martian rocks may contain S2--only or mixed S6+/S2- in their apatite. Apatite in Shergotty show spectral evidence of S2- (peaks at 2470 and 2477 eV), corresponding to S6+/S=0. This is consistent with fO2 indicated by Fe-Ti oxides and DEuCpx/melt of IW+1.9-IW+2.8. At these fO2 we expect S to be present only as S2-, suggesting that apatite preserve the fO2 of igneous crystallization of Shergotty. In contrast, QUE94201 apatite have spectral evidence for both S2- (peaks at 2470, 2477 eV) and S6+ (peak at 2482 eV), varying in S6+/S from 6-91%. This is inconsistent with other mineralogical records of fO2 in this meteorite, i.e. magmatic fO2 of IW1.5-IW+0.9 calculated by DEuCpx/melt and DVCpx/melt , where S should be predominantly S2-. Our new S6+/S measurements suggest that QUE94201 apatite have been reset by some metamorphic event after igneous crystallization, either on Earth or on Mars. The D compositions of QUE94201 apatite suggest fluid interaction with the parent rock on Mars. QUE94201, an Antarctic find, contains veins of jarosite that crosscut the fusion crust, indicating subsequent alteration of the meteorite on Earths surface. Because D compositions of apatite reflect Martian values, we conclude that our oxidized S6+/S measurements of apatite in QUE94201 reflect the fO2 of alteration on the Martian surface, indicating that the alteration process(es) oxidized susceptible phases substantially.