Magnetite is commonly found at sites on Mars explored by robotic spacecraft, yet is rare in martian meteorites and in experimental studies of martian magma compositions. Iron redox systematics of the high-FeO shergottitic liquids are poorly known, yet have a fundamental control on stability of phases such as magnetite, ilmenite, and pyroxenes. We undertook experiments to constrain the Fe3+/∑Fe in high-FeO (15–22 wt%) glasses as a function of fO2, melt P2O5, temperature and pressure. We also performed a series of sub-liquidus experiments between 1100 and 1000 °C and FMQ+0.5 to FMQ−1 to define magnetite stability. Run products were analyzed for Fe3+ and Fe2+ by Mössbauer spectroscopy and micro-X-ray absorption near edge structure (micro-XANES) spectroscopy. One bar liquids equilibrated at FMQ−3 to FMQ+3 show a much lower Fe3+/∑Fe than terrestrial basalts at the same conditions. As melt P2O5 contents increase from 0 to 3 wt% (at fixed pressure, temperature, and fO2), Fe3+/∑Fe decreases from 0.07 to 0.05, but this is within error on the measurements. Temperature increases between 1200 and 1500 °C cause little to no variation in Fe3+/∑Fe. Pressure increases from 1 to 4 GPa cause a 0.06 decrease in Fe3+/∑Fe. The trends with pressure and temperature are in agreement with results of previous studies. Combining our new series of data allows derivation of an expression to calculate Fe3+/Fe2+ for high-FeO melts such as martian magmas.
This expression can be used to show that decompressed melts become slightly more oxidized at the surface (compared to 4 GPa). Magnetite stability is suppressed by the lower Fe3+/Fe2+ of the high-FeO melts. Magnetite stability is a function of Fe2O3 and temperature and is stable ~50 °C lower than typical terrestrial basalt. Difficulty in producing magnetite as a liquidus phase in magmatic systems suggests either that many martian basalts are more oxidized than FMQ (but not represented among meteorite collections), that the titano-magnetite only forms upon cooling below ~1000 °C at FMQ, or that the magnetite has a secondary origin.