Redox systematics of martian magmas with implications for magnetite stability

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 Fe 3+ / ΣFe in high-FeO (15-22 wt%) glasses as a function of f O2 , melt P 2 O 5 , 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 Fe 3+ and Fe 2+ 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 Fe 3+ /ΣFe than terrestrial basalts at the same conditions. As melt P 2 O 5 contents increase from 0 to 3 wt% (at fixed pressure, temperature, and f O2 ), Fe 3+ /ΣFe 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 Fe 3+ /ΣFe. Pressure increases from 1 to 4 GPa cause a 0.06 decrease in Fe 3+ /ΣFe. 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 Fe 3+ /Fe 2+ for high-FeO melts such as martian magmas. ln(X Fe3+ /X Fe2+ ) = a lnf O2 + b/T + cP/T + dX FeO + eX Al2O3 + fX CaO + gX Na2O + hX K2O + iX P2O5 + j 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 Fe 3+ /Fe 2+ of the high- FeO melts. Magnetite stability is a function of Fe 2 O 3 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.