Redox Systematics of Martian Magmas with Implications for Magnetite Stability

Introduction: 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 [1]. Experiments: We undertook experiments to constrain the Fe 3+ /Fe(tot) in high FeO glasses as a function of fO2, melt P2O5, temperature and pressure. We also performed a series of sub-liquidus experiment between 1100 and 1000 °C and FMQ+0.5 to FMQ-1 to define magnetite stability. Experiments were performed at JSC in either the 1 bar gas mixing lab, or in a piston cylinder or multi-anvil presses in the high pressure experimental petrology lab. Analyses: Run products were analyzed for Fe 3+ and Fe 2+ by either Mössbauer spectroscopy [2] or microXANES (x-ray absorption near edge structure) spectroscopy [3]. Results: One bar glasses equilibrated at FMQ-3 to FMQ+3 show a much lower Fe 3+ /Fe(tot) than terrestrial basalts at the same conditions (Figure 1). As melt P2O5 contents increase from 0 to 3 wt% (at fixed pressure, temperature and fO2), Fe 3+ /Fe(tot) decreases from 0.07 to 0.05. Temperature increases between 1200 and 1500 °C cause little to no variation in Fe 3+ /Fe(tot). Pressure increases from 1 to 4 GPa cause a slight decrease in Fe 3+ /Fe(tot). The trends with pressure and temperature are in agreement with results of previous studies on terrestrial compositions [4-6]. Implications: Combining our new series of data allows derivation of an expression to calculate Fe 3+ /Fe(tot) for high FeO melts such as martian magmas.