Temperature dependence of magmatic iron redox speciation to 2100 °C

The iron redox speciation of silicate melts, quantified by Fe^3þ/Fe^2þ ratios, is of significance to redox evolution during magmatism and is temperature dependent, but exploration of temperature effects has been limited by the narrow range of investigable super-liquidus conditions accessible in gas mixing furnaces (∼1200–1600 °C). This study combines gas mixing and aerodynamic-laser-levitation furnace experiments to quench glasses from a single martian basaltic melt composition at fixed f_O2 and 100 kPa from 1250–2100 °C. This 850 °C span allows improved quantification of temperature dependence on Fe^3þ/Fe^2þ ratios. Fe^3þ/Fe^2þ ratios, measured by XANES, are used to calibrate thermodynamic parameterisations. Results indicate that models calibrated from smaller intervals have variable success quantifying the influ ence of temperature on magmatic Fe^3þ/Fe^2þ ratios. A survey of experimental results suggests that the temperature dependence of Fe^3þ /Fe^2þ does not vary appreciably with melt composition. The new data improve constraints on iron redox systematics during igneous differentiation of martian magmas by quantifying the effect of cooling on magma Fe^3þ /Fe^2þ and, in extrapolation, refine predictions of Fe^3þ/Fe^2þ ratios established in magma oceans at extreme temperatures, by constraining the temperature dependence of Fe^3þ/Fe^2þ at a fixed f_O2 relative to melt-alloy equilibrium.