Controls on Iron‐Redox State in Martian Magmas Quantified by Mössbauer Spectroscopy, Colorimetric Wet Chemistry, and XANES Spectroscopy

Abstract To elucidate the relationship between oxygen fugacities ( f O2 ) recorded in martian basalts and redox processes in the martian interior, superliquidus 100‐kPa furnace experiments on a composition similar to Humphrey (Adirondack basalt) were conducted at variable f O2 and temperature. Quenched glasses were analyzed by EPMA, Mössbauer spectroscopy, colorimetric wet chemistry, and microbeam X‐ray absorption near edge structure (XANES) spectroscopy. The experiments reveal Mössbauer and wet chemical determinations of silicate glass Fe 3+ /Fe T agreeing within uncertainty, supporting the accuracy of extended‐Voigt‐based fitting of Mössbauer spectra when recoil‐free fraction is considered. Fe 3+ /Fe T ratios determined from Mössbauer spectroscopy from Humphrey and previously studied martian‐relevant glass compositions are combined to calibrate models that characterize the relationship between Fe 3+ /Fe T , f O2 , temperature, and composition in martian silicate liquids. The models demonstrate, similar to previously investigated silicate liquids, that the correlation between and log f O2 in martian magmas has a slope less than the value (0.25) expected if ferric and ferrous iron oxide mixed ideally. Martian magma Fe 3+ /Fe T ratios are more temperature‐sensitive compared to non‐martian compositions, suggesting that temperature variations may contribute to comparatively large f O2 variations in martian basalt. The models are applied to demonstrate that the Fe 3+ /Fe T increases required to explain multiple‐log unit changes in f O2 in shergottite magma would not increase terrestrial magma f O2 as effectively. To aid in future investigations of martian magma redox, a XANES technique that allows for non‐destructive, microanalytical characterization of Fe 3+ /Fe T in natural martian materials and martian‐relevant experiments is introduced.