Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts

Abstract

We performed a series of experiments at 1 atm pressure and temperatures of 1300–1500 °C to determine the effect of oxygen fugacity on the oxidation state of Fe in a synthetic martian basalt. Ferricferrous ratios were determined on the quenched glasses using Mössbauer spectroscopy. Following the conventional doublet assignments in the spectrum, we obtain a Fe3+/ΣFe value of 0.19 at 1450 °C and an oxygen fugacity corresponding to the QFM buffer. If we apply the Berry et al. (2018) assignments the calculated Fe3+/ΣFe drops to 0.09, and the slope of log(XFeO1.5melt/XFeOmelt) vs. log(fO2) changes from 0.18 to 0.26. Combining oxidation state data together with results of one additional olivine-bearing experiment to determine the appropriate value(s) for the olivine (Ol)-liquid (liq) exchange coefficient, KD,Fe2+-Mg = (FeO/MgO)Ol/(FeO/MgO)liq (by weight), suggests a KD,Fe2+-Mg of 0.388 ± 0.006 (uncertainty is one median absolute deviation) using the traditional interpretation of Mössbauer spectroscopy and a value of 0.345 ± 0.005 following the Mössbauer spectra approach of Berry et al. (2018). We used our value of KD,Fe2+-Mg to test whether any of the olivine-bearing shergottites represent liquids. For each meteorite, we assumed a liquid composition equal to that of the bulk and then compared that liquid to the most Mg-rich olivine reported. Applying a KD,Fe2+-Mg of ~0.36 leads to the possibility that bulk Yamato 980459, NWA 5789, NWA 2990, Tissint, and EETA 79001 (lithology A) represent liquids.