The primary fO2 of basalts examined by the Spirit rover in Gusev Crater, Mars: Evidence for multiple redox states in the martian interior

The primary oxygen fugacity (fO(2)) of basaltic melts reflects the mantle source oxidation state, dictates the crystallizing assemblage, and determines how the magma will evolve. Basalts examined by the Spirit Mars Exploration Rover in Gusev Crater range from the K-poor Adirondack class (0.02 wt% K2O) to K-rich Backstay class (up to 1.2 wt% K2O) and exhibit substantially more variation than observed in martian basaltic meteorites. The ratios of ferric to total iron (Fe3+/Fe-T) measured by the Mossbauer spectrometer are high (equivalent to -0.76 to +2.98 Delta QFM; quartz-fayalite-magnetite buffer as defined by Wones and Gilbert, 1969), reflecting secondary Fe3+ phases. By combining the Fe3+/FeT of the igneous minerals (olivine, pyroxene, and magnetite) determined by Mossbauer spectrometer, we estimate primary fO(2) for the Gusev basalts to be 3.6 to 0.5 QFM. Estimating the fO(2) as a function of the dependence of the CIPW normative fayalite/magnetite ratios on Fe-3_/Fe-T yields a slightly smaller range of -2.58 to +0.57 Delta QFM. General similarity between the fO(2) estimated for the Gusev basalts and ranges in fO(2) for the shergottitic meteorites (-3.8 to 0.2 Delta QFM; Herd, 2003; Goodrich et al., 2003) suggests that the overall range of fO(2) for the martian igneous rocks and mantle is relatively restricted. Like the shergottites (Herd, 2003), estimated fO(2) of three Gusev classes (Adirondack, Barnhill and Irvine) correlates with a proxy for LREE enrichment (K2O/TiO2). This suggests mixing between melts or fluids derived from reservoirs with contrasting fO(2) and REE characteristics. Oxygen fugacity estimates for the martian interior suggest that tectonic processes have not led to sufficient recycling of oxidized surface material into the martian interior to entirely affect the overall oxidation state of the mantle. (C) 2013 Published by Elsevier B.V.