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 (fO2) 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+/FeT) measured by the Mössbauer spectrometer are high (equivalent to −0.76 to + 2.98 Δ 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 Mössbauer spectrometer, we estimate primary fO2 for the Gusev basalts to be −3.6 to 0.5 Δ QFM . Estimating the fO2 as a function of the dependence of the CIPW normative fayalite/magnetite ratios on Fe3+/FeT yields a slightly smaller range of −2.58 to + 0.57 Δ QFM. General similarity between the fO2 estimated for the Gusev basalts and ranges in fO2 for the shergottitic meteorites (−3.8 to 0.2 Δ QFM ; Herd, 2003; Goodrich et al., 2003) suggests that the overall range of fO2 for the martian igneous rocks and mantle is relatively restricted. Like the shergottites (Herd, 2003), estimated fO2 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 fO2 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.