Revisiting the oxygen fugacity of martian meteorites: implications for the redox history of the mantle of marsOPEN ACCESS
Christopher D.K. Herd, Sophie Benaroya
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 3 October 2025
“We provide an updated compilation of oxygen fugacity (fO2) estimates for martian meteorites, with a specific focus on the shergottites. The compilation includes estimates from over 70 distinct lithologies from the martian meteorite suite, calculated from olivine-pyroxene-spinel and Fe-Ti oxide oxybarometers. Olivine-pyroxene-spinel oxybarometry was recalculated from original data sources using an updated model. Results from V- in-olivine and Eu/Gd oxybarometry from the literature are provided for comparison. Oxygen fugacity data are plotted against chondrite-normalized La/Yb ratio to critically examine the correlation between fO2 and incompatible trace element (ITE) enrichment previously postulated. We find that the correlation holds, when factors including differences in petrogenetic histories, distinctions between shergottite petrologic types, and early vs. late crystallizing assemblages, are taken into consideration. We model the degassing of H, C and S species from primitive martian magmas using the MAGEC model (Sun and Lee, 2022) and successfully reproduce the 2–3 log unit increase recorded within olivine-phyric shergottites between early and late crystallizing assemblages. We find that volatile degassing can account for most of the fO2 increase in the olivine-phyric shergottites, without requiring extensive auto-oxidation, as long as their fO2 remains at or below a value equivalent to the fayalite-magnetite-quartz (FMQ) equilibrium throughout their crystallization. With these considerations in mind, we propose a martian mantle redox-ITE trend defined by shergottite sources: a depleted source (La/Yb ∼ 0.1) with fO2 = FMQ-4 ± 0.7, an intermediate source (La/Yb ∼ 0.5) at fO2 = FMQ-3 ± 0.75 and an enriched source (Lab/Yb ∼ 1) at fO2 = FMQ-2 ± 0.75. The depleted/reduced source is likely graphite saturated.
Comparisons with compilations of fO2 from basaltic eruptives on Earth highlight fundamental differences between the two planets ultimately attributable to differences in degree of mantle convective mixing throughout their histories: terrestrial mantle sources produce basaltic eruptives with a relatively limited range of fO2, within ±1 log unit of FMQ; any degassing from these magmas results in reduction, not oxidation. The mantle sources of the shergottites – while represented by a similarly limited range of fO2, ∼FMQ-4 to FMQ-2 – produce basaltic eruptives with a range of low initial (magmatic) fO2; the more reduced nature of these magmas make them more susceptible to overprinting by degassing of H-C-S species during eruption and emplacement. Whether the mantle sources inferred from the shergottites apply to other martian meteorites (or other martian igneous rocks) remains to be tested; however, post-magma ocean crystallization processes would have acted to oxidize and overprint initial mantle sources defined by the shergottite fO2-ITE trend.”
