The redox state of the martian interior: insights from experimentally calibrated V/Sc oxybarometry

Constraining the oxygen fugacity ( f O 2 ) of the mantle of Mars is critical for understanding planetary differentiation processes and magmatic evolution. The degree to which the shergottite martian meteorites faithfully record the redox states of their mantle sources remains obscured by several factors. One of these factors is the various methods used to estimate f O 2 : Fe-based oxybarometers require multiple minerals to be found in chemical equilibrium, which can be challenging to obtain, while previous V-based values rely on estimated parental melt compositions. Here, we present new, mineral-specific V/Sc oxybarometers calibrated for olivine and pyroxene in shergottites using experimentally determined partition coefficients. This method obviates the need for parental melt V concentrations and allows for f O 2 determination from single mineral phases, bypassing the equilibrium constraints that limit Fe-oxybarometry. We applied these calibrations to a petrologically diverse suite of geochemically depleted, intermediate, and enriched shergottites. Our results reveal that: (1) basaltic shergottites, previously estimated at f O 2 ∼FMQ-1, record a significantly lower initial/magmatic f O 2 of ∼FMQ-1.7; (2) the magmatic f O 2 of shergottites is correlated with their geochemical enrichment; and (3) all shergottites show oxidation of a magnitude of >0.5 log units with progressive crystallization. The V/Sc oxybarometers provide a robust tool for estimating the magmatic f O 2 of shergottites and tracking their redox changes throughout their petrogenetic histories.