Martian meteorites are the only available samples that can be directly measured to constrain the geological evolution of Mars. It has been suggested that the oxygen fugacity (fO2) of martian shergottite meteorites, which have low (∼7 wt.%) to high-MgO (∼30 wt.%) compositions, correlates with incompatible trace element enrichment (i.e., La/Yb), and 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and 176Hf/177Hf at the time of crystallization. These relationships have been interpreted to result from early magmatic processes segregating enriched and more oxidized from depleted and more reduced reservoirs in Mars. Here we use the V-in-olivine oxybarometer to constrain the fO2 of shergottites and the dunitic chassignites. These data, utilizing early crystallizing silicate phases, constrain the shergottite fO2 range to between −3.72 ± 0.07 and −0.21 ± 0.55 ΔFMQ (log units relative to the fayalite-magnetite-quartz buffer), with no correlation with trace element enrichment or Nd isotope systematics. Previously employed oxybarometers that use later-formed or multiple mineral phases, and that show such correlations, likely differ from the V-in-olivine oxybarometer in that they record effects from late-stage magmatic processes. In contrast to shergottites, chassignites are relatively oxidized, at +2.1 ± 0.4 to +2.2 ± 0.5 ΔFMQ. The chassignites, along with the nakhlites, have been proposed to be sourced from metasomatized lithospheric mantle, and their high fO2 strengthens this model. The new data implies that the martian mantle sources of shergottites have fO2 of −2.1 ± 1.8 ΔFMQ. This estimate indicates that the mantle and core of Mars are not in redox equilibrium and therefore that oxidation of the martian mantle following core formation is required.
