Uniform oxygen fugacity of shergottite mantle sources and an oxidized martian lithosphere

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 (fO(2)) of martian shergottite meteorites, which have low (similar to 7 wt.%) to high-MgO (similar to 30 wt.%) compositions, correlates with incompatible trace element enrichment (i.e., La/Yb), and Sr-87/Sr-86, Nd-143/Nd-144, Os-187/Os-188 and Hf-176/Hf-177 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 fO(2) of shergottites and the dunitic chassignites. These data, utilizing early crystallizing silicate phases, constrain the shergottite fO(2) range to between -3.72 +/- 0.07 and -0.21 +/- 0.55 Delta FMQ (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 Delta FMQ. The chassignites, along with the nakhlites, have been proposed to be sourced from metasomatized lithospheric mantle, and their high fO(2) strengthens this model. The new data implies that the martian mantle sources of shergottites have fO(2) of -2.1 +/- 1.8 Delta FMQ. 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. (C) 2021 Elsevier B.V. All rights reserved.