Redox evolutions of planetary mantle reservoirs constrained by titanium isotopes

Abstract The oxygen fugacity (hereafter referred as f O 2 ) of terrestrial planets is key in defining the outcome of planetary-scale differentiation and the planets’ potential habitability. Reconstructing the initial f O 2 records in the mantle right after core formation for terrestrial planets remain challenging due to frequent secondary modifications. Here, we show based on studies of ureilites and diogenites that measurable Ti isotope fractionation occurred during melt extraction from planetary mantle in the presence of Ti 3+ , and demonstrate that Ti isotopes can serve as a f O 2 tracer of planetary mantle reservoirs. We also show a positive correlation between the 49 Ti/ 47 Ti and La/Yb ratios for shergottites, which, when integrated with chronological constraints, imply the occurrence of Ti isotope fractionations arising from the presence of Ti 3+ during early mantle differentiation of Mars. Further constraints define reducing conditions of ~ΔIW–0.8 to ~ΔIW–1.6 for early-formed martian mantle reservoirs at ~4.5 Ga. This f O 2 estimate coincides with the lowest recommended values from martian meteorites, that of core-mantle differentiation for Mars and of diogenites, but is more oxidizing than that of ureilites. Mantle outgassing under these conditions would result in a reducing primordial atmosphere that is in stark contrast with Mars’ current CO 2 -dominated atmosphere.