Unraveling planetary mantle redox histories with titanium isotopes

Variations in oxygen fugacity (fO₂) influence the geochemical and mineralogical evolution of a planet, directly impacting its atmosphere, surface conditions, and capacity to sustain life. Despite its importance, reconstructing the primordial fO₂ conditions for planets such as Earth and Mars remains a significant challenge, primarily due to the overprinting of original signals by subsequent geological processes and secondary modifications. Here we reveal that measurable titanium (Ti) isotope fractionation occurs during melt extraction from planetary mantles under reducing conditions involving trivalent Ti. This finding underscores the potential of Ti isotopes as a reliable tracer for fO₂ in planetary interiors, with the preliminary application of this novel proxy offering valuable insights into the redox conditions of Mars' mantle during its early differentiation around 4.5 billion years ago.