January 8th, 2026

Mantle redox, which has far-reaching consequences for our planet and its evolution, may be approached from the perspective of oxidation state (e.g., Fe3+/ΣFe ratios) or the perspective of thermodynamic equilibrium (i.e., oxygen fugacity). Over the past 15 years, improvements in analytical techniques for measuring oxidation state, as well as improvements in thermodynamic/empirical models for calculating oxygen fugacity (fO2), have highlighted the complex interplay between these two approaches. While oxidation state and oxygen fugacity are closely linked, they cannot be treated interchangeably. In particular, the dependence of oxygen fugacity on factors such as pressure, temperature, and phase stability can lead to counterintuitive effects on system behavior, with important implications for the redox-depth profile of the upper mantle. A combination of natural samples, modeling, and experiments suggests that as pressure decreases towards the surface, a garnet-bearing peridotite with constant bulk oxidation state will first increase in fO2, reach an apex around 4 GPa, and then decrease in fO2 as it transitions into the spinel stability field. This fO2 turnover has important implications for deep, hot melting, such as in the early Earth or at modern hotspots, as melting that initiates at pressures near this turnover may produce melts and residues with highly variable fO2 from an initial mantle of constant bulk oxidation state.