Effects of Subsolidus Cooling on the Redox State of the Oceanic Upper Mantle

Abstract Oxygen fugacity (fO2) of the oceanic upper mantle is commonly determined using the olivine–orthopyroxene–spinel (ol–opx–sp) oxybarometer based on Fe2+/Fe3+ equilibrium among these minerals. However, rapid Fe redistribution during subsolidus cooling questions how subsolidus cooling influences ∆fO2 values recorded by this oxybarometer. Harzburgites from the Luobusa ophiolite (Tibet) exhibit mineral and whole-rock compositions consistent with those of abyssal peridotites, and these are interpreted as residues of the oceanic upper mantle after partial melting. The Mg–Fe closure temperatures between olivine and spinel are 875 ± 26 °C, significantly lower than typical melting temperatures. This temperature difference results from rapid diffusion of Mg and Fe during subsolidus cooling. Based on such Mg–Fe closure temperatures, the ol–opx–sp oxybarometer yields ∆fO2 0.4 ± 0.9 log units relative to the fayalite–magnetite–quartz (FMQ) buffer (∆FMQ 0.4 ± 0.9). This ∆fO2 range is similar to that of abyssal peridotites (∆FMQ 0.3 ± 1.5) calculated using the same oxybarometer, but it is higher than that (∆FMQ −2 to −1) estimated from whole-rock V/Sc ratios. The latter were unaffected by element exchange between minerals during subsolidus cooling. Thermodynamic modeling predicts that ∆fO2 is elevated by ~1 log unit from solidus (1300–1400 °C) to Mg–Fe closure temperatures (800–900 °C), reasonably explaining the discrepancy in ∆fO2 values calculated using different methods. Our study suggests that ∆fO2 values calculated using the ol–opx–sp oxybarometer possibly represent the upper limit of the upper mantle ∆fO2. This needs to be considered in determining the true redox state of the oceanic upper mantle.