Effects of subsolidus cooling on the redox state of the oceanic upper mantle

Oxygen fugacity (fO2) of the oceanic upper mantle is commonly determined by the olivine-orthopyroxene-spinel 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 are interpreted as residues of oceanic upper mantle after partial melting. The Mg-Fe closure temperatures between olivine and spinel are 875 ± 13 °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 olivine-orthopyroxene-spinel oxybarometer yields ∆fO2 0.4 ± 0.4 log units relative to the fayalite-magnetite-quartz (FMQ) buffer (∆FMQ 0.4 ± 0.4). This ∆fO2 range is similar to that of abyssal peridotites (∆FMQ 0.3 ± 0.7) calculated by the same oxybarometer, but 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 by different methods. Our study confirms that subsolidus cooling considerably affects the redox state of the oceanic upper mantle, and that ∆fO2 values calculated by the olivine-orthopyroxene-spinel oxybarometer represent the upper limit of ∆fO2 under upper mantle conditions.