Upper mantle oxygen fugacity recorded by spinel lherzolites

The oxygen fugacities (fO₂s) recorded by rocks from the Earth's upper mantle have been the subject of much recent study and controversy^1-7. Discussion has been stimulated by reported differences of several orders of magnitude between measurements made by different methods (Fig. 1). Oxygen fugacity is an important parameter because, together with temperature, pressure and composition, fO₂ controls the petrogenesis of mantle-derived magmas, affects the composition and speciation of mantle fluids, and is an initial input into geochemical models of the evolution of the Earths crust, mantle and hydrosphere. Here we report the determination of the fO₂ recorded by upper mantle spinel-lherzolite xenoliths entrained in alkaline magmas. This was done by experimentally calibrating the activity of the Fe₃O₄ (magnetite) component in MgAl₂O₄-rich synthetic spinel and applying these data to calculate thermobarometric fO₂s for the three-phase assemblage livine-orthopyroxene-spinel. Results for appropriate model xenolith compositions, corrected to ISkbar total pressure ⁸ and plotted in temperature-fO₂ space, fall at or above the synthetic quartz-fayalite-magnetite buffer (Fig. 3). In contrast to earlier studies^2,3, we conclude that the shallow upper mantle does not retain an fO₂ signature of equilibrium with the metallic core, and that gaseous species in the C-H-O system will be dominated by CO₂ and H₂O (refs 9, 10), rather than CH₄ and H₂.