Implications for the primitive atmosphere of the oxidation state of Earth's upper mantle

Abstract

Knowledge of the oxidation state of the Earth's mantle is critical for understanding the processes of magma genesis and the composition of deep-seated volatiles. Measurements reported here of the intrinsic oxygen fugacities ($f_{{\text{O}}_{\text{2}}}$s) of mantle-derived spinels from peridotite and megacryst assemblages show that the Earth's upper mantle is close to the synthetic iron-wustite (IW) buffer curve in $f_{{\text{O}}_{\text{2}}}$ versus T space. It seems likely that most erupted volcanics are oxidized after their formation, perhaps by diffusive H₂ loss¹. There is a strong possibility that the mantle was in equilibrium with a core-forming metal phase, and that subsequent oxidation of the upper mantle resulted from an interaction between the present oxidizing atmosphere–hydrosphere and the mantle by subduction processes. Evidence in support of this mechanism has been supplied by rare-gas analyses of deep-seated nodules from kimberlites².