Earth’s past and present mantle oxygen fugacity

Abstract

Oxygen is the most abundant element in Earth’s mantle. Oxygen fugacity (fO2), which quantifies the availability of oxygen to mediate oxidation–reduction reactions, affects important mantle processes, such as depth of melting, extraction of volatiles to the atmosphere, crustal composition and ore body generation. Debate continues over modern and past variations in mantle fO2. In this Review, we compile thermobarometric data from mafic and ultramafic rocks at ridges, back-arcs, and arcs and show that the fO2 of subduction-influenced arc mantle is appreciably higher than the mantle supplying ocean ridges. We review the timing and mechanisms that might transfer redox budget to the arc mantle wedge. A new proxy for the redox-sensitive element vanadium confirms the higher oxidation state of arc mantle and can be used to show there is no conclusive evidence for oxidation of the ambient mantle since the Archaean (2,500–4,000 million years ago). Earlier, in the Hadean magma ocean (>4,000 million years ago), liquid silicate equilibrated with liquid metal alloy while the upper mantle was rapidly oxidized to higher fO2. Future research should focus on how mantle fO2 coevolved with Earth’s primitive atmosphere during core formation, magma ocean crystallization and degassing. Mantle oxygen fugacity is set by phase equilibria and is intimately linked to geochemical and geodynamic processes. This Review explores the possible mechanisms that have controlled mantle oxygen fugacity from Earth’s early beginnings to the present day.