Iron isotope fractionation and the oxygen fugacity of the mantle

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (δ57/54Fe) of mantle spinels. Spinel δ57/54Fe values correlate with relative oxygen fugacity, Fe3+/ΣFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling