Archaean chromitites show constant Fe³⁺/ΣFe in Earth's asthenospheric mantle since 3.8 Ga

Theoretical and planetary studies show that the Earth's upper mantle is more oxidised than it should be. The mechanism by which this took place and the timing of the oxidation is contested. Here we present new Mössbauer spectroscopy measurements of the ionic ratio Fe ^3 +/(Fe ^3 + + Fe ^2 +) in the mineral chromite hosted in mantle-derived melts to show that there is no change in mantle Fe ^3 +/(Fe ^3 + + Fe ^2 +) ratio before and after the oxidation of the Earth's atmosphere at ca. 2.4 Ga and over Earth history from 3.8 Ga to 95 Ma. Our finding supports the view that the oxidation of the asthenospheric mantle was very early and that the oxygenation of the Earth's atmosphere was not directly coupled to mantle processes.