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

•New Mossbauer measurements of Fe3 +/ΣFe in Archaean chromite•Samples from layered intrusions in Zimbabwe (2.7Ga), Greenland (3.0, 3.3 and 3.8 Ga)•Measurements identical to 95 Ma Oman ophiolite and 2.1 Ga Bushveld intrusion•No change in mantle redox since 3.8 Ga•No evidence for mantle influence on GOE at 2.4 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 Fe3 +/(Fe3 + + Fe2 +) in the mineral chromite hosted in mantle-derived melts to show that there is no change in mantle Fe3 +/(Fe3 + + Fe2 +) 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.