Chromium isotope constraints on deep mantle oxidation from the Early Archean to Paleoproterozoic.

The evolution of Earth's mantle redox state is central to understanding the rise of atmospheric oxygen and the development of a habitable planet. However, whether the deep mantle has become increasingly oxidized over geological time and its underlying mechanisms remain debated. Here, we analyze chromium isotopes in 66 ancient volcanic rocks (komatiites and picrites) spanning from 3.48 to 1.87 billion years ago combined with an oxygen fugacity (fO2) estimate (Fe3+/ΣFe) derived from thermodynamic simulations. We find that older Archean samples possess distinct isotopic signatures compared to younger Paleoproterozoic ones. Our modeling indicates that Archean magmas formed under more reducing conditions, driven by less recycled surface material in the deep Earth. These results provide evidence for an oxidation of the mantle from Archean to Paleoproterozoic. Our results link deep geodynamic transitions to the surface Great Oxidation Event through the progressive evolution of the mantle redox state.