Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event

Before the Archean to Proterozoic Transition (APT) the tectonic regime was dominated by microplates floating on a low viscosity mantle. Such a regime restricted chemical exchange between the shallow and deeper mantle reservoirs. After the APT, a more global convection regime led to deep subduction of slabs. We propose that the improved vertical mixing of the mantle favoured the release to the Earth's surface of an oxygen excess initially trapped in the deep mantle. This excess built up when the primordial lower mantle was left with a high Fe 3+ /(Fe 2+ +Fe 3+ ) ratio (#Fe 3+ ), after metallic iron segregated down into the core. Our synchrotron-based in situ experiments suggest a primordial Fe 3+ excess of ~20 % for the mantle iron. By comparison with the #Fe 3+ of the present mantle, this Fe 3+ excess would correspond to 500-1000 times the O 2 content in the Earth's atmosphere. The tectonic transition greatly facilitated the ascent of oxidised lower mantle material towards the Earth's surface, inducing a continuous arrival of O 2 at the Earth's surface and into the atmosphere.