COEVOLUTION OF IRON AND WATER IN THE EARTH'S MANTLE

The disproportionation reaction links three valence states of iron in the mantle—ferrous (Fe²⁺), ferric (Fe³⁺), and metallic (Fe⁰)—and has been observed experimentally in silicate minerals including pyroxene, garnet, and bridgmanite. The abundance of such metallic iron disproportionated from mantle silicates is influenced by oxygen fugacity and controls the fate of redox-sensitive light elements recycled into the mantle via subduction. Here, we quantify the iron disproportionation reaction occurring in the cooling mantle through thermodynamic modeling and present a new perspective on the reaction and coevolution of disproportionated iron and subducted water in the Earth's interior, affecting both the oxidation state of the mantle and the size of the surface oceans. Our results suggest that up to 3.1×10²¹ kg of disproportionated metallic iron in the mantle has reacted with subducted water, which could explain the redox evolution observed in mantle rocks since the Archean. This interaction implies that the volume of Earth's early oceans may have been up to 80% larger than today. In addition, we find that the Earth's middle mantle, between 500 and 1000 km depth, has likely remained metal-saturated with 0.1–0.3% native iron since its formation. This metal-saturated layer acts as a water filter, keeping the lower mantle dry while hydrating the upper mantle.