Progressive oxidative weathering raised sub-arc mantle fugacity on the Archaean Earth

Abstract In the modern Earth, sub-arc mantle oxygen fugacity is intrinsically associated with addition of sulfur species by slab fluids. However, it is unclear to what extent recycling of volatiles controlled redox changes of the sub-arc mantle in the early Earth. Here we use sulfur K-edge micro-X-ray absorption near-edge structure spectroscopy to measure the relative abundances of S 6+ , S 4+ and S 2− in apatite inclusions hosted by geochemically characterized and dated zircons from sub-arc sanukitoid magmas that span 3.4 billion years of Earth’s geological history. The sub-arc mantle shows marked increase in oxygen fugacity between 3.4 and 3.0 billion years ago (Ga). It is substantially more oxidized than unaltered ambient upper mantle, a characteristic that cannot be explained by fractionation, degassing or intrinsic mantle processes. We propose that progressive oxidative weathering enhanced the proportion of oxidized sulfur recycled to mantle wedge sources, establishing feedback between subduction of surface materials and incrementally oxidized arc magmatism. Early subduction involving inorganic oxidation and ‘whiffs’ of oxygen between 3.4 and 2.7 Ga may have regulated an early increase in fugacity of the sub-arc mantle before the Great Oxidation Event, matching similar conditions to the modern Earth by 2.1 Ga.