Redox melting in the mantle

Redox melting is caused by a change in oxidation state that is not necessarily coupled to a change in temperature or pressure, but hinges on the effects of COH volatiles on depressing the melting point. Hydrous redox melting (HRM) operates when fluids containing CH₄ and H₂O react to deposit solid carbon, and the increasing activity of water (aH₂O) that results causes a severe drop in the melting temperature. In more oxidized conditions, carbonated redox melting (CRM) results from the solidus with both H₂O and CO₂ being lower than that with H₂O alone. In many cases, carbon deposited during HRM provides the carbon reservoir for later carbonate-rich magmatism in the same or adjacent areas. HRM especially causes fractionation of redox conditions, with oxidized, H₂O-rich melts leaving behind reduced, dry residues, explaining cratonic lithosphere keels. This was the main driver of the oxidation of surface conditions in the late Archean. Incipient melting was restricted in the Archean, so that initial melting was not extensive despite higher mantle temperatures. The subduction environment became gradually more oxidized between 3.3 and 2.3 Ga. The CRM mechanism was inoperable until the late Archean, and redox melting of recycled lithospheric blocks in the deep mantle had to await oxidized conditions in subduction zones.