Redox variables and mechanisms in subduction magmatism and volcanism

The redox state of magma produced at arcs affects the nature of continental crust, likelihood of ore deposit formation, geochemical cycling of key elements for life, and environmental impacts via magma degassing. Arc magmas are more oxidized than magmas produced at mid-ocean ridges. Proposed explanations include a source region oxidized by inputs from the subducting slab, effects of slab-derived components such as water, and/or processes that occur subsequent to magma production, such as differentiation and degassing. To distinguish between these possibilities, the capacity of the slab to oxidize or modify sub-arc mantle and the potential of differentiation and degassing to produce the observed oxidation are reviewed. Uncertainties remain, particularly relating to the application of experimental results to natural systems, but there is a growing consensus that the effects of differentiation and degassing cannot produce the observed degree of oxidation. It is harder to determine the relative contribution of the different slab-derived components to the production of oxidized arc magmas, but slab-derived sulfate produced by oxidation of sulfide-hosted sulfur by reduction of ferric iron remains a strong contender, as does the effect of water on melting relationships. Further work is necessary to test and refine these relationships.