Redox processes in subduction zones: Progress and prospect

Abstract

Oxygen fugacity (fO2) is an intensive variable that describes the redox state of a system. By controlling the valence state of multivalent elements, fO2 affects the stability of iron-bearing minerals, dominants the species of volatile elements (e.g., carbon and sulfur), and controls the partitioning behaviors of multivalent elements (e.g., iron, vanadium, cerium, europium). Thus, fO2 plays a key role in understanding the generation and differentiation of arc magmas, the formation of magmatic-hydrothermal deposits, and the nature of magmatic volatiles. Subduction zones are an important site for arc magmatism and fluid action, and the study of redox processes is indispensable in subduction zone geochemistry. In this paper, we first introduce the concept, expression, and estimation methods of fO2. Then we retrospect the history and progress about the oxidation state of the metasomatized mantle wedge, summarize the redox property of slab-derived fluids, and review the latest progress on redox evolution of arc magmas during magma generation and differentiation. The main conclusions include: (1) despite its wide variation range, fO2 of the mantle wedge is generally higher than that of the oceanic mantle; (2) the redox property of the subducting slab-derived fluids is still controversial and the mechanism for the oxidization of the mantle wedge remains unclear; (3) how the fO2 varies during the generation and differentiation of the arc magmas is debated. We propose that the crux in deciphering the oxidization mechanism of the mantle wedge is to determine the mobility of iron, carbon and sulfur in subducting slab-derived fluids (especially solute-rich fluid or supercritical fluid); the key in understanding the redox evolution during arc magma generation and differentiation is to determine the partition coefficients of Fe3+ and Fe2+ between ferromagnesian minerals and silicate melts.