Redox processes in subduction zones: Progress and prospect
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dox processes in subduction zones: Progress and prospect 1*
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Jintuan WANG , Xiaolin XIONG , Yixiang CHEN & Fangfang HUANG 1
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State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, CAS, Guangzhou 510640, China; 2 School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China Received December 31, 2019; revised July 8, 2020; accepted July 16, 2020; published online September 28, 2020
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 magmatichydrothermal 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 3+ 2+ magma generation and differentiation is to determine the partition coefficients of Fe and Fe between ferromagnesian minerals and silicate melts. Keywords Citation:
Subduction zones, Oxygen fugacity, Mantle wedge, Slab dehydration, Arc magma differentiation
Wang J, Xiong X, Chen Y, Huang F. 2020. Redox processes in subduction zones: Progress and prospect. Science China Earth Sciences, 63, https://doi. org/10.1007/s11430-019-9662-2
1. Introduction Subduction zones are an important site for mass transport and interaction between the Earth’s crust and mantle (Zheng and Chen, 2016). During plate subduction, slabderived fluids migrate upwards and react with the mantle wedge, resulting in a series of compositional changes and subsequent partial melting in the mantle wedge (Zheng and Zhao, 2017; Zheng et al., 2020). Partial melt
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