The oxidation of arc magmas: A critical review

Arc magmas are notably more oxidized than those from mid-ocean ridges, back arcs, and mantle plumes, yet the primary controls on this oxidation— source characteristics or magmatic fractionation processes—remain debated. This review synthesizes geochemical data from 56,450 volcanic rocks across 29 Cenozoic arcs and 1242 zircon analyses from 6 Mesozoic-Cenozoic arcs worldwide to clarify the roles of source and fractionation processes in shaping arc magma oxidation states, with a focus on distinctions between continental and oceanic arc settings. We assess oxygen fugacity using Fe³⁺/ΣFe ratios of volcanic rocks at varying MgO contents to evaluate the fractionation effects, while Ti, Ce, and initial U contents in zircons provide insights into oxygen fugacity trends in evolved magmas. High-MgO volcanic rocks (MgO > 8 wt%) reflect the redox state of arc magma sources, whereas low-MgO rocks (MgO < 8 wt%) and zircons reveal fractionation influences. Thermodynamic modeling with MELTS and Monte Carlo mass balance simulations of cumulates and residual melts further constrain the impact of crustal thickness, partial melting, and mineral fractionation on magma oxidation. Our findings indicate that primitive magmas (MgO > 8 wt%) in continental arcs exhibit slightly more oxidized (average Fe³⁺/ΣFe = 0.33 ± 0.01, 1σ) than those in oceanic arcs (averaging 0.30 ± 0.01). Oxidation increases with differentiation, reaching Fe³⁺/ΣFe values of 0.55 ± 0.01 in continental arcs and 0.49 ± 0.01 in oceanic arcs at low MgO contents (< 1 wt%). Zircon data confirm rising oxygen fugacity with decreasing temperature, with a steeper trend in continental arcs. Higher slab-derived fluxes, lower degrees of partial melting, and fractionation of minerals such as garnet and amphibole, driven by greater crustal thickness, enhance oxidation in continental arcs. This review emphasizes the critical role of magma differentiation on oxidation of arc magmas and provides a comprehensive framework for understanding arc magma oxidation, highlighting distinct processes in continental and oceanic arcs and identifying key avenues for future research.