On the oxidation state of arc magmas

Arc magmas have long been considered significantly more oxidised than their ocean island and mid-ocean ridge counterparts, a characteristic widely attributed to infusion of the mantle wedge by fluids from subducted lithologies. However, here we show that at comparable degree of differentiation and sulfur content, arc magmas have comparable oxidation state to ocean island magmas. Our study is based on measurements of Fe3+/∑Fe along with major and volatile elements in olivine and plagioclase hosted melt inclusions and matrix glasses from eleven volcanic systems located in arc settings worldwide. Accounting for fractional crystallisation (to MgO = 6 wt. %) we find that all systems lie on a reducing trend accompanying sulfur degassing, from QFM +0.9 (±0.2, 1σ) when S > 2000 ppm to QFM −0.2 (±0.6, 1σ) when S < 100 ppm (where QFM stands for the Quartz-Fayalite-Magnetite buffer). These findings reconcile the observed discrepancy between the oxidation states of xenoliths in arc magmas and gas emissions from arc volcanoes. We further show that fractional crystallisation influences the redox evolution of arc magmas to a comparable extent as, and sometimes counteracting, sulfur degassing.