Subduction zone basalts are more oxidized than basalts from other tectonic settings (e.g., higher Fe3 /â^'Fe), and this contrast may play a central role in the unique geochemical processes that generate arc and continental crust. The processes generating oxidized arc magmas, however, are poorly constrained, although they appear inherently linked to subduction. Near-surface differentiation processes unique to arc settings might drive oxidation of magmas that originate in equilibrium with a relatively reduced mantle source. Alternatively, arc magmas could record the oxidation conditions of a relatively oxidized mantle source. Here, we present new measurements of olivine-hosted melt inclusions from a single eruption of Agrigan volcano, Marianas, in order to test the influence of differentiation processes vs. source conditions on the Fe3 /â^'Fe ratio, a proxy for system oxygen fugacity (fO2). We determined Fe3 /â^'Fe ratios in glass inclusions using μ-XANES and couple these data with major elements, dissolved volatiles, and trace elements. After correcting for post-entrapment crystallization, Fe3 /â^'Fe ratios in the Agrigan melt inclusions (0.219 to 0.282), and their modeled fO2s (Î"QFM 1.0 to 1.8), are uniformly more oxidized than MORB, and preserve a portion of the evolution of this magma from 5.7 to 3.2 wt.% MgO. Fractionation of olivine ± clinopyroxene ± plagioclase should increase Fe3 /â^'Fe as MgO decreases in the melt, but the data show Fe3 /â^'Fe ratios decreasing as MgO decreases below 5 wt.% MgO. The major element trajectories, taken in combination with this strong reduction trend, are inconsistent with crystallization of common ferromagnesian phases found in the bulk Agrigan sample, including magnetite. Rather, decreasing Fe3 /â^'Fe ratios correlate with decreasing S concentrations, suggesting that electronic exchanges associated with SO2 degassing may dominate Fe3 /â^'Fe variations in the melt during differentiation. In the case of this magma, the dominant effect of differentiation on magmatic fO2 is reduction rather than oxidation. Tracing back Agrigan melts with MgO > 5 wt.% (i.e., minimally degassed for S) along a modeled olivine fractionation trend to primary melts in equilibrium with Fo90 olivine reveals melts in equilibrium with the mantle beneath Agrigan at fO2s of Î"QFM 1 to 1.6, significantly more oxidized than current constraints for the mantle beneath mid-ocean ridges.
