LAUSR

Elevated H2O concentrations and oxygen fugacities are two fundamental properties that distinguish magmas formed in subduction zones from new crust generated at mid-ocean ridges. However, the mechanism of magma oxidation and how it relates to the increase in H2O remain unclear. In this study, we used infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during the transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene H2O concentrations for the peridotite samples studied allowed the assignment of specific, commonly observed absorption bands to redox-sensitive crystallographic defects. H2O content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidized conditions during the migration of hydrous melts through the mantle wedge. A separate but related process of reaction with H2 occurring primarily during the earliest stages of melt–mantle reaction may be fundamental in generating the oxidized nature of hydrous melts. Our study proposes that the oxidized nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle. Oxidation of arc magmas may be a secondary feature, acquired as hydrogen from magmatic water is incorporated into anhydrous minerals in the mantle wedge, according to analyses of orthopyroxenes in mantle xenoliths from an arc setting.