Arc magmas oxidised by water dissociation and hydrogen incorporation in orthopyroxene.

Elevated HO 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 HO remains unclear. In this study, we use infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene HO concentrations for the peridotite samples studied allowed the assignment of specific, commonly-observed absorption bands to redox-sensitive crystallographic defects. HO content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidised conditions during the migration of hydrous melts through the mantle wedge. A separate, but related process of reaction with H occurring primarily during the earliest stages of melt-mantle reaction may be fundamental in generating the oxidised nature of hydrous melts. Our study proposes that the oxidised nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle.