SUBDUCTION ZONE REDOX: EXTRACTING FO2 FROM THE ROCK RECORD

The oxidation state of Earth’s mantle is a fundamental chemical variable that influences the speciation of gaseous volatile outputs from volcanoes, alters the stability of minerals, and governs igneous differentiation pathways during crust production. Oxygen fugacity (fO₂) is a parameter used to quantify the degree of oxidation in an Earth system and can be determined using oxybarometers (fO₂-dependent phase equilibria) and trace and major element fO₂-proxies. The calibration of oxybarometers for mantle assemblages has proliferated in pursuit of understanding mantle fO₂, particularly as recorded by volcanic glasses, mantle xenoliths, and oceanic lithosphere. Intriguingly, the fO₂ of arc volcanoes above subduction zones is higher (more oxidized) than that of typical oceanic basalt produced at spreading ridges. The cause of this globally observed trend is contested. One hypothesis proposes that the magma source, the sub-arc mantle, is oxidized by fluids from underlying subducted oceanic lithosphere. Another suggests that the oxidized signature can be explained by shallow-level differentiation processes and that oxidized subducted slab components do not alter the bulk redox state of the mantle. A fundamental unknown persists: are subducted slabs a net oxidizing entity? This question is difficult to answer owing to the challenge of calibrating oxybarometers for complex metamorphic phase assemblages, and because exhumed metamorphic terranes vary in composition and degrees of metamorphism and retrogression, generating significant complexity. Nevertheless, fO₂ estimates for metamorphic rocks have been made: some qualitatively assessed using thermodynamic modeling and others quantified using a variety of nascent oxybarometers. We here provide an overview of the current state of fO₂ measurements on metamorphic rocks, with an emphasis on comparing methods used to quantify results and appraising associated errors. As a proof-of-concept case study, we present electron microprobe analyses on a suite of blueschist and eclogite from the metamorphic terrane of New Caledonia and calculate fO₂ using different methods, evaluating the compatibility of these results with each other and assessing the extent of fO₂ variability recorded in the same suite of rocks.