Subducting serpentinites release reduced, not oxidized, aqueous fluids

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO 2 and CO 2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity ( f O 2 ) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain f O 2 conditions...