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 SO2 and CO2 that metasomatize and oxidize the subarc
mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen
fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction.
Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM)
buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain
minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids
carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are
the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as
for redox processes controlling subduction zone geodynamics.