Seminar Patrick Beaudry

Super-critical aqueous fluids released by subducting slabs drive chemical mass transfer and metasomatism of the subarc mantle, influencing the geochemistry, oxidation state, and volatile inventory of arc magmas. Sulfur is thought to be an important component of slab fluids, contributing to the oxidation of the arc mantle source, and to the high sulfur contents and enriched S isotopic signatures (i.e. high δ34S) of arc magmas. Thermodynamic modeling using the Deep Earth Water (DEW) model suggests that oxidized and high δ34S sulfur preferentially partitions into slab fluids and can oxidize mantle mineral assemblages to levels similar to those recorded by subarc mantle xenoliths. In comparison, CO2-rich fluids only result in marginal oxidation, owing to the relative stability of oxidized carbon species in the subarc mantle. Modeling predictions of sulfur solubilities in slab fluids compare well with estimates of sulfur fluxes into and out of subduction zones, and are consistent with primitive melt sulfur contents based on melt inclusions and solubility experiments. Because sulfur in the subducted oceanic crust is dominantly reduced (i.e. as pyrite and pyrrhotite), the release of oxidized sulfur-bearing fluids and subsequent oxidation of the sub-arc mantle is dependent on the fraction of oxidized iron (Fe3+) in the subducting crust, which likely varied over geological time.