Tracking the fate of sulfur in Earth's crust using sulfur valence in magmatic apatite

Degassing of sulfur from magmas causes explosive volcanism, the genesis of economically important copper deposits and atmospheric cycling of sulfur dioxide. Sulfur dissolved in ascending magmas transitions from S2- at the base of the crust to S6+ at the surface, however pinpointing the location and nature of this transition is challenging due to the lack of preservation of melt S valence state. This proposal aims to measure the valence state of S in apatite (which contains S2-, S4+ and S6+) in magmatic rocks from Famatina, Argentina by X-ray Absorption Spectroscopy (XAS) at the S K-edge to track the evolution of S in magmas with depth. Peak positions and areas will be fitted to quantify integrated S6+/Stotal and infer melt S redox evolution. Identifying a shift from S2- to S6+ in the deep crust, may provide a mechanism by which metal-rich sulphides are recycled into ascending magmas, explaining the paradoxical occurrence of sulfur- and metal-rich magmas at Earth’s surface.