Kinetics of Sulfide Dissolution Controlled by Sulfur Radical Diffusion: Implications for Sulfur Transport and Triggering of Volcanic Eruptions

Chemical mixing of different types of magma, such as basaltic magma and silica-rich, hydrous magma, often triggers volcanic eruptions. However, the kinetics, mechanisms, and rates of sulfide dissolution reactions in hydrous melts are currently unknown, despite the fact that these reactions can influence the sulfur budget in the crust and mantle. I experimentally model dissolution of pyrrhotite minerals in hydrous rhyolite melt at conditions corresponding to the sulfate–sulfide transition field at 1 GPa pressure. The reaction results in the production of FeO, SO42−, H2, H2S and di- and tri-sulfur radical ions, (S2− or S3−) in fluid/melt. The calculated sulfur diffusion coefficient implies extremely fast sulfur diffusion in the hydrous hybrid melt. The production of S-rich magma is controlled by the fastest-ever-recorded chemical diffusion of sulfur in the form of S2− or S3− in hybrid magma under sulfate-sulfide transition conditions. I demonstrate that such dissolution reactions can be responsible for triggering explosive volcanic eruptions (e.g., the 1991 Mount Pinatubo eruption) in volcanic arc settings. The sulfide dissolution reaction can also promote the production of chalcophile metal (sulfur-loving Au, Cu and Pt) ore deposits associated with the formation of volcanic arcs.