Water-rich magmas optimise volcanic chalcophile element outgassing fluxes

Magmatic-hydrothermal fluids transport chalcophile metals to the atmosphere as volcanic gases; and to the crust, where they may play a role in the formation of ore deposits. Global volcanic gas datasets show considerable variability in the flux and composition of metals outgassed between volcanoes, but the controls on this variability are unclear. Magmatic chloride is a key ligand for metal transport but magmatic water dominates the exsolved fluid reservoir into which metals partition during crystallisation and decompression. Here we develop models simulating decompression-driven degassing (‘first boiling’) and isobaric crystallisation-driven degassing (‘second boiling’) of magmas to show that while moderate concentrations of chlorine are essential for metal partitioning into the magmatic fluids, magmatic water contents have the greatest potential to control the mass yield of metals carried by exsolving fluids. Our models explain why water-rich magmatic-volcanic systems like Mount Etna (Italy) deliver the largest mass fluxes (per unit of degassing magma) of metals to the atmosphere, whereas relatively dry magmatic-volcanic systems like Yasur (Vanuatu) deliver the smaller mass fluxes. Our results establish the important role of magmatic water in generating large reservoirs of metal-rich aqueous fluids in the shallow crust.