Sulfide breakdown controls metal signature in volcanic gas at Kawah Ijen volcano, Indonesia

• Deep-trapped melt inclusions at Ijen preserve evidence for sulfide liquid
• This sulfide liquid is a dominant host for metals.
• The sulfide liquid breaks down during ascent and releases these metals.
• Present-day fumarolic emissions have the same metal signature as this sulfide.

Immiscible sulfide liquid is thought to be an important intermediary in volcanic degassing of sulfur and chalcophile elements by concentrating and transferring metals from magma to hydrous fluid. Here, we track the interaction of sulfide liquid with a fluid exsolving from basalt at Kawah Ijen volcano in Indonesia. As in many other volcanic systems, neither sulfide nor fluid is preserved. Instead, the reaction is recorded in changes in metal and sulfur concentrations in the melt during magma ascent, and shows a two-stage evolution; deep-seated progressive breakdown of sulfide during which metal concentrations in the melt are largely controlled by the sulfide, followed by fluid–melt partitioning controlling metal concentrations at shallow depth once this sulfide has been exhausted. Present-day fumarole gases have similar Zn/Cu, Pb/Cu and Mo/Cu ratios to the reconstructed sulfide, but are enriched in Tl, As and Sb. These enrichments are also observed in melt inclusions in the most recent dacitic deposits at Kawah Ijen. This suggests that the fumarole gases are sourced from a deep, degassing sulfide-saturated basalt, preserved in Zn/Cu, Pb/Cu and Mo/Cu ratios, which subsequently interact with a shallow dacitic melt that enhances Tl, As and Sb emissions.