Williams-Jones, A.E.

The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo81) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe2+ loss by diffusion during cooling. Melt inclusions in Fo81 olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo81 olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe2+ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.

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.