Experimental constraints on the origin of chlorine emissions at the Soufrière Hills volcano, Montserrat

Abstract.

Experimental study of Cl solubility in a rhyolitic melt composition corresponding to the phenocryst-free matrix of the recently erupted Soufrière Hills andesite provides insights into the origin of Cl emitted to the atmosphere during the eruption. The experiments were done at 25–250 MPa, 860–890°C, and they involved equilibration of the estimated matrix composition [~71 wt.% SiO₂, molar (Na₂O+K₂O)/Al₂O₃ ~ 0.6] with a Cl-rich aqueous fluid or aqueous fluid+hydrosaline brine. The Cl concentrations in glasses from the experimental run products (0.48–0.68 wt.% Cl) are significantly higher than those observed in natural matrix glasses (~0.20 wt.% Cl) and melt inclusions (0.30±0.07 wt.% Cl). These results indicate that if the pre-eruptive Montserrat magma was vapour saturated, the vapour phase would have been Cl poor and there was not a co-existing hydrosaline brine. This interpretation, based on Cl solubility data, is supported by the observed coincidence between remote sensing measurements of Cl emissions (as HCl) and petrological estimates of Cl emissions based on the volume of magma extruded and Cl contents of melt inclusions and matrix glasses (with correction for groundmass crystallization during ascent). The Cl emissions during the Soufrière Hills eruption are thus proposed to simply reflect syn-eruptive degassing of magma during ascent.