Degassing at the Soufriere Hills Volcano, Montserrat, Recorded in Matrix Glass Compositions

Abstract

Compositions of matrix glasses from the current eruption of Soufrière Hills Volcano indicate that decompression-driven crystallization results in 20–70 wt % groundmass crystallization during eruption and variable degassing. Variations in crystallinity and volatile contents (water and chlorine) of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome. Residual water contents in pumice clasts (0·2–0·6 wt %) indicate minimum pressures of 1·1–3·7 MPa in 1997 Vulcanian explosions. Residual water contents of 1·6 wt % in a ballistic block ejected in sub-Plinian explosive activity on 17 September 1996 imply larger pressure drops (∼20 MPa). Variable residual water contents in dome samples are consistent with pressure variations of up to 9 MPa in the lava dome interior. Large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow-level processes dominate degassing. Low melt chlorine contents of dome samples cannot be explained by open- or closed-system degassing, especially when crystallization is taken into account. Instead, heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed. Variable and elevated matrix glass δD values can also be attributed to interaction with isotopically heavy surface waters. HCl emission during the current eruption can be accounted for by Cl loss from the melt, consistent with the melt being undersaturated in chlorine.