Sulphur and chlorine degassing at the Soufrière Hills, Volcano, Montserrat, West Indies

This work is based on gas monitoring data: COSPEC-derived sulphur dioxide emission rates and Open-path FTIR-derived HCl:SO₂ mass ratios in the volcanic plume at Soufrière Hills Volcano, Montserrat, West Indies, collected during the 1995-2000 eruption, as well as geochemical analyses of phenocryst-hosted melt inclusions and matrix glass of the erupted products. The following chapters deal with various aspects of the behaviour of the elements sulphur, chlorine and water in the pre-, syn- and post-eruption melt and the gas species sulphur dioxide and hydrogen chloride in the volcanic plume. Melt inclusion and matrix glass compositions (major, volatile and trace elements) have been used to build a picture of melt evolution with respect to sulphur and chlorine. Concentrations of chlorine and sulphur in the melt; both pre- and syn-eruption, indicate that these species are derived from different sources and from different depths. Surface emission data are used to constrain these models (chapter 3). Open-path FTIR measurements have yielded HCl emissions throughout the eruption of Soufrière Hills Volcano, with the most recent measurements being obtained during dome building in November and December 1999 and January, August, September and October 2000. These data have corroborated theories on chlorine degassing from the melt on ascent derived from analytical results and we have recognised clear dome building and "residual period" HCl emission signatures.