Pre- and syn-eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies

International audienceSoufriere Hills fumaroles contained magmaderived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal coinposition. Chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at Galway's Soufriere, the most active fumarolic field. Hydrothermal buffering of the fumaroles has been favoured by their remote location (!-2 km) froin the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater. High temperature (720øC) gas collected froin the extruding lava dome in Feb. 1996 was chemically and isotopically representative of the magmatic gas stream. Its coinposition allows assessment of average eruptive fluxes of H20, CO 2 and HCI which require the degassing of only 2.5-3 times more magma than erupted