Comparison of the fluid geochemical signatures between the hydrothermal systems of Bouillante and Soufrière (Guadeloupe, French West Indies)

Abstract : This study presents a comparison of the fluid geochemical signatures between the hydrothermal systems of Bouillante and Soufrière, carried out from numerous works existing on these two systems. If the thermal springs located around the Soufrière volcano discharge low salinity (< 2 g/l) waters with different chemical compositions and for which the chemical geothermometers indicate no deep temperature higher than 80°C, those emerging from the Bouillante area can be divided into two groups following their chemistry and salinity: - the first group is constituted of Na-HCO3 waters, with low salinities (< 1 g/l) and low emergence temperatures (< 40°C), which are warmed by thermal conduction at surface as those located around the Soufrière volcano; - the second group is represented by Na-Cl waters, with salinities higher than 1 g/l and partially constituted of deep Na-Cl fluid of the Bouillante geothermal reservoir. This fluid which can be collected from the geothermal production wells has salinity close to 20 g/l and results from a mixing of 58% of seawater and 42% of freshwater reacting with volcanic rocks at temperatures close to 260°C. The fumaroles located in the Bouillante area are associated with this geothermal fluid. According to the delta D and delta 18O values, their water steam would result from vaporization at 100°C of a mixing of this fluid with surface cold freshwater. The chemical composition of their non-condensable gases (predominance of CO2 with different air contributions) is in agreement with this assumption. The fumaroles located around the Soufrière volcano are constituted of 93 to 99% of acidic water steam and corresponding non-condensable gases. These gases have a magmatic chemical and isotopic signature much more marked than that of the gases associated with the Bouillante geothermal fluid, less abundant and acid, of mixed origin and which could rather result from a tectonic source.