Helium isotope systematics of volcanic gases and thermal waters of Guadeloupe Island, Lesser Antilles

The island of Guadeloupe is located in the middle of the 850km long Lesser Antilles island arc. Present-day volcanic and geothermal activity is concentrated in two systems both located in the southwestern part of the island (Basse Terre): the La Soufrière volcanic complex and the Bouillante hydrothermal system, some 20km to the northwest of the volcano. We report here the largest isotopic data set for helium isotopes in hydrothermal gases and waters from both systems, acquired between 1980 and 2012. 3He/4He ratios in the fumarolic gases of La Soufrière volcano have been quite homogeneous and stable over the last thirty years. The average ratio of 8.2±0.2 Ra confirms that the volcano is tapping a MORB-like mantle source. In contrast, the nearby Bouillante geothermal system displays a much lower 3He/4He ratio (4.5±0.1 Ra). He-C elemental and isotopic relationships show that both systems are actually fed by the same magmatic source, and that their marked difference in 3He/4He results from...