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

• 3He/4He ratios confirm that La Soufrière volcano is tapping a MORB-like mantle source.
• Nearby Bouillante geothermal system displays a much lower 3He/4He ratio of 4.5 ± 0.1 Ra.
• 3He/4He ratios of both systems have been stable over the last thirty years.
• He–C relationships show that both systems are fed by the same magmatic source.
• Radiogenic 4He from the host rocks increases with distance from the volcanic edifice.

The island of Guadeloupe is located in the middle of the 850 km 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 20 km 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 the 4He contamination of the Bouillante deep aquifer by the surrounding wallrock. This conclusion is strengthened by the spatial distribution of 3He/4He ratios which shows that La Soufrière fumaroles and the Bouillante geothermal system are the two end-members of a spatial trend of decreasing 3He/4He ratio with distance from La Soufrière summit dome, implying an increasing addition of radiogenic 4He from the host rocks away from the present-day active volcanic edifice.