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RESEARCH PRODUCT

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

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Abstract 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 Soufriere 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. 3 He/ 4 He ratios in the fumarolic gases of La Soufriere volcano have been quite homogeneous and stable over the last thirty years. The average ratio of 8.2 ± 0.2 R a confirms that the volcano is tapping a MORB-like mantle source. In contrast, the nearby Bouillante geothermal system displays a much lower 3 He/ 4 He ratio (4.5 ± 0.1 R a ). He–C elemental and isotopic relationships show that both systems are actually fed by the same magmatic source, and that their marked difference in 3 He/ 4 He results from the 4 He contamination of the Bouillante deep aquifer by the surrounding wallrock. This conclusion is strengthened by the spatial distribution of 3 He/ 4 He ratios which shows that La Soufriere fumaroles and the Bouillante geothermal system are the two end-members of a spatial trend of decreasing 3 He/ 4 He ratio with distance from La Soufriere summit dome, implying an increasing addition of radiogenic 4 He from the host rocks away from the present-day active volcanic edifice.