Boron isotope ratios of surface waters in Guadeloupe, Lesser Antilles

Large variations are reported in the B concentrations and isotopic ratios of river and thermal spring waters in Guadeloupe, Lesser Antilles. Rivers have δ11B values around 40‰ and B concentrations lower than 30 μg/L, while thermal springs have δ11B of 8–15‰ and B concentrations of 250–1000 μg/L. River samples strongly impacted by hydrothermal inputs have intermediate δ11B and B contents. None of these surface water samples have δ11B comparable to the local unweathered volcanic rocks (around 0‰), implying that a huge isotopic fractionation of 40‰ takes place during rock weathering, which could be explained by preferential incorporation of 10B during secondary mineral formation and adsorption on clays, during rock weathering or in the soils. The soil-vegetation B cycle could also be a cause for such a fractionation. Atmospheric B with δ11B of 45‰ represents 25–95% of the river B content. The variety of the thermal spring chemical composition renders the understanding of B behavior in Guadeloupe hydrothermal system quite difficult. Complementary geochemical tracers would be helpful.

► Rivers outer of hydrothermal areas have d11B around 40‰ and [B] of 10–31 μg/L.
► Thermal springs have d11B of 8–15‰ and [B] between 250 and 1000 μg/L.
► With Na, SO4 and Cl, boron shows mixing of rain, low and high-T weathering inputs.
► Guadeloupe rivers and thermal springs have d11B 20–40‰ higher than the local rocks.
► Solid–solution fractionation during weathering pathways may explain this gap of d11B.