Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring

La Soufrière (Guadeloupe, Lesser Antilles) is one of the most active and dangerous volcanoes of the Lesser Antilles arc. The renewal of the volcanic activity at La Soufrière requires replenishment of the volatile-poor andesitic stored magma with more primitive, volatile-rich basaltic magma. This event will trigger early release of large quantities of highly volatile species, like CO2 and helium, in the hydrothermal system. To use these precursors, first their baseline abundances and their behaviour in the hydrothermal system must be known during the current phase of dormancy. In this study, we have combined a previous systematic investigation of noble gases with carbon data of hydrothermal fluids flowing out of La Soufrière. sults for dissolved gases reveal that an intense interaction between rising magmatic volatiles and groundwaters currently takes place at La Soufrière. We show that the pristine magmatic composition, with a δ13CCO2 value of − 3.19 ± 0.03‰, a 3He/4He ratio of 8.1 ± 0.2 Ra and a CO2/3He ratio of 1.03 ± 0.18 × 1010, may vary during the ascent of magmatic fluids due to either fractionation or mixing processes with meteoric water and/or an organic component. Here, we provide a baseline for CO2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios.