Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc

The ascending magmatic component in La Soufrière volcano (Guadeloupe, Lesser Antilles) was investigated by measuring noble gas concentrations and isotopic ratios in thermal springs and fumaroles. A clear magmatic helium signal (4He/3He ~ 89,000; R/Ra ~ 8), equivalent to the MORB value, is recorded in most fluids. 3He fluxes in both fumarole and springs have been estimated and related to 3He content in the magma chamber. In order to explain the 3He flux measured at the surface, we conclude that the magma chamber must be regularly fed by fresh magma batches. Using our new results and data from literature, we propose that the historical activity of La Soufrière volcano can be explained by both abnormal energy inputs from new batches of magma in the chamber and cycles of clogging/opening of the hydrothermal system. We propose a new scenario for the origin of the 1976–1977 crisis whereby a fresh batch of magma could have been emplaced prior to 1968 (possibly between 1959 and 1962) in the magma chamber. The resulting heat flux is not stored in the different aquifers but preferentially evacuated through fractures reactivated or created during the 1956 phreatic eruption. Only when the self-sealing of the hydrothermal system is sufficiently developed, can pressure and temperature within the aquifers rapidly increase to trigger a crisis.