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

Abstract

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 CO 2 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. The results 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 δ 13 C CO2 value of −3.19 ± 0.03‰, a 3 He/ 4 He ratio of 8.1 ± 0.2 R a and a CO 2 / 3 He ratio of 1.03 ± 0.18 × 10 10 , 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 CO 2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios.

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

“…Because of this low residence time and of the low tritium (Pascaline et al, 1982) and high helium contents of the waters, the tritiugenic 3 He contribution to the measured 3 He/ 4 He ratio is negligible. Our results are in excellent agreement with those published by Ruzié et al, 2013. Helium isotope data are plotted in Fig.…”

“…He ratios with distance to the crater for gases (black circles) and waters (black squares). In red, literature data for gases (+) by Van Soest et al, 1998;Pedroni et al, 1999;Ruzié et al, 2012 and for waters (x) (Ruzié et al, 2013).…”

“…Marty, 1995). The gases from the Bains Chauds Matouba hot spring were not taken into account in the calculation of the (CO 2 / 3 He) and δ 13 C mean values because of their peculiar composition (low CO 2 and very high N 2 concentrations) which indicates a strong CO 2 elemental and isotopic fractionation during previous gas-water separation (Ruzié et al, 2013). 3.1.2.…”

“…Previous studies of helium isotopes in Guadeloupe (van Soest et al, 1998;Pedroni et al, 1999;Ruzié et al, 2012Ruzié et al, , 2013 have concentrated mostly on La Soufrière system and have shown that the volcano is tapping a MORB-like source with 3 He/ 4 He values around 8 R a . In contrast, two results from the Bouillante geothermal power plant (van Soest et al, 1998;Pedroni et al, 1999) indicate a much lower 3 He/ 4 He value (4.5 R a ), raising interrogations as to whether the Bouillante hydrothermal system is fed by a magmatic source distinct from that of La Soufrière.…”

“…Since then, intense hydrothermal activity has persisted at La Soufrière, including six phreatic eruptive phases. Volcanic gases are persistently released as fumaroles on and around the lava dome (Brombach et al, 2000;Ruzié et al, 2012) and as dissolved and/or bubbling gases in thermal springs around the volcanic edifice (Villemant et al, 2005;Ruzié et al, 2013).…”