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

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 Soufrière 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 Soufrière 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 Soufrière 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 Soufrière summit dome, implying an increasing addition of radiogenic 4 He from the host rocks away from the present-day active volcanic edifice.

Spatio-Temporal Relationships between Fumarolic Activity, Hydrothermal Fluid Circulation and Geophysical Signals at an Arc Volcano in Degassing Unrest: La Soufrière of Guadeloupe (French West Indies)

“…The highly conductive body detected below the southern part of the summit extends southwards and downwards to a few hundred meters depth below the base of the dome, but also rises vertically along a structural contact to reach the surface at the level of the Galion thermal springs (Figure 1b) [29]. This is in agreement with geochemical data that suggest a direct connection between the fluids reaching the dome summit and the fluids that ultimately feed these thermal springs [10,27]. It is noteworthy that the electrical conductivity is much smaller (<0.05 S•m −1 ) in the upper region extending between the bottom of the central pits and the surface of the lava dome, where no stagnant liquid water occurs and where volcanic gas pathways are instead located (Figure 7).…”

“…Seismic data and petro-geochemical investigations indicate that the volcano is fed by an andesitic magma reservoir located at about 6-7 km depth beneath the summit [32,33,[39][40][41]. According to C, He and Cl isotopic ratios of the hydrothermal fluids, persistent degassing of this magma reservoir continuously supplies fluids and heat to a shallower hydrothermal system [10,12,[24][25][26][27][28]42,43]. [6,29], the main faults (blue-green), historical eruptive fractures and craters (black) and collapse structures (purple triangle on trace), the region of highest electrical conductivity (>1 S/m, light purple area) determined by Rosas-Carbajal et al [29], active fumaroles (small and big yellow circles), 10 m DEM from GeoEye image, Latitude Geosystems; c) 1 m resolution orthophoto (GeoEye) of the lava dome showing the main active fumaroles of the summit area (yellow circles) TAS: Tarissan crater; NAPN: Napoléon Nord; NAP: Napoléon 1 NPE1: Napoleon Est 1; NPE2: Napoléon Est 2; CS: Cratère Sud, which is divided into northern (CSN), central (CSC) and southern (CSS) vents; G56: Gouffre-56; LCS: Lacroix Supérieur, that is divided into LCS-1 and LCS-2; BLK1: Breislack fumarole; TY: Morne-Mitan fumarole along the Ty fault; d) aerial photo of La Soufrière lava dome (October 2016) showing vegetation impacted by prolonged H2S-and HCl-rich acid gas emissions, photo taken by A. Anglade, OVSG-IPGP, with a drone from OBSERA and with permission by the Parc National de Guadeloupe).…”

“…Acid gas emissions from both vents over the past two decades have considerably impacted the vegetation growing on the downwind summit and W-SW flanks of the dome (Figure 1b). of the hydrothermal fluids, persistent degassing of this magma reservoir continuously supplies fluids and heat to a shallower hydrothermal system [10,12,[24][25][26][27][28]42,43]. After the 1976-1977 events, fumarolic degassing around and on top of the lava dome strongly diminished to ultimately disappear by 1984, synchronously with a gradual decline of seismic activity (Figure 2a).…”

“…By coupling the measured gas composition to the horizontal and vertical distribution of H 2 S in the plume cross-sections and then scaling to the wind speed measured at the vent, Allard et al [12] found that the total gas flux from La Soufrière had increased by a factor of~3 in 2012 compared to a first measurement made in 2006 (using one simple H 2 S electrochemical sensor). Because isotopic tracers demonstrate a persistent supply of magma-derived volatiles and heat to La Soufrière hydrothermal system (e.g., [12,23] and references therein; [24][25][26][27]), such an increase in the gas discharge, together with other phenomena recorded by the local volcano Observatory [28], have raised concerns about the evolution of current unrest and, therefore, deserves further investigations.…”

Steam and gas emission rate from La Soufriere volcano, Guadeloupe (Lesser Antilles): Implications for the magmatic supply during degassing unrest

“…This activity represents the surface expression of a well-developed hydrothermal system (Le Guern et al, 1980;Zlotnicki et al, 1992;Brombach et al, 2000;Villemant et al, 2005) that receives heat and gas from a magma reservoir probably confined at 6-7 km depth beneath the summit (Hirn and Michel, 1979;Pozzi et al, 1979;Semet et al, 1981;Feuillard et al, 1983;Poussineau, 2005). Isotopic investigations actually demonstrate a persistent contribution of magma-derived volatiles to La Soufrière fumarolic gas emissions and thermal waters (Allard et al, 1982;Allard, 1983Allard, , 2006Van Soest et al, 1998;Pedroni et al, 1999;Li et al, 2012;Ruzié et al, 2012Ruzié et al, , 2013Jean-Baptiste et al, 2013). Accordingly, the volcano is closely monitored by the local Volcanological and Seismological Observatory (OVSG-IPGP).…”