Gas Monitoring of Volcanic-Hydrothermal Plumes in a Tropical Environment: The Case of La Soufrière de Guadeloupe Unrest Volcano (Lesser Antilles)

Abstract

Fumarolic gas survey of dormant volcanoes in hydrothermal activity is crucial to detect compositional and mass flux changes in gas emissions that are potential precursors of violent phreatic or even magmatic eruptions. Here we report on new data for the chemical compositions (CO2, H2S, SO2) and fluxes of fumarolic gas emissions (97–104°C) from La Soufrière volcano in Guadeloupe (Lesser Antilles) obtained from both mobile MultiGas measurements and permanent MultiGas survey. This paper covers the period 2016–2020, encompassing a period of enhanced hydrothermal unrest including an abrupt seismic energy release (M 4.1) on April 27, 2018. Our dataset reveals fumarolic CO2/H2S and SO2/H2S gas trends correlated to the evolution of surface activity and to other geochemical and geophysical parameters. We demonstrate that, even under tropical conditions (high humidity and rainfall), MultiGas surveys of low-T fumarolic emissions permit to distinguish deeply sourced signals of volcanic unrest from secondary changes in degassing due to shallow forcing processes such as water-gas-rock interactions in the hydrothermal system and meteorological effects.

Monitoring Hydrothermal Activity Using Major and Trace Elements in Low-Temperature Fumarolic Condensates: The Case of La Soufriere de Guadeloupe Volcano

“…After that, La Soufriere experienced frequent phreatic explosions (e.g., 1797-1798, 1836-1837, 1956, 1976-1977), which were restricted to the volcano summit, with the latter leading to the evacuation of 76,000 people from the volcano's surroundings [49]. Later, the volcano remained in post-eruptive quiescence until 1992, when fumarolic and seismic activity reactivated and evolved, as described in Brombach et al [50], Allard et al [51], Villemant et al [41,42], Rosas-Carbajal et al [52], Tamburello et al [53], Moretti et al [11], Jessop et al [54], and Moune et al [10].…”

“…The main fumarolic emissions (Figure 1c) are in punctual areas, along with fractures (such as Cratère Sud, hereafter "CS") and open pits (such as Gouffre 56 and Tarissan) caused by past phreatic explosions (Figure 1c). Additionally, numerous minor emissions (i.e., pervasive soil degassing) emerge from the ground in an expanding hot zone on the northeast section of the dome (Figure 1c), which includes the Napoleon Nord (NapN) vent formed in 2014 [10,11,53,54]. La Soufriere's hydrothermal system appears to be distributed throughout all the volcanic edifice down to 2-3 km below the summit [11,50,52].…”

“…Thus, volcanic-hydrothermal systems (hereafter called hydrothermal systems) are constituted by a heat source (i.e., magma body) and a network of circulating groundwater with recharge and discharge areas (e.g., [5][6][7][8][9]). Hydrothermal systems are then governed by the equilibrium between internal (e.g., the input of heat and magmatic fluids) and external (e.g., variable input of meteoric water, permeability changes) forces, which strongly influence the physicochemical characteristics of fluids discharged at the surface (e.g., [2,10,11]).…”

“…For example, it was recently reported that the significant input of meteoric waters due to heavy rainfalls into shallow hydrothermal systems can decrease the outlet gas temperatures of fumarolic emissions (e.g., [19,20]). Conversely, extended dry seasons seem to decrease scrubbing processes, amplifying the magmatic signal of discharged fluids (e.g., [10,21]). On one hand, changes in the chemistry of fumarolic gases are essential to discriminate between the deep magmatic and shallow hydrothermal contributions and evaluate the nature and deep causes of volcanic unrest phenomena (e.g., [22][23][24][25][26]).…”

“…Here, we present the major and trace element concentrations in condensates collected between 2017 and 2021 from the Cratère Sud fumarole (95 • C-111 • C) at La Soufriere de Guadeloupe volcano, one of the most dangerous in the Lesser Antilles volcanic arc, with recent records of phreatic eruptions (e.g., [44,45]). Our dataset encompassed a peak in activity in April 2018 and an amplification of the magmatic signal usually screened by the hydrothermal system in the second half of 2019 [10,11]. This work aims to constrain the chemical behavior of major and trace elements in condensates during fluctuating fumarolic activity and evaluate changes in their concentration based on the influence of internal and external forces.…”