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)

Tamburello Tamburello,Moune Moune,Le Marchand Le Marchand,Allard Allard,Komorowski Komorowski,Venugopal Venugopal,Rosas-Carbajal Rosas-Carbajal,Didier Didier,Beauducel Beauducel,Le Friant Le Friant,Deroussi Deroussi,Robert Robert,Dechabalier Dechabalier,Moretti Moretti,Bonifacie Bonifacie,Dessert Dessert,Kitou Kitou

doi:10.3390/geosciences9110480

Abstract

： Over the past two decades, La Soufrière volcano in Guadeloupe has displayed a growing degassing unrest whose actual source mechanism still remains unclear. Based on new measurements of the chemistry and mass flux of fumarolic gas emissions from the volcano, here we reveal spatio-temporal variations in the degassing features that closely relate to the 3D underground circulation of fumarolic fluids, as imaged by electrical resistivity tomography, and to geodetic-seismic signals recorded over the past two decades. Discrete monthly surveys of gas plumes from the various vents on La Soufrière lava dome, performed with portable MultiGAS analyzers, reveal important differences in the chemical proportions and fluxes of H2O, CO2, H2S, SO2 and H2, which depend on the vent location with respect to the underground circulation of fluids. In particular, the main central vents, though directly connected to the volcano conduit and preferentially surveyed in past decades, display much higher CO2/SO2 and H2S/SO2 ratios than peripheral gas emissions, reflecting greater SO2 scrubbing in the boiling hydrothermal water at 80–100 m depth. Gas fluxes demonstrate an increased bulk degassing of the volcano over the past 10 years, but also a recent spatial shift in fumarolic degassing intensity from the center of the lava dome towards its SE–NE sector and the Breislack fracture. Such a spatial shift is in agreement with both extensometric and seismic evidence of fault widening in this sector due to slow gravitational sliding of the southern dome sector. Our study thus provides an improved framework to monitor and interpret the evolution of gas emissions from La Soufrière in the future and to better forecast hazards from this dangerous andesitic volcano.

Highlights

Active arc volcanoes that erupt with evolved magmas commonly display long periods of dormancy between their magmatic eruptions
We present new results for the chemical composition and the mass flow rate of fumarolic gases emitted from different vents on top of La Soufrière volcano in 2016–2017, during an ongoing phase of degassing unrest that has developed since 1992
Our results reveal a wide range in gas compositions, reflecting the variable influence of shallow processes (SO2 scrubbing in liquid water and near-surface sulfur precipitation in the volcanic ground), that closely relates to the evolution of the fumarolic activity with respect to the underground circulation of hydrothermal fluids inside the lava dome, as imaged from a recent electrical tomography [29]

Summary

Introduction

Active arc volcanoes that erupt with evolved magmas commonly display long periods (decades to centuries) of dormancy between their magmatic eruptions. These quiescent periods are often characterized by hydrothermal manifestations (fumaroles, boiling pools, thermal springs, etc.). While interacting with shallow groundwater and host rocks, this continued magmatic supply sustains a hydrothermal system (e.g., [1])) and generates acid hydrothermal fluids that promote intense alteration of the host rocks. This could lead to mechanical weakening of the volcanic edifices and their potential collapse. As recorded by the cases of Ontake in Japan in 2014 [4]

Results

Discussion

Conclusion