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)

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.

The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption

“…These vent speeds and the temperature measurements noted above suggest that the total steam flux at CS dropped from a maximum of around 8.0±1.0 kg/s at the beginning of April to about 3.5±0.5 kg/s soon after the 27 April. This latter value is of same order as CS steam fluxes previously estimated from MultiGAS traverses in 2006 (0.87 kg/s), 2012 (1.72 kg/s) and May 2016 (0.52 kg/s) (Allard et al, 2014;Tamburello et al, in 2019), indicating a slow but significant over the past decade or so. Instead, our values of steam and enthalpy flux are substantially lower than those of Gaudin et al, (2016), who estimated the CS steam and enthalpy fluxes (thermal camera data collected in 2010) to be 19.5±4.0 kg/s and 48.0±9.8 MW, respectively.…”

“…Instead, increased boiling will make it decreasing because of steam addition. As regards the C/S ratio, it can increase either due to either the uprise of deep magmatic gas (often associated with a temperature increase), because CO 2 in magmas is much less soluble than sulfur-bearing gas species and then degasses much earlier (e.g., Moretti et al, 2003 and reference therein) or a loss of sulphur in the hydrothermal system (scrubbing of SO 2 and H 2 S, as well as precipitation of sulfides and/or native sulphur; Allard et al, 2014;Villemant et al, 2005;Tamburello et al, 2019),this latter process being often associated with a decrease in temperature.…”

“…The OVSG-IPGP uses routinely a portable MultiGAS station (Aiuppa et al, 2005;Shinohara, 2005) to measure the concentration of gas emitted by major craters and structures, and also perform gas flux measurements along traverses through main fumarole plumes (e.g., Allard et al, 2014;Tamburello et al, 2019).…”

“…From 2012 to 2016, gas fluxes increased by a factor 3 and 2 at CS and Tarissan, respectively, while gas fluxes from G56 have varied from below detection limit to values that are comparable to those from Tarissan (e.g., Allard et al, 2014;Tamburello et al, 2019). Since 2016, measurements show constant gas fluxes at Tarissan and South Crater, with mean values of 5.7 (± 1.6) and 7.5 (± 1) t/d, respectively.…”

WebObs: The Volcano Observatories Missing Link Between Research and Real-Time Monitoring

“…Extensometers are useful to monitor active cracks and fractures. The superproc makes time series of linear distance, air temperature and wind strength for each station, a summary graph of all distances grouped in geographic zones and a map showing extensive or compressive oriented arrows of displacements (Tamburello et al, 2019). A specific form is available to manage manual data input in a dedicated database (see section 3.3.3).…”