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

Abstract

Since its last magmatic eruption in 1530 AD, La Soufrière andesitic volcano in Guadeloupe has displayed intense hydrothermal activity and six phreatic eruptive crises. Here we report on the first direct quantification of gas plume emissions from its summit vents, which gradually intensified during the past 20 years. Gas fluxes were determined in March 2006 and March 2012 by measuring the horizontal and vertical distributions of volcanic gas concentrations in the air-diluted plume and scaling to the speed of plume transport. Fluxes in 2006 combine realtime measurements of volcanic H 2 S concentrations and plume parameters with the composition of the hot (108.5°C) fumarolic fluid at exit. Fluxes in 2012 result from MultiGAS analysis of H 2 S, H 2 O, CO 2 , SO 2 and H 2 concentrations, combined with thermal imaging of the plume geometry and dynamics. Measurements were not only focused on the most active South crater (SC) vent, but also targeted Tarissan crater and other reactivating vents. We first demonstrate that all vents are fed by a common H 2 O-rich (97-98 mol%) fluid end-member, emitted almost unmodified at SC but affected by secondary shallow alterations at other vents. Daily fluxes in 2012 averaged 200 tons of H 2 O, 15 tons of CO 2 ,~4 tons of H 2 S and 1 ton of HCl, increased by a factor~3 compared to 2006. Even though modest, such fluxes make La Soufrière the second most important volcanic gas emitter in the Lesser Antilles arc, after Soufriere Hills of Montserrat. Taking account of other hydrothermal manifestations (hot springs and diffuse soil degassing), the summit fumarolic activity is shown to contribute most of the bulk volatile and heat budget of the volcano. The hydrothermal heat output (8 MW) exceeds by orders of magnitude the contemporaneous seismic energy release. Isotopic evidences support that La Soufrière hydrothermal emissions are sustained by a variable but continuous heat and gas supply from a magma reservoir confined at 6-7 km depth. By using petro-geochemical data for La Soufrière magma(s) and their dissolved volatile content, and assuming a magmatic derivation of sulfur, we estimate that the volcanic gas fluxes measured in 2012 could result from the underground release of magmatic gas exsolved from~1400 m 3 d −1 of basaltic melt feeding the system at depth. We recommend that fumarolic gas flux at La Soufrière becomes regularly measured in the future in order to carefully monitor the temporal evolution of that magmatic supply.

Volcanic plume measurements using a UAV for the 2014 Mt. Ontake eruption

“…Unlike the remote sensing for SO 2 flux measurements, both the above methods need to bring sensors into a plume. The former method is only applicable when the SO 2 flux can be constrained by spectroscopic method and is usually impossible for low-temperature fumaroles with low SO 2 and predominant H 2 S (Allard et al 2014). These are the reasons that number of reported H 2 S fluxes for volcanoes are limited compared to that for SO 2 fluxes (McGee et al 2001;Aiuppa et al 2005a, b).…”

“…In order to retrieve H 2 S flux in a plume, two types of methods have been used in volcanic surveys. One is a combination of SO 2 flux and SO 2 /H 2 S ratio in a plume (e.g., Bandy et al 1982;Aiuppa et al 2005a, b) and the other is the H 2 S concentration profiling for a plume cross section by an aircraft for huge plumes (e.g., Radke et al 1976;McGee et al 2001) or by walk for small plumes (Allard et al 2014). Unlike the remote sensing for SO 2 flux measurements, both the above methods need to bring sensors into a plume.…”

Improvement of density models of geological structures by fusion of gravity data and cosmic muon radiographies

“…The last magmatic eruption occurred in AD 1530 when the present lava dome formed, and the last phreatic eruption occurred in 1976. Recent field measurements in its vents (Allard et al, 2014) and sources (Villemant et al, 2014) show a significant regain of activity in the 2006-2012 period. More recently, new vents appeared during our muon tomography experiments.…”

Tunable diode laser measurements of hydrothermal/volcanic CO<sub>2</sub> and implications for the global CO<sub>2</sub> budget

“…The figure is a log-log plot of the cumulative number of volcanoes (N) having measured CO 2 flux of ≥ f . The diagram is based upon the data set of Burton et al (2013), implemented with new results from this study (Table 1) and additional data for Turrialba (1140 t d −1 ; Conde et al, 2014) and Poás (24.7 t d −1 ; Aiuppa et al, 2014) in Costa Rica, Telica (132 t d −1 ; Conde et al, 2014) and San Cristóbal (523 t d −1 ; Aiuppa et al, 2014) in Nicaragua, Lastarria (973 t d −1 ) and Láscar (534 t d −1 ) in Chile , and La Soufriere volcano in Guadeloupe, Lesser Antilles (14.9 t d −1 ; Allard et al, 2014). This implemented CO 2 flux population (43 volcanoes in total) clearly departs from a linear trend, as would be expected for a power-law distribution (see Eq.…”