Remote and in situ plume measurements of acid gas release from La Soufrière volcano, Guadeloupe

Marie-Lise Bernard, Jack Molinié, Rose-Helen Petit, François Beauducel, Gilbert Hammouya, Guy Marion

J. Volcanol. Geotherm. Res., 150, 395-409, 2006.

Abstract. This paper presents the first remote measurements of La Soufrière gas emissions since the fumarolic and seismic reactivation in 1992. The chemical composition of the plumes has been measured from May 2003 to September 2004 using an Open Path Fourier Transform InfraRed (OP-FTIR) spectrometer, up to 15 m downwind the South Crater. HCl is clearly detected (concentration between 2.4 and 12 ppmv) whereas SO2 and H2S generally remain below thedetection limit of the OP-FTIR. Direct measurements of SO2 and H2S near the South Crater with a Lancom III analyzer show a fast decrease of their concentrations with the distance. Calculated Cl/S mass ratios are high:from 9.4±1.7 at 15 m from the vent to 2.8±0.6 at 140 m. The enrichment in HCl of the gas emitted at La Soufrière, observed since 1998, corresponds to the degassing of a magma enriched in Cl and depleted in S. This result agrees with isotopic measurements which suggest a magmatic origin of the gases. Readjustments inside the volcanic system may have taken place during the seismic activity beginning in 1992 and enhance the transfer of magmatic gases to the summit.

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