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Geological Society, London, Memoirs; 2002; v. 21; p. 621-639;
DOI: 10.1144/GSL.MEM.2002.021.01.31
© 2002 Geological Society of London

Geophysical and gas studies

Variation in HCl/SO2 gas ratios observed by Fourier transform spectroscopy at Soufrière Hills Volcano, Montserrat

C. Oppenheimer1, M. Edmonds2, P. Francis3 & M. Burton1,4

1 Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK co200{at}cam.ac.uk
2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
3 Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK
4 Present address: Istituto Nazionale di Geofisica e Vulcanologia, Sezione de Catenia, U.F. Sistema Poseidon, Via Monti Rossi 12, 95030 Nicolosi, Catania, Italy

We present here the results of open-path Fourier transform infrared (FTIR) spectroscopy of gases emitted from the lava dome of Soufrière Hills Volcano. Although measurement campaigns have been discontinuous, they do span a three-year period and provide strong evidence of secular change in the HCl/SO2 molar ratio from ≥5 in 1996 to <0.5 in 1999. The post-1996 spectral data represent the only available measurements of gas ratios for the volcano's summit emissions, and complement SO2 emission rate data obtained by ultraviolet correlation spectroscopy (COSPEC), enhancing the interpretation of degassing at the volcano. The long-term decreasing HCl/SO2 ratio accompanied an increasing SO2 emission rate, and suggests a transition from degassing of andesitic to basaltic magma, or progressive tapping of a sulphur-rich vapour phase that was introduced by mafic magma, or that was already resident within the andesite magma reservoir. On timescales of minutes to hours, we observed variations in HCl/SO2 ratios associated with dome collapses. On 27 July 1998, for example, the HCl/SO2 ratio dropped from about 0.7 to 0.4 within minutes of a minor dome collapse. A much larger collapse event on 26 October 1998 was followed by a decrease in HCl/SO2 from 0.6 to 0.1, some 14 hours later. These changes are suggestive of transient degassing from a sulphur-rich source region - larger collapses resulted in tapping of deeper sources, with exsolved gases taking longer to reach the surface. The sustained degassing and response to dome collapse events suggest a permeable upper conduit system established by syneruptive vesiculation, and efficient transfer of volatiles out of the magma chamber by degassing-driven convection in the lower part of the central conduit. Open-path FTIR spectroscopy represents a means for remote geochemical surveillance when access to vent regions is restricted for safety reasons, yielding valuable insights into degassing mechanisms.