MONITORING SO2 EMISSION AT THE SOUFRIERE HILLS VOLCANO - IMPLICATIONSFOR CHANGES IN ERUPTIVE CONDITIONS

Correlation spectrometer measurements of sulfur dioxide (SO2) emission rates during the current eruption of the Soufriere Hills volcano, Mon tserrat, have contributed towards identifying different phases of volc anic activity. SO2 emission rate has increased from <200 tonnes per da y (td(-1); <2.3 kgs(-1)) in the early stages of dome growth to >550 td (-1) (>6.4 kgs(-1)) after July 1996, with the uncertainty associated w ith any individual measurement ca. 30%. Significantly enhanced SO2 emi ssion rates have been identified in association with early phreatic er uptions (800 td(-1) (9.3 kgs(-1))) and episodes of vigorous dome colla pse and pyroclastic flow generation (900 to 1500 td(-1) (10.4 to 17.4 kgs(-1))). SO2 emission rate has proved a useful proxy measurement for magma production rate. Observed SO2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenoc rysts, implying the existence of a S-rich magmatic vapour phase.