Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufriere Hills magma

New experimental results are used to constrain the P, T, X(H2O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO2 corresponding to ∼NNO+1 with PH2O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at PH2O >115 MPa and temperatures 115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H2O and CO2 contents