Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufreiere 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 <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO2 in residual melt) at PH2O >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 <60 ppm. The evolved Soufriere Hills magma would therefore be H2O-saturated at pressures <130 MPa.

These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at PH2O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5–6 km depth. Magma temperatures were initially low (820–840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.