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) condit ions of the Soufriere Hills magma prior to ascent and eruption. The ex periments were performed on a powdered andesite erupted in January, 19 96, at an fO(2) corresponding to similar to NNO+1 with P-H2O and tempe ratures in the range 50 to 200 MPa and 800 to 940 degrees C. Amphibole is stable at P-H2O >115 MPa and temperatures <875 degrees C. Quartz o nly becomes stable at low temperatures and after high degrees of cryst allization (T <840 degrees C, >72 wt% SiO2 in residual melt) at P-H2O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioc lase from recently erupted samples indicate melt water contents of 4.2 7 +/- 0.54 wt% H2O and CO2 contents <60 ppm. The evolved Soufriere Hil ls 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 an d ilmenite was stored at P-H2O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5-6 km depth. Magma temp eratures were initially low (820-840 degrees 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 up per limit (similar to 880 degrees C) for the extent of this reheating.