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

Abstract

New experimental results are used to constrain the P, T, X(H 2 O) 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 fO 2 corresponding to ∼NNO+1 with P H 2 O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at P H 2 O >115 MPa and temperatures <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO 2 in residual melt) at P H 2 O >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% H 2 O and CO 2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H 2 O‐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 P H 2 O 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.