PETROLOGIC EVIDENCE FOR PRE-ERUPTIVE PRESSURE-TEMPERATURE CONDITIONS,AND RECENT REHEATING, OF ANDESITIC MAGMA ERUPTING AT THE SOUFRIERE HILLS VOLCANO, MONTSERRAT, WI

The recent eruption of the Soufriere Hills Volcano in Montserrat (July , 1995, to present; September, 1997) has produced an andesitic dome (S iO2 similar to 59-61 wt.%). The eruption has been caused by invasion o f mafic magma into a preexisting andesitic magma storage region (P sim ilar to 130 MPa; greater than or equal to 5 km depth). The composition of the andesite has remained essentially constant throughout the erup tion, but heating by the mafic magma increased the andesite temperatur e from less than or equal to 830 degrees C to less than or equal to 88 0 degrees C. Prior to being heated, the stable mineral assemblage in t he andesite was plagioclase + amphibole + orthopyroxene + titanomagnet ite + ilmenite + quartz. The rise in temperature from less than or equ al to 830 degrees C to less than or equal to 880 degrees C (f(O2) simi lar to 1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe-Ti oxide mineral compositi ons. The andesitic magma is likely saturated with an H2O-rich vapor ph ase in the upper part of the magma storage region. Melt H2O content is similar to 4.7 wt.%.