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 (SiO2 similar to 59-61 wt.%). The eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region (P similar to 130 MPa; greater than or equal to 5 km depth). The composition of the andesite has remained essentially constant throughout the eruption, but heating by the mafic magma increased the andesite temperature from less than or equal to 830 degrees C to less than or equal to 880 degrees C. Prior to being heated, the stable mineral assemblage in the andesite was plagioclase + amphibole + orthopyroxene + titanomagnetite + ilmenite + quartz. The rise in temperature from less than or equal to 830 degrees C to less than or equal to 880 degrees C (f(O2) similar to 1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe-Ti oxide mineral compositions. The andesitic magma is likely saturated with an H2O-rich vapor phase in the upper part of the magma storage region. Melt H2O content is similar to 4.7 wt.%.