The kinetics of degassing-induced crystallization at Soufrière Hills Volcano, Montserrat

Abstract

A series of decompression experiments at 875° C, with a 16 h anneal period at 160 MPa, and depressurizations to a final water pressure (Pf) of 125-30 MPa, were run for between 1 and 504 h. Using an experimentally derived plagioclase liquidus, the depressurizations are estimated to impose an undercooling (ΔT) of between 38 and 151° C. The experiments show that there is a delay of ∼1-4 h in the nucleation of plagioclase crystals after decompression. Over the range of pressures considered, nucleation rates vary up to 32 sites/mm3 per s, and increase with undercooling. The growth rate of plagioclase increases with ΔT, reaching a maximum of 1·7 × 10-6 mm/s at a Pf of 75 MPa. Comparison of the experimental results with natural samples of the Soufrière Hills andesite suggests that pumice samples did not experience any decompression-induced crystallization and therefore ascent from the magma chamber must have been completed within 4 h. Dome samples are variable in texture, but all contain numerous small microlite crystals with low plagioclase anorthite contents and larger microlites with albitic overgrowth rims. Experimental results show that these textural differences can be related to variations in ascent rate.