Vesiculation and Microlite Crystallization Induced by Decompression: a Case Study of the 1991^1995 Mt Unzen Eruption ( Japan)

Isothermal decompression experiments were performed to simulate magma ascent at Unzen volcano from the depths of magma storage to shallow crustal levels, corresponding to pressure decrease from 300 to 50MPa. A partially crystallized synthetic rhyodacitic magma (representing equilibrium conditions at 8508C and 300MPa) was used as a starting material; this has a composition identical to the groundmass of Unzen rocks erupted in 1991^1995. Decompression rates were varied from 0·0002 to 20MPa s1. Experiments conducted with decompression rates 0·1MPa s1 were decompressed continuously; a multi-step decompression approach was used at decompression rates 0·1MPa s1. The ex-periments were fluid-saturated, either containing only water as a fluid component (H2O-bearing) or containing a water and carbon dioxide mixture (H2OþCO2; initial mole fraction of H2O in the fluid 0·6).The experimental products of the H2O-bearing experi