The prevalence of andesitic and dacitic volcanic eruptions over the past 20 years has led to a new appreciation of processes typical of magmas of intermediate composition. Extensive syn-eruptive crystallization, driven by decompression and volatile exsolution, is one such process. A water-saturated melt that is decompressed isothermally from its liquidus must crystallize in response to the diminishing capacity of the melt to retain volatiles (particularly H2O). Only rapid magma ascent allows such a melt to reach the Earth's surface without crystallizing. Intermediate rates of ascent permit varying amounts of syn-eruptive crystallization, which in turn changes magma rheology and affects continued magma progress toward the surface. Feedback among magma decompression, vesiculation, and crystallization is poorly understood, particularly with regard to the kinetics of crystallization.
Here we present two complementary approaches to the study of syn-eruptive, degassing-induced crystallization. The first involves projection of matrix glass compositions onto the well-understood Qz-Ab-Or ternary, which allows relative (quartz-undersaturated melt) or absolute (quartz-saturated melt) determination of magma equilibration (or ‘closure’) pressure. We show that glass composition (groundmass crystallinity) changes as a function of decompression rate, and that either very slow ascent or rapid ascent followed by arrest and shallow cooling can lead to extensive cotectic precipitation of quartz + feldspar. The second approach involves quantification of plagioclase textures, which provides a direct measurement of the relative importance of crystal nucleation and growth (J/G). This parameter can, in turn, be linked to the effective undercooling (supersaturation) experienced during decompression. Finally, we use phenocryst melt inclusion data to suggest that a substantial amount of phenocryst crystallization may also be explained by decompression of water-saturated melt.