Degassing and crystallization of ascending andesite and dacite

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 inte rmediate composition. Extensive syn-eruptive crystallization, driven by dec ompression and volatile exsolution, is one such process. A water-saturated melt that is decompressed isothermally from its liquidus must crystallize i n response to the diminishing capacity of the melt to retain volatiles (par ticularly H2O). Only rapid magma ascent allows such a melt to reach the Ear th's surface without crystallizing. Intermediate rates of ascent permit var ying amounts of syn-eruptive crystallization, which in turn changes magma r heology and affects continued magma progress toward the surface. Feedback a mong magma decompression, vesiculation, and crystallization is poorly under stood, 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) d etermination of magma equilibration (or 'closure') pressure. We show that g lass composition (groundmass crystallinity) changes as a function of decomp ression 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 plagiocla se textures, which provides a direct measurement of the relative importance of crystal nucleation and growth (J/G). This parameter can, in turn, be li nked to the effective undercooling (supersaturation) experienced during dec ompression. 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.