Crystallization Kinetics in Continuous Decompression Experiments: Implications for Interpreting Natural Magma Ascent Processes

Decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma. These experiments differ from previous studies in that they employ a continuous, rather than stepwise, pressure^time trajectory. Four series of experiments were performed at rates of 0·5^10MPa h1, corresponding to ascent rates of 0·007^0·14 m s1. Experiments quenched along each decompres-sion path allow snapshot-style monitoring of progressive crystalliza-tion. As expected, rates of plagioclase nucleation and growth depend strongly on decompression rate. Regardless of decompression rate, ap-proximately equal volumes of feldspar crystallize during a given decompression interval, even when the observed crystallinity is lower than the equilibrium crystallinity. Apparently, a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existin