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

Abstract

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 h , corresponding to ascent rates of 0·007^0·14 m s . Experiments quenched along each decompression path allow snapshot-style monitoring of progressive crystallization. As expected, rates of plagioclase nucleation and growth depend strongly on decompression rate. Regardless of decompression rate, approximately 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 existing crystals. Other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks. Feldspar microlites do not maintain chemical equilibrium with melt. They are more anorthite-rich than equilibrium plagioclase, suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static (phase equilibrium) experiments. Although decompression rate is an important element controlling final textures in volcanic rocks, no single compositional or textural parameter uniquely records this rate. Of the criteria examined, microlite number density and morphology are the best indicators of magma decompression rate. Based on a small number of comparisons between continuous and multi-step decompression style at the same time-integrated decompression rate (1MPa h ), decompression path evidently influences crystal texture, with stepwise decompression yielding textures correlative with faster decompression than indicated by the time-integrated decompression rate. Thus, consideration of magma ascent style, in addition to rate, will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes.