Crystallization of hydrous rhyodacite magma during continuous decompression
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2011-12
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University of Hawaii at Manoa
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This dissertation examines plagioclase crystallization kinetics in a series of continuous decompression experiments on H2O-saturated rhyodacite magma. Experimental studies provide a contextual link between observations in natural volcanic samples and interpretations of magma ascent processes because they allow examination of the kinetic response of the system to decompression and devolatilization through crystal nucleation, growth, and maturation processes. Comparisons of plagioclase crystallization in samples decompressed at different rates reveals a strong dependence of nucleation and growth rates on decompression rate. However, a shifting balance is maintained between nuclei formation and growth of existing crystals such that a constant (non-equilibrium) total volume of plagioclase is observed in all samples at the same quench pressure, regardless of decompression rate. Comparisons of multi-step and continuous decompression experiments at the same integrated rate reveal that decompression path influences crystal textures. Thus, considerations of ascent style, in addition to rate, are necessary to strengthen the interpretive power of experimental studies for constraining natural magma ascent processes. Crystal nucleation and growth rates derived from 3D crystal size distribution (CSD) analysis are substantially lower than corresponding values determined using 2D measurements of bulk crystal populations (batch methods). Although plagioclase growth rates are relatively constant during decompression at a given rate, the average growth rate in a rapidly decompressed sample is approximately five times faster than the growth rate in a more slowly decompressed sample.
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magma
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Geology and Geophysics.
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