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Title: Experimental constraints on volatile-driven crystallisation in volcanic systems : a case study at Mount St. Helens
Author: Riker, Jenny Michelle
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Degassing and crystallisation are concomitant processes in volcanic systems that together modulate the physico-chemical properties of magmas. The compositions and textures of crystal-bearing volcanic rocks therefore preserve valuable information about the degassing paths experienced by their host magmas. Accurate interpretation of the rock record, however, requires a detailed interpretive framework. This thesis presents the results of high-temperature, high-pressure experiments designed to clarify the mechanisms and manifestations of volatile-driven crystallisation in natural magmas. The well-characterised :Mount St. Helens volcanic system is taken as a case study. Isothermal phase equilibria experiments in P- XH20 space establish the fluid-composition dependence of phase relations in Mount St. Helens (Sugar Bowl) rhyodacite. Parameterisation of experimentally derived data enables simulation of magmatic properties across a wide range of parameter space. This approach has been applied to investigate the equilibrium evolution of phase assemblages, abundances, and compositions along different H20 - C02 degassing trajectories. Experimental decompression of magmas saturated with H20 and H20 -C0 2 fluids further demonstrates that plagioclase textures are strongly modulated by the isothermal decompression (PH20 - t) path. The presence of CO2 increases crystal nucleation rates relative to the pure-water case, while growth on pre-existing crystals contributes significantly to added crystallisation at a range of decompression conditions. Comparison of crystal textures in experimental samples and natural pyroclasts are used to interpret changing magma storage conditions during the summer 1980 explosive- effusion transition at Mount St. Helens. Finally, the behaviour of magmatic volatiles in the shallow crust is ultimately modulated by the nature and abundance of vapour exsolved at depth. To this end, this research presents new constraints on water solubility in silicic melts at lower crustal pressures, where experimental determinations are sparse. Together with data from lower pressures, these experiments define a continuous solubility curve in the Sugar Bowl rhyodacite from 0.1- 1.1 GPa. Saturation water contents suggest that the presence of fluid-saturated, evolved melts is likely in the deep crust.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available