This project builds on previous experimental and analytical studies to constrain the evolution of of volatile-rich magma phenocryst-melt assemblages. The first task will involve the calibration of plagioclase-amphibole-melt equilibria at pressures up to 600 MPa and temperatures up to the amphibole breakdown for representative mafic andesite and dacite rock compositions. The amphibole-plagioclase assemblage is widespread in intermediate composition magmas, and their compositions contain significant P-T information based on previous work. The proposed experiments will be in the 300-600 MPa range, rarely obtained in previous studies, and are designed to obtain a close approximation to an equilibrium assemblage of these minerals. The results will greatly improve the information about subvolcanic magma systems that can be obtained from analyses of natural volcanic rocks. The new data will be specifically used to interpret the subvolcanic magma history of the Mount St, Helens magma system.
The second task will be to determine the composition of the CO2-rich gas phase that is certainly generated at pressures in excess of 400 MPa in mafic arc magma systems either as a result of decompression during magma ascent or by cooling. Study of the H2O, S and Cl content of this gas phase will provide data to assess models that have been proposed to explain the origin of very oxidized and S-rich silicic magmas in arc environments. It will also be useful in assessing the importance of the relative volatile abundances measured at volcanic vents and often used in eruption prediction. A third general task is also proposed that involves the continued petrologic and analytical study of the materials being erupted in the ongoing eruptions of Mount St. Helens and Soufrierre Hills, Montserrat.
