Magmas containing high concentrations of SiO2 are responsible for some of the most catastrophic eruptions on Earth yet they are rarely detected in the crust. Consequently, little is known about the longevity and dynamics of such magmas prior to their eruption. Much like tree-rings, the chemical zoning of crystals in such magmas contain a sequential record of magmatic evolution; unlike tree-rings, unraveling the practical significance of this zoning has proved to be elusive. For silicic magmas in particular, crystal-scale zoning profiles may provide essential insights into magmatic history.
This project will exploit a newly calibrated geothermometer that is based on the titanium content of zircons (Watson and Harrison, 2005) with the goal of using crystal zoning to determine whether crystals spend much of their history suspended in mostly liquid magma, isolated in cumulates piles, or solidified as a part of granitoids. The work will initially focus on rhyolites erupted at three important centers of silicic volcanism (Long Valley, Toba, & Yellowstone) for which U-Th-Pb crystal ages have already determined. Because this new geothermometer has a high potential for many geologic applications, ion microprobe trace element analyses of the zircons will be performed in order to evaluate how crystal growth kinetics, crystal-melt partitioning, and magma mixing might affect the thermometer. When considered in the context of the age data obtained, it will be possible to delimit the time-temperature-compositional evolution of silicic magmas at caldera volcanoes. It is expected that the results will provide a better understanding of how silicic magmas evolve prior to eruption, the mechanisms of pluton growth, and the evolution of Earth's continental crust. The research will engender collaboration between researchers and students at two institutions that serve significant numbers of ethnic minority students, and will support the research projects of two M.Sc. students and several undergraduates.
