Sierra Nevada Aquatic Research Laboratory

17 May, 11 AM - 24 May, 02 AM

Formation of crystal-poor rhyolite is poorly understood because it is a high-SiO2 volcanic rock with <<1% crystals. Magma crystallizes during ascent and cooling, so producing a rock with negligible crystal content requires unusual circumstances. Many interpret crystal-poor, high-SiO2 volcanic rocks as such: high-SiO2 magma fractionally crystallizes from mafic magma, and after 40-50% crystallization, the liquid is extracted through crystal compaction and gravitational settling (Bowen, 1956; Bachmann and Bergantz, 2004). This is problematic because viscosity increases exponentially with SiO2 content (~109 Pa s, about the viscosity of glass at 600? C, at SiO2 = 75 wt. %). It is difficult to separate crystals from such a viscous liquid. This research will test an alternative hypothesis: that shear heating of high-silica magma during ascent can heat magma enough to cause crystal dissolution, producing crystal-poor magma. The objectives of this study are to: 1) create a thermal model of a high-silica dike, 2) mimic production of crystal-poor rhyolite as closely as possible through high-temperature furnace experiments and 3) use microanalytical instruments to examine crystal boundaries in both crystal-poor rhyolites and experimental runs. The proposed field sites for this study are Glass Mountain (northeast rim of Long Valley caldera), Casa Diablo (south-central part of Long Valley caldera), and the Mono-Inyo volcanic chain. The geology of the eastern side of the Sierra Nevada Mountains near the Valentine Eastern Sierra Reserve is dominated by the Long Valley Caldera, which formed during the climactic eruption of the rhyolitic Bishop tuff ~0.76 m.y. ago (Bogaard and Schirnick, 1995). Nearby Glass Mountain is ~ 50 km^3 of crystal-poor, rhyolitic lava erupted during the ~ 1 m.y. preceding the Bishop tuff (Metz and Mahood, 1991). The Mono-Inyo chain is a relatively young (<0.3 m.y.) volcanic chain extending north from the caldera (Bailey et al., 1976). All of these lavas are dominantly rhyolitic in composition, and much of this rhyolite is crystal-poor. This research aims to provide insight into formation of these types of lavas. A better understanding of the formation of crystal-poor rhyolite is is essential to understanding the geology of the land surrounding the Valentine Eastern Sierra Reserve. The numerical modeling program COMSOL Multiphysics will be used to construct the thermal model. This program has been used to model geologic phenomena such as magma reservoir failure (Grosfils, 2007), and is suitable for application to fluid dynamics of volcanic conduits. The thermal model will simulate high-SiO2 magma flow in a conduit undergoing simple shear in Poiseuille flow. It will use values appropriate to high-SiO2 magma for thermal conductivity, initial viscosity and temperature, and magma density. Values for velocity and extrusion rate will be estimated from observations of eruptions (Kaneko et al., 2002). The aim of this model is to understand whether shear heating can overcome the loss of heat to the surrounding rock and produce a net increase in temperature. Temperature increases derived from this model will inform the experiments outlined below. Experiments in this study will be constructed to simulate a high-SiO2 magma undergoing a rise in temperature due to shear heating. Powdered crystal-poor rhyolite will be seeded with ~100-200 μm crystals of minerals observed in crystal-poor rhyolite (e.g. hypersthene and plagioclase). These crystal-powder mixtures will be heated to superliquidus temperatures at atmospheric and 0.1 GPa pressure, simulating near-surface conditions. One-atmosphere experiments will be conducted at UNC-Chapel Hill using a Deltech DT31VT gas-mixing, vertical-quench furnace with an atmosphere of CO2/H2 gas held at the Ni-NiO buffer. Pressurized (≤0.1 GPa) experiments will be conducted at the NC State Museum of Natural Science using a cold-seal furnace with Pt capsules. At both pressures, charges will be held at a static temperature for various durations of time (e.g. 24, 72, 144 hours) before being rapidly quenched. Electron probe microanalysis (EPMA) will be performed on the JOEL JXA-8530F field emission electron probe microanalyzer at Fayetteville State University. EPMA yields precise chemical analyses of polished probe-quality thin sections. Mineral dissolution involves the removal of ions from the crystal lattice and the transport of ions away from the crystal-liquid boundary (e.g. Donaldson, 1985). EPMA will be used to observe and quantify the degree to which these processes are or are not occurring in experimental charges and natural samples. To clarify, this study has three main components: 1) a thermal model built in a numerical modeling program 2) experimental runs simulating temperature increase during crystal-poor rhyolite formation, and 3) microanalysis of crystal boundaries in both natural rocks and experimental runs. The model will inform the temperature rise to which experimental runs are taken. The end products of the experiments will be plagioclase or hypersthene crystals showing varying degrees of dissolution. The textures and chemical diffusion of the natural rocks and experimental runs will be precisely characterized using EPMA. The geology of the eastern side of the Sierra Nevada Mountains near the Valentine Eastern Sierra Reserve is dominated by the Long Valley Caldera, which formed during the climactic eruption of the rhyolitic Bishop tuff ~0.76 m.y. ago (Bogaard and Schirnick, 1995). Glass Mountain is ~ 50 km^3 of crystal-poor, rhyolitic lava erupted in the ~ 1 m.y. preceding the Bishop tuff (Metz and Mahood, 1991). The Mono-Inyo chain is a relatively young (<0.3 m.y.) volcanic chain extending north from the caldera (Bailey et al., 1976). These lavas are dominantly rhyolite in composition, and much of this rhyolite is crystal-poor. Thus, a better understanding of the formation of crystal-poor rhyolite is is essential to understanding the geology of the Long Valley region.

Visit #28161 @Sierra Nevada Aquatic Research Laboratory

Approved

Under Project # 25045 | Research

The role of shear heating in producing crystal-poor rhyolite

graduate_student - University of North Carolina

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Adam Curry May 17 - 24, 2012 (8 days)
Adam Curry May 17 - 24, 2012 (8 days)

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