Collaborative Research: Phenocrysts and Bubbles in the Bishop Tuff Rhyolitic Magma

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Most of the crust of continents is made of intrusive igneous rocks rich in silica. The bulk earth composition is relatively silica poor and continental crust is not in equilibrium with earth's interior. Rather, continental crust is a differentiated product that has developed and evolved over billions of years of earth history. Silicic rocks appear to be rare on other planets, and it is thought that water plays an important role in the formation and evolution of silica-rich igneous rock such as granite. Granite forms kilometers below the surface. The volcanic counterpart of granite, rhyolite, erupts as deposits that are easily eroded and make almost no lasting contribution to continental crust. A planetary enigma is why some silicic magmas erupt and form ephemeral rhyolites whereas others crystallize at depth and form granites. The formation of continental crust requires intrusive granite, not surficial rhyolite. Water vapor is the main source of energy for eruption of rhyolitic magma and it is likely that water plays a key role in determining whether silicic magma will erupt and form rhyolite rather than crystallize at depth and form granite. The PI's will investigate this by studying the textures and compositions of crystals and bubbles in the Bishop Tuff rhyolite, one of the most thoroughly studied, large silicic bodies. In their previous work they found an apparent accumulation of gas in the earliest-erupted part of the Bishop magma body. It is likely that accumulation of gas facilitates eruption, and gas accumulation requires motion of bubbles relative to melt. The extent to which and process whereby gas accumulates in pre-eruptive silicic magma is uncertain. The PI's will document the extent of crystal motion and mixing within individual parcels of magma (pumice clasts) that were rich and poor in pre-eruptive gas. Their work will test the idea that rhyolitic magmas are stably stratified (have minimal mixing), accumulate gas and thus erupt. In contrast, granitic magmas may convect and release accumulated gas, and therefore, end up crystallizing at depth and becoming part of the continental crust. The broader impacts of the proposed study include the involvement of both graduate and undergraduate students at the University of Chicago and the University of Oregon. The students will learn a broad range of approaches to develop and test new concepts based on observations in the field and lab. They will learn to use a number of sophisticated techniques including X-ray tomography, infrared spectroscopy, cathodoluminescense imaging, and gas porisimetry, and will have an opportunity to present their results at national meetings.