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

大陆的大部分地壳是由富含二氧化硅的侵入火成岩组成的。整体地球成分相对贫硅，大陆地壳与地球内部不平衡。相反，大陆地壳是一种经过数十亿年地球历史发展和演化的差异化产品。硅质岩在其他星球上似乎很少见，人们认为水在花岗岩等富硅火成岩的形成和演化中发挥了重要作用。花岗岩形成于地表以下几公里处。与花岗岩相对应的火山岩流纹岩，以沉积物的形式喷发，很容易被侵蚀，对大陆地壳几乎没有持久的贡献。一些硅质岩浆喷发形成短暂的流纹岩，而另一些硅质岩浆在深处结晶形成花岗岩，这是一个行星之谜。陆壳的形成需要侵入花岗岩，而不是表面流纹岩。水蒸气是流纹岩岩浆喷发的主要能量来源，水可能在决定硅质岩浆是否喷发形成流纹岩而不是在深部结晶形成花岗岩方面起着关键作用。PI将通过研究毕晓普凝灰岩流纹岩中晶体和气泡的结构和组成来研究这一点，凝灰岩流纹岩是研究最彻底的大型硅质体之一。在他们之前的工作中，他们在毕晓普岩浆体最早喷发的部分发现了明显的天然气聚集。气体的聚集很可能促进喷发，而气体的聚集需要气泡相对于熔体的运动。天然气在喷发前硅质岩浆中聚集的程度和过程尚不确定。PI将记录单个岩浆(浮石碎屑)中晶体运动和混合的程度，这些岩浆在喷发前的气体中有富有贫。他们的工作将检验流纹岩岩浆稳定分层(混合最少)、积累气体从而喷发的观点。相反，花岗岩浆可能会对流并释放积聚的气体，因此最终在深处结晶，成为大陆地壳的一部分。这项拟议研究的更广泛影响包括芝加哥大学和俄勒冈大学的研究生和本科生的参与。学生将学习广泛的方法，以开发和测试基于实地和实验室观察的新概念。他们将学习使用许多复杂的技术，包括X射线断层扫描、红外光谱、阴极发光成像和气体孔隙度测量，并将有机会在全国会议上展示他们的成果。

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.