Collaborative Research: Origins of Phenocrysts and Xenocrysts in the Bishop Tuff Rhyolitic Magma

本研究的目的是记录主教凝灰岩中晶体的来源，主教凝灰岩是一种体积大、碎屑状的硅火山岩沉积，是由一次巨大的爆炸喷发形成的，大约是1980年圣海伦火山喷发的500倍。这些晶体很重要，因为它们包含了与岩浆如何形成、如何以及为何喷发有关的过程记录。对这种晶体起源的传统解释是，它们从熔体中生长出来，并在其中喷发（这种晶体被称为“斑晶”）。最近，这种传统观点受到了能够提供独立时间测量方法的放射性同位素系统的挑战。新的解释是，大多数晶体形成于喷发前很长一段时间，是从更古老的、先前结晶的岩浆体中回收的（这种晶体被称为“异晶”）。这些古老的、大部分是结晶体的再加热被假设为释放了被喷发岩浆夹带的晶体。在这种解释中，晶体包含很少或根本没有关于岩浆如何形成或是什么使它喷发的信息。大型硅岩浆体是行星分化的重要组成部分，检验与它们起源有关的概念是至关重要的。在晶体上的连续生长区和在晶体生长过程中被捕获的熔体夹杂物（现在是玻璃）中包含了明显的斑晶形成记录。我们将用这个记录来检验关于主教凝灰岩岩浆中晶体来源的想法。本研究涉及火山沉积物的野外关系、结构信息和地球化学，并将为学生提供学习和理解对假设想法进行实地测试的必要性的机会。

Anderson/WallaceEAR-0001154EAR-0001182The objectives of this study are to document the sources of crystals in the Bishop Tuff, a large volume, fragmental deposit of silicic volcanic rock that formed as a result of a huge explosive eruption, about 500 times larger than the 1980 Mt. St. Helens eruption. The crystals are important because they contain a record of processes related to how the magma formed and how and why it erupted. The traditional interpretation for the origins of such crystals is that they grow from the melt in which they erupt (such crystals are termed 'phenocrysts'). Recently, this traditional view has been challenged by those working with radiometric isotopic systems capable of providing independent measures of time. New interpretations are that most of the crystals formed a long time before eruption and were recycled from older, previously crystallized magma bodies (such crystals are termed 'xenocrysts'). Reheating of these older, largely crystalline bodies is hypothesized to have liberated crystals that became entrained in the erupting magma. In this interpretation, crystals contain little or no information about either how the magma formed or what made it erupt. Large bodies of silicic magma are an important part of planetary differentiation, and it is vital to test concepts that are related to their origin. A record of apparent formation of phenocrysts is contained in successively grown zones on crystals and in melt inclusions (now glass) that were trapped during crystal growth. We will use this record to test ideas regarding the sources of crystals in the Bishop Tuff magma. This study involves field relations of volcanic deposits, textural information, and geochemistry, and will provide an opportunity for students to learn and appreciate the need for field tests of hypothetical ideas.