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

Anderson/WallaceEAR-0001154EAR-0001182这项研究的目的是记录毕晓普凝灰岩的晶体来源，毕晓普凝灰岩是一种巨大的硅质火山岩碎屑沉积物，由一次巨大的爆炸性喷发形成，大约是1980年山体的500倍。圣海伦斯火山喷发。这些晶体很重要，因为它们包含了与岩浆如何形成以及如何和为什么喷发有关的过程的记录。对这种晶体起源的传统解释是，它们是从喷发的熔体中生长出来的(这种晶体被称为“斑晶”)。最近，这一传统观点受到了那些使用放射性同位素系统的人的挑战，这些系统能够提供独立的时间测量。新的解释是，大多数晶体在喷发前很长一段时间就形成了，是从较老的、先前结晶的岩浆体中循环而来的(这种晶体被称为“捕虏晶”)。重新加热这些较老的、主要是结晶的天体被认为是释放了进入喷发岩浆中的晶体。在这种解释中，晶体很少或根本没有关于岩浆是如何形成的，或者是什么促使它喷发的信息。大型硅质岩浆是行星分化的重要组成部分，检验与其起源相关的概念至关重要。在晶体的连续生长区和晶体生长过程中捕获的熔融包裹体(现在的玻璃)中，都有明显斑晶形成的记录。我们将利用这一记录来测试有关主教凝灰岩中晶体来源的想法。这项研究涉及火山沉积、结构信息和地球化学的野外关系，并将为学生提供一个机会来学习和理解实地测试假设想法的必要性。

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.