Volcanic-Plutonic Evolution Evaluated Using Zircon Growth Histories at Mt. St. Helens Volcano

智力优点：岩浆被输送到上地壳，储存在其中，并从上地壳排出（喷发）的过程是理解火山作用和地壳构造的关键。最近的研究表明，岩浆系统可能是长期存在的，在空间和时间上的成分，温度和熔体分数（因此流变学）变化很大。寿命和时空波动部分是反复充电的结果，可能是喷发的原因，也可能是喷发的结果。了解单个系统如何工作，或得出一般结论并预测岩浆行为，取决于监测和比较侵入（古代冻结）和喷发产物的时间-温度-成分历史。锆石矿物为这种监测提供了独特的机会，因为它将高精度的现场放射性测年与分析和解释元素分带的新方法相结合。建议在圣海伦斯山测试和应用这种新的综合方法-铀钍不平衡定年法和元素分析法，包括钛测温法。这样的研究将为它的长期历史和岩浆“管道系统”提供重要的见解，否则研究得很好的火山方面仍然知之甚少。我们将描述和分析来自英安岩的锆石，英安岩是该系统30万年喷发历史的主要产物。锆石可以提供它们所遇到的温度和熔体成分序列的可靠记录。此外，锆石数据将提供火山下方相对缓慢的深成过程（晶体可能在此过程中生长）与它们从岩浆库中提取并喷发的快速过程之间的重要联系。预计，他们将记录机械，热，和化学之间的相互作用的岩浆批次与不同的历史内的深成管道，这些交换的程度的差异将阐明系统的演变，通过time.Broader影响：这个项目将提供一个博士，一个MS和两个本科生的研究培训。博士生已经大量参与了为这个项目奠定基础，她将获得分析化学和火山学方面的专业知识，以及与USGS和其他大学的领导者在该领域的宝贵合作经验。同样，硕士和本科生将受益于一系列领域和地球化学学科的经验，并从广泛的合作。 该研究还将在PI和圣海伦斯火山学家和地球化学家之间建立新的合作。他的职业生涯一直致力于岩浆系统的侵入性根源，并将利用这个机会获得，也许增强，火山的观点。在圣海伦斯山的有效合作特别受到不断扩大的美国地质调查局-大学伙伴关系的青睐，这种伙伴关系已经发展到调查目前的火山爆发和国家纪念碑，其使命的一部分是促进对火山的研究和有关知识的传播。拟议的研究将有助于了解一个危险火山的管道系统内的过程，该系统曾被用来照亮一般火山活动的各个方面更好地了解该系统的更深部分将有助于完善岩浆行为模型，并有助于解释该火山和其他火山的地球物理监测数据。

Intellectual merit: The processes by which magma is transported into, stored within, and expelled (erupted) from the upper crust are critical to understanding volcanism and crustal construction. Recent studies suggest that magmatic systems may be long-lived and highly variable in composition, temperature, and melt fraction (hence rheology) in both space and time. Longevity and spatial and temporal fluctuations are in part a consequence of repeated recharging and can be either cause or consequence of eruption. Understanding how individual systems work, or drawing general conclusions about and predicting magmatic behavior, hinges on monitoring and comparing the time-temperature-composition histories of both intruded (ancient frozen) and erupted products. The mineral zircon provides unique opportunities for such monitoring by combining high-precision in situ radiometric dating with new methods of analysis and interpretation of elemental zoning. It is proposed to test and apply this new combined methodology - U-Th disequilibria dating and elemental analysis, including Ti thermometry - at Mount St. Helens. Such a study will provide critical insights into its long-term history and magmatic "plumbing system," aspects of the otherwise well-studied volcano that remain poorly known. We will characterize and analyze zircons from the dacites that are the principal products of the 300,000-year eruptive history of the system. The zircons can provide a reliable record of the sequence of temperatures and melt compositions that they have encountered. Furthermore, the zircon data will provide an important link between relatively slow plutonic processes beneath the volcano, during which the crystals are likely to have grown, and the rapid processes by which they were extracted from magma chambers and erupted. It is expected that they will document mechanical, thermal, and chemical interaction among magma batches with differing histories within the plutonic plumbing, and that differences in the extent of these exchanges will elucidate evolution of the system through time.Broader impacts: This project will provide research training for one PhD, one MS, and two undergraduate students. The PhD student has already been heavily involved in laying the groundwork for this project, and she will gain expertise in analytical chemistry and volcanology as well as invaluable collaborative experience with leaders in the field at the USGS and other universities. Likewise, the Masters and undergraduate students will benefit from experience in a range of field and geochemical disciplines and from the broad collaboration. The study also will establish new collaborations between the PI and Mount St. Helens volcanologists and geochemists. He has spent a career working on the intrusive roots of magmatic systems and will take advantage of this opportunity to gain, and perhaps augment, the volcanic perspective. Effective collaboration at Mount St. Helens is especially favored by the expanding USGS-university partnership that has developed to investigate the current eruption and by the National Monument, whose mission is in part to facilitate study of the volcano and dissemination of knowledge about it. The proposed research will contribute to understanding of processes within the plumbing system of a dangerous volcano that has been used to illuminate general aspects of volcanic behavior. Better understanding of the deeper portions of the system will help to refine models of magma behavior and aid in interpretation of geophysical monitoring data at this and other volcanoes.