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

知识价值：岩浆被输送、储存和从上地壳喷出(喷发)的过程对于理解火山作用和地壳结构至关重要。最近的研究表明，岩浆系统可能是长期存在的，在组成、温度和熔体比例(因此是流变学)在空间和时间上都是高度可变的。长寿和时空波动在一定程度上是反复补给的结果，可以是喷发的原因或结果。了解单个系统是如何工作的，或就岩浆行为得出一般结论和预测，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。矿物锆石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一新的组合方法-U-Th不平衡测年和元素分析，包括钛测温。这样的研究将为这座火山的长期历史和岩浆“管道系统”提供关键的见解，这些系统在其他方面得到了很好的研究，但仍然鲜为人知。我们将对英安岩中的锆石进行表征和分析，英安岩是该系统30万年喷发历史的主要产物。这些锆石可以提供它们所遇到的温度和熔体组成序列的可靠记录。此外，锆石数据将提供火山下相对缓慢的深成过程与快速过程之间的重要联系，在此过程中，晶体很可能已经长大，而快速过程是从岩浆室中提取并喷发的。预计他们将记录深成管道内不同历史的岩浆批次之间的机械、热和化学相互作用，这些交换程度的差异将阐明该系统随时间的演变。广泛影响：该项目将为一名博士、一名硕士和两名本科生提供研究培训。这位博士生已经积极参与了这个项目的基础工作，她将获得分析化学和火山学方面的专业知识，以及与美国地质调查局和其他大学该领域领导人的宝贵合作经验。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以及广泛的合作。这项研究还将在PI和圣海伦斯火山的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研究岩浆系统的侵入根源，并将利用这个机会获得，或许是增强火山的视角。圣海伦斯火山的有效合作尤其受到为调查目前的火山喷发而发展的不断扩大的美国地质调查局-大学伙伴关系和国家纪念碑的支持，国家纪念碑的任务之一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理解危险火山管道系统内的过程，该系统已被用来阐明火山行为的一般方面。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.