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.
