A Study of Magma Dynamics Using Short-Lived Radionuclides

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Intellectual Merit: Lavas from arcs, ocean islands, and mid-ocean ridges will be analyzed for shortlived nuclides in the 238U and 232Th decay series to investigate the time-scales ofmagmatic processes occurring over the decades, years, and weeks leading to volcanic eruptions. A primary goal of the project is to chart differences in degassing andcrystallization patterns between magmas resulting from differences in tectonic settings, initial water contents, and oxygen fugacities. This project will provide time series analysis of eruptive products from ongoing activity at Mount St. Helens (MSH) and Kilauea volcanos, that in turn will help understand magma system dynamics, and recgharge and storage histories. Such information will lead to more realistic models for past behavior and future activity at these volcanoes.. The first study aims to understand magma chamber processes at MSH based on samples of lavas and tephras erupted since October, 2004. Repeat analyses of these samples for 210Po by alpha spectrometry will allow initial 210Po and 210Pb activities to be determined. Activities of 214Pb and 228Ac will be determined by gamma spectrometery and used as proxies for 226Ra and 228Ra. These data will be interpreted in collaboration with the MSH working group, who are collecting other major element, trace element, isotope, and mineral composition data on the same samples. Preliminary data on groundmass material provide constraints on depths and timing of magma degassing, and initial data for plagioclase mineral provide information on the growth history of these minerals. Additional analyses will delineate the proportions of old and new plagioclase and test models of magma chamber development and crystallization. The second project is a collaborative study of 210Po-210Pb-226Ra disequilibrium in lavas erupted from Pu'u O'o and nearby vents at Kilauea. The goal of this project is to chart the persistence and timing of degassing for Kilauean basalts before they erupt. The crustal residence times of magmas that were stored in the shallow crust and erupted during the early 1980s and during the 1997 also will be determined. Broader Impacts. This study will enhance the database for short-lived U-series nuclides in diverse lavas from a wide array of tectonic settings and degrees of fractionation. This will lead to a greater understanding of the nature of volatile fluxing and crystallization that occurs within a century of volcanic eruptions, and also provide information about the depth and volumes of magma reservoirs, how magma viscosity and explosivity changes with time, and what triggers of magma migration from deep chamber systems toward the surface. This study also will look for geochemical predecessors of change in eruption activity. It will continue an international collaboration between the University of Iowa, the Mount St. Helens working group, and researchers from a number of other institutions in the USA and oversees. Undergraduate- and graduate-level research at the University of Iowa will be supported by this project.