SGER: 210Po-210Pb Constraints on Time-Scales of Degassing and Crystallization for Young Lavas and Tephras

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Zero-aged" volcanic samples with a broad range of compositions are being gathered for comprehensive U-series analyses (i.e., 238U-230Th-226Ra-210Pb-210Po and (230Th)/(232Th)). The principal goals for conducting these analyses are to constrain the time scales of crystallization and degassing in magmas with widely diverse compositions. The longer-lived nuclide analyses will be used to investigate processes occurring on the millenium or longer time-scale before eruption. This greatest effort for this project will be directed toward enhancing the meager database for 210Pb and especially 210Po in volcanic arc lavas. Therefore, this study has a primary focus on the magmatic processes that occur during the last century before a volcanic eruption. By focusing on the period just before eruption, we hope to gain an understanding of the triggers of magma migration from deep chamber systems toward the surface, as well as the nature and timing of mass fractionation processes in the rising magma. The first samples to be analyzed for this study are from Mount St. Helens, Tristan da Cunha, and the Izu arc. Our preliminary analyses of the October 4, 2004 tephras from Mount St. Helens for 210Po indicate that most of the ejecta contains 210Po that was volatilized at magmatic temperatures, then sublimated in the shallow fumarolic system in the year leading up to the eruption. The potential for recognizing a juvenile magmatic component in this early tephra awaits a second set of analyses, which also will provide the initial 210Pb values for all of these samples. The samples from all locations will be analyzed for 226Ra and longer-lived nuclides in the 238U and 232Th decay series at UC Santa Cruz and the University of Bristol. Funding for this SGER will also support the training of undergraduates and a graduate student ath the University of Iowa.