HR: 1340h
AN: V53D-1599    [Abstracts]
TI: U-Th-Ra Disequilibria in Lavas from the 2004-2005 Eruption of Mt. St. Helens
AU: * Donnelly, C T
EM: ctd3@u.washington.edu
AF: Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Cooper, K M
EM: kmcooper@u.washington.edu
AF: Department of Earth and Space Sciences University of Washington, Box 351310, Seattle, WA 98195 United States
AB: Decay of several nuclides in the U-series decay chain occurs on timescales similar to those of a variety of magmatic processes. Thus, analyses of the disequilibria between different parent/daughter pairs within this series have been used successfully to trace crystal populations and to describe the time scales of crystallization and magma storage in volcanic systems. Previous work with six samples erupted from Mt. St. Helens from ~2ka to 1982 showed ²²⁶Ra-²³⁰Th crystal ages of a few ka. In two of these samples, the Ra-Th ages were discordant with ²³⁰Th-²³⁸U ages of tens of ka, indicating a longer, more complex history than either age alone would reveal. We are now analyzing a suite of samples from the ongoing eruption of Mt. St. Helens for U-Th-Ra disequilibria in whole rocks as well as in plagioclase and amphibole mineral separates. Our preliminary data show that 1) though the Th isotopic composition (IC) of three whole rock samples are within the range of Th ICs from samples spanning the past 2kyr, they are markedly different from the Th IC of the 1982 dacite dome. 2) MSH304 (erupted Oct 2004) plagioclase has a Th IC significantly lower than any MSH samples previously analyzed and differs greatly from its corresponding WR. This indicates either that this plagioclase was dominated by crystals foreign to the matrix within which they were erupted, or that they are tens of thousands of years old. 3) A plagioclase separate from MSH305-1 (erupted Nov 2004) has Th IC similar to that of its corresponding WR, indicating that this plagioclase separate is dominated by recent (10ka) growth. 4) Ra excesses in all of the WR and plagioclase samples indicate that a significant mass of total crystallization occurred within the past ~10kyr. We are currently processing several more samples spanning the first eight months of the ongoing eruption. Several size fractions of plagioclase separates from MSH304 and MSH319 (erupted June 2005) will help to elucidate what appears to be a complex crystallization history. Additional WR samples should form a more complete picture of the range of Th isotopic compositions currently being erupted, and Th ICs of additional 1980's samples will test the possibility that portions of this new material are related to the lavas of the 1980's.
DE: 1036 Magma chamber processes (3618)
DE: 1120 Isotopic disequilibrium dating
DE: 3640 Igneous petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005