238U-230Th-226Ra Disequilibria in Dacite and Plagioclase from the 2004–2005 Eruption of Mount St. Helens

Kari M. Cooper; Carrie T. Donnelly

doi:10.3133/pp175036

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238U-230Th-226Ra Disequilibria in Dacite and Plagioclase from the 2004–2005 Eruption of Mount St. Helens

Kari M. Cooper·Carrie T. Donnelly

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Abstract

Uranium-series disequilibria in whole-rock samples and mineral separates provide unique insights into the time scales and processes of magma mixing, storage, and crystallization. We present 238 U- 230 Th- 226 Ra data for whole-rock dacite and gouge samples and for plagioclase separated from two dacite samples, all erupted from Mount St. Helens between October 2004 and April 2005. We also present new 238 U- 230 Th disequilibria for a suite of four reference samples from the 1980-86 eruption of Mount St. Helens. We use the U-series data to evaluate the origin of the 2004-5 magma, its relation to the 1980-86 magma, and the relation of 2004-5 phenocrysts to their host magmas. Dacite samples from 2004-5 show variable ( 230 Th)/( 238 U), ranging from 238 U-enriched to 230 Th-enriched. ( 230 Th)/( 232 Th) ratios in 2004-5 dacite and gouge samples do not vary outside of analytical error and are within the range of ( 230 Th)/( 232 Th) measured for the 1980s reference suite. However, ( 230 Th)/( 232 Th) for plagioclase separates for dome samples erupted during October and November 2004 are significantly different from corresponding whole-rock values, which suggests that a large fraction (>30 percent) of crystals in each sample are foreign to the host liquid. Furthermore, plagioclase in the two 2004 samples have U-series characteristics distinct from each other and from plagioclase in dacite erupted in 1982, indicating that (1) the current eruption must include a component of crystals (and potentially associated magma) that were not sampled by the 1980-86 eruption, and (2) dacite magmas erupted only a month apart in 2004 contain different populations of crystals, indicating that this foreign component is highly heterogeneous within the 2004-5 magma reservoir.

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Source-reported bounding extent: 46.161175° to 46.233792° latitude; -122.238678° to -122.131489° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Kari M. Cooper, Carrie T. Donnelly. 2008. 238U-230Th-226Ra Disequilibria in Dacite and Plagioclase from the 2004–2005 Eruption of Mount St. Helens. https://doi.org/10.3133/pp175036

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