238U-230Th-226Ra disequilibria in dacite and plagioclase from the 2004-2005 eruption of Mount St. Helens: Chapter 36 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

Professional Paper 1750-36
This report is Chapter 36 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006. For more information, see: Professional Paper 1750
By:  and 
Edited by: David R. SherrodWilliam E. Scott, and Peter H. Stauffer



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 238U- 230Th-226Ra 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 238U-230Th 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 ( 230Th)/(238U), ranging from 238U-enriched to 230Th-enriched. ( 230Th)/(232Th) ratios in 2004-5 dacite and gouge samples do not vary outside of analytical error and are within the range of ( 230Th)/(232Th) measured for the 1980s reference suite. However, (230Th)/(232Th) 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.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title 238U-230Th-226Ra Disequilibria in Dacite and Plagioclase from the 2004–2005 Eruption of Mount St. Helens
Series title Professional Paper
Series number 1750-36
DOI 10.3133/pp175036
Year Published 2008
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Hazards Program
Description 20 p.
Larger Work Type Report
Larger Work Subtype USGS Numbered Series
Larger Work Title A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006 (Professional Paper 1750)
First page 827
Last page 846
Country United States
State Washington
Other Geospatial Mount St. Helens
Google Analytic Metrics Metrics page
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