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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. Sherrod, William E. Scott, and Peter H. Stauffer

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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 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.

Geospatial Extents

Additional Publication Details

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: Chapter 36 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006
Series title:
Professional Paper
Series number:
1750-36
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