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: Kari M. Cooper and 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 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 |