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

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.