Trace element and Pb isotope composition of plagioclase from dome samples from the 2004-2005 eruption of Mount St. Helens, Washington

We report the results of in-situ laser ablation ICP–MS
analyses of anorthite content, trace-element (Li, Ti, Sr, Ba, La,
Pr, Ce, Nd, Eu, Pb) concentrations, and Pb-isotope compositions
in plagioclase from eight dome-dacite samples collected from
the 2004-5 eruption of Mount St. Helens and, for comparison,
from three dome samples from 1981-85. For 2004-5 samples,
plagioclase phenocrysts range in composition from An₃₀ to
An₈₀, with the majority An₄₂-An₆₅. With the exception of Li, the
range of trace-element abundances in plagioclase phenocrysts is
largely constant in material erupted between October 2004 and
April 2005 and is broadly consistent with the 1983-85 dome
samples. Anomalously high Li contents in the early stage of the
eruption are thought to reflect addition of Li to the upper part
of the magma chamber immediately before eruption (within
~1 year) by transfer of an alkali-enriched, exsolved vapor from
deep within the magma chamber. Other trace elements show
significant correlations (at >99 percent confidence limits) with
anorthite content in plagioclase phenocrysts--Ba, light rareearth elements (LREE), and Pb show positive correlations,
whereas Ti and Sr correlate negatively. Variations in plagioclase-melt partitioning as a function of anorthite content cannot
explain trace-element variations--in particular predicting
trends for Ti and Sr opposite to those observed. A simple model
involving closed-system fractional crystallization of plagioclase
+ hypersthene + amphibole + oxides largely reproduces the
observed trends. The model requires no gain or loss of plagioclase and is consistent with the lack of europium anomalies
in bulk dacite samples. Analytical traverses within individual
plagioclase phenocrysts support this model but also point to a
diversity of melt compositions present within the magma storage zone in which plagioclase crystallized.
Plagioclase crystals from gabbronorite inclusions in three
dacite samples have markedly different trace-element and Pbisotope compositions from those of plagioclase phenocrysts,
despite having a similar range of anorthite contents. Inclusions show some systematic differences from each other but
typically have higher Ti, Ba, LREE, and Pb and lower Sr and
have lower ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios than coexisting
plagioclase phenocrysts. The compositions of plagioclase from
inclusions cannot be related to phenocryst compositions by
any reasonable petrologic model. From this we suggest that
they are unlikely to represent magmatic cumulates or restite
inclusions but instead are samples of mafic Tertiary basement
from beneath the volcano.