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

Abstract

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 An 30 to An 80 , with the majority An 42 -An 65 . 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 rare-earth 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 Pb-isotope 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 208 Pb/ 206 Pb and 207 Pb/ 206 Pb 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.