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Vapor transfer prior to the October 2004 eruption of Mount St. Helens, Washington

Adam J.R. Kent^*,1, Jon Blundy², Katharine V. Cashman³, Kari M. Cooper⁴, Carrie Donnelly⁵, John S. Pallister⁶, Mark Reagan⁷, Michael C. Rowe^,8 and Carl R. Thornber⁹

¹ Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA
² Department of Earth Sciences, University of Bristol, Bristol BS81RJ, UK
³ Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA
⁴ Department of Geology, University of California at Davis, Davis, California 95616, USA
⁵ Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA
⁶ U.S. Geological Survey, David A. Johnson Cascade Volcano Observatory, Vancouver, Washington 98683, USA
⁷ Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA
⁸ Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA
⁹ U.S. Geological Survey, David A. Johnson Cascade Volcano Observatory, Vancouver, Washington 98683, USA

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 µg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess (²¹⁰Pb)/(²²⁶Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing.

Diffusion modeling shows that Li enrichment occurred within 1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

Key Words: Mount St. Helens • diffusion • plagioclase • lithium • vapor transport

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