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Magmatic Interactions on Short Time Scales: Magma Mixing During the Kalama Eruptive Period at Mount St. Helens, Washington

A. E. Lieuallen1, F. J. Tepley III3, and M. Clynne2
1Department of Geosciences, Oregon State University, Corvallis, Oregon
2United States Geological Survey, Menlo Park, California
3College of Atmospheric and Oceanic Sciences, Oregon State University, Corvallis, Oregon

Insight into the evolution of magmatic systems of arc volcanoes is critical in understanding eruptive histories, which are used to estimate the capability for future eruptions and identify potential hazards. Mount St. Helens, with its easy accessibility, long history and frequency of eruption is an ideal location to synthesize observations of recent eruptions with investigations of older volcanics. The Kalama eruptive period of Mount St. Helens, A.D. 1479 - ~1750, was bracketed by interludes of quiescence and thus likely represents an entire eruptive cycle within a span of 300 years. Study of the magmatic evolution during this short time period provides key information regarding inputs and the plumbing system of Mount St. Helens.

The rocks erupted during the Kalama period were a result of magma mixing events, but the number, source and composition of each end member is unclear. Current magma mixing models do not adequately explain existing major and trace element patterns relative to the dacite-andesite-basaltic andesitedacite progression of erupted products, nor do they consider the tertiary basement rocks and mafic enclaves found as inclusions in Kalama rocks as potential components of the system. To improve existing models of mixing relationships during the Kalama period, a suite of 32 rocks were analyzed to 1) determine the composition of end members and 2) detail their interactions through textural study of observed phases in order to describe the progression of mixing events. Preliminary results indicate a wide compositional variance in middle and early Kalama plagioclase, particularly from pyroclastic flows and less so for middle Kalama lavas. The compositional variation, along with correlation of crystal textures with new major and trace chemical data, suggests the influence of basement rock on the system.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas