HR: 1340h
AN: V33C-1517    [Abstracts]
TI: Trace Element and Textural Variation in Plagioclase Phenocrysts from Prehistoric Eruptions at Mount St. Helens
AU: * Kyriazis, S F
EM: stephanie.kyriazis.83@csun.edu
AF: California State University Northridge, Department of Geological Sciences, 18111 Nordhoff Street, Northridge, CA 91330-8266, United States
AU: Vazquez, J A
EM: jvazquez@csun.edu
AF: California State University Northridge, Department of Geological Sciences, 18111 Nordhoff Street, Northridge, CA 91330-8266, United States
AU: Cooper, K M
EM: kmcooper@geology.ucdavis.edu
AF: University of California Davis, Department of Geology, One Shields Avenue, Davis, CA 95616-8605, United States
AU: Clynne, M A
EM: mclynne@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Ra-Th dating of major phases in lavas from Mount St. Helens (MSH) has indicated that plagioclase phenocrysts resided for up to several thousands of years before finally being erupted. In most cases, these plagioclase contain significant compositional zoning related to changing magmatic conditions. Hence, individual and populations of plagioclase may preserve an important record of crystallization and thermochemical evolution beneath MSH. In order to quantify the record of magmatic evolution recorded by prehistoric plagioclase at MSH, we have measured major and trace element compositions via LA-ICPMS and electron microprobe in thirty single crystals from dacite and andesite lavas that erupted over the last four millennia (during the Smith Creek, Pine Creek, Sugar Bowl, Kalama, and Goat Rocks episodes of the Spirit Lake stage). In each episode, a significant proportion of the plagioclase contain distinct cores, some with sieved and/or resorbed textures, that are surrounded by rims with oscillatory zoning of variable thickness. Inclusion-free cores have compositions that are similar between plagioclase from the different episodes, with an average of An₄₀ (range of An₂₇ - An₄₇), while sieved cores have a range from An₂₀ to An₈₀. Rim compositions also show significant variability, with a range of An₃₁ to An₆₅. Ba and Sr concentrations in plagioclase across the eruptive episodes contain up to 10-fold variation in concentration (~30-300 ppm, ~600-1800 ppm, respectively). Plagioclase from Pine Creek, Sugar Bowl (East Dome) and Kalama dacites have marked decreases in Sr concentration within the rim-most 50 to 100 micrometers. Near-rim concentrations of Ba are decoupled from Sr, with variation between crystals from the same sample as well as eruptive episodes. Similar isotopic ages, major and trace element compositions, and disequilibrium textures for plagioclase cores between eruptive episodes suggest that recycling of crystal material was significant in the MSH magmatic system over the last 4000 years. Near-rim changes in Sr and Ba concentrations and covariation likely result from the kinetics of ascent-driven crystallization, as is recognized from other studies at Mount St Helens (e.g. Berlo et al., 2007). This rim-ward zoning preserves an important record of magmatic evolution prior to eruption, which may be amenable to geospeedometry.
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting