HR: 10:55h
AN: V32C-03    [Abstracts]
TI: Sr Isotope and Compositional Zoning of Plagioclase in the Active Mount St. Helens Lava Dome
AU: * Hart, G L
EM: ghart@wsu.edu
AF: Washington State University, 1228 Webster, Pullman, WA 99164,
AU: Streck, M J
EM: StreckM@pdx.edu
AF: Portland State University, 17 Cramer Hll, Portland, OR 97207,
AU: Wolff, J A
EM: jawolff@mail.wsu.edu
AF: Washington State University, 1228 Webster, Pullman, WA 99164,
AB: Isotopic and compositional patterns preserved in zoned plagioclase provide a record of processes occurring within a magma during phenocryst growth. Such information is most valuable when the analyses are paired from individual grains and placed in a chronological framework. In this study, we present new paired Sr-isotope and trace element data on plagioclase crystals from samples collected from the actively growing dacite lava dome at Mt. St. Helens. Plagioclase crystals from sample SH321C, collected Aug 19, 2005, fall into four broad textural and compositional groups defined primarily by their An contents, resorption features and the presence of pyroxene inclusions. The first group has fairly uniform core compositions of An_55-60 that monotonically decline toward the rim to An_30-35. The second group displays oscillatory zoning from An₄₅ to An₅₅ within a 30-50 micron spacing separated by dissolution boundaries. The third group has unusually sodic (An_25-30) core compositions overgrown by oscillatory rims that are similar to the second group. The fourth group includes all grains that are large, sieved, or xenocrystic in appearance. Sr-isotope variations within single plagioclase grains in these four groups were obtained by LA-MC-ICPMS. The first group of feldspars have nearly homogenous Sr isotope compositions of ~0.7032-0.7034, with the majority at 0.7034, which matches the average whole-rock Sr isotope composition. The second group has slightly more variable Sr compositions in the oscillatory-zoned sections with values extending up to 0.7036. The oscillatory zones most likely record larger Sr isotope variations, but at a scale less than the resolution of the laser ablation sampling technique. The third group of feldspars shows the most variability with values reaching ~0.7050 in the cores, decreasing to ~0.7035 at the rims. The forth group displays intermediate Sr isotope compositions of ~0.7037-0.7039. Sr isotope compositions of the plagioclase rims from all groups have values ranging from ~0.7032-0.7034, suggesting overgrowth formation from dominant magma of similar Sr isotope composition to the group one plagioclase and the whole rock. Nonetheless the `crystal cargo` carried by the dome lavas was assembled from a variety of sources and incorporated into the magma at different times. These isotopic and compositional variations suggest a complex magmatic history that involved periods of crystal growth, resorption, magma injection, and growth of plagioclase rims. The crystals with high- ⁸⁷Sr/⁸⁶Sr cores record an earlier period of crystal growth from a distinct magma, or, alternatively, the incorporation and assimilation of an older rock into a younger magma chamber.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting