HR: 0800h
AN: V41D-0796    [Abstracts]
TI: The Dynamics of Magma Chamber Processes at Popocatepetl Volcano, as Recorded by Plagioclase Phenocrysts
AU: * Sosa-Ceballos, G
EM: giovannisosa@mail.utexas.edu
AF: Department of Geological Sciences Jackson School of Geosciences The University of Texas at Austin, University Station C1100 Austin, TX, Austin, TX 78712,
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AU: Housh, T
EM: housh@mail.utexas.edu
AB: Popocatépetl Volcano had 5 Plinian eruptions in the last 23,000 years, and is currently undergoing passive degassing and small-scale explosions. We seek to clarify the roles of crystallization, magma mixing, and assimilation of the calcareous basement in controlling magma composition by studying the compositional and ⁸⁷Sr/⁸⁶Sr isotopic variations in plagioclase phenocrysts. Compositional variations were analyzed by electron microprobe (EPMA) and zones based on An content analyzed for Sr isotopes using a 213 nm laser ablation system coupled with an IsoProbe multicollector, magnetic sector ICP-MS. Based on variations of An content we can classify crystals into three groups. The first group has albite-rich cores that are mantled by nearly constant composition plagioclase of An₅₀. The second group has anorthite-rich cores that are mantled by nearly constant composition plagioclase of An₄₀. The third group lacks cores with disparate compositions. Despite the relatively simple grouping, isotopic trends are variable among groups. For those with albite-rich cores, some have constant isotopic values from core to rim, whereas others have more radiogenic cores than their rims (e.g. Δ _c-r⁸⁷Sr/⁸⁶Sr > 0.0015), and yet others have less radiogenic cores than rims (e.g. Δ _c-r⁸⁷Sr/⁸⁶Sr < 0.0022). Equally complicated zonings are found in the other two groups. Variable correlations between isotopic trends and An contents show that the magma system of Popocatepetl has been quite dynamic. Crystals with albite-rich cores related to less radiogenic rims indicate mixing with a more mafic magma. Crystals with variable An content and more radiogenic rims might reflect local basement assimilation. Crystals with minor variations of An content and nearly constant isotopic values reflect crystallization. Crystals with variable An content and nearly constant isotopic values may reflect processes as decompression, changes in temperature, or changes in water content.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting