HR: 10:20h
AN: V52B-01    [Abstracts]
TI: Not so fast: Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc
AU: * Jicha, B R
EM: bjicha@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Singer, B S
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Beard, B L
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Johnson, C M
AF: Department of Geology and Geophysics University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AB: U-Th isotope data from Pleistocene-Recent basaltic to rhyolitic lavas and their phenocrysts from Seguam Island, Aleutian Island arc, reveal a monotonic evolution consistent with radiogenic ingrowth of $^{230}$Th in a long-lived magma reservoir between 142 and 9 ka. Internal U-Th mineral isochrons from six lavas and tephras are indistinguishable from their eruption ages as constrained by $^{40}$Ar/$^{39}$Ar dating, which implies a short period of crystallization. These results can be reconciled if small batches of magma are repeatedly extracted from a deep, thermally buffered, basaltic reservoir and rapidly cool and differentiate in shallow, ephemeral chambers or conduits immediately prior to eruption. Cone collapse and caldera formation at 9 ka correspond to influx of new basaltic magma into the deep reservoir. Our interpretation, that integrates petrologic, geophysical, and now U-series observations from Seguam, is at odds with the conclusion that magma residence times in the crust are short ($<$ 10$^{3}$ years) and that most variation in the transit time between partial melting and eruption originates in the mantle wedge, a widely held view that is based mainly on U-Th-Ra series isotope data from whole rocks and Sr and Mg diffusion profiles of plagioclase in but a few arc magmas. If correct, our model implies that bodies of basaltic magma can reside undisturbed within the lower crust of an island arc for $>$ 10$^{5}$ years. Suppression of crystallization for 10$^{5}$ years requires minimal heat loss; suggesting either storage in unusually hot wall rocks, or heating from still deeper magma ponded below this reservoir. In any case, the U-series data from Seguam vividly illustrate the importance of a suite of well-dated lavas spanning a significant period of the eruptive history of a single volcano when attempting to constrain rates of magma ascent, crystallization, and differentiation.
DE: 8400 VOLCANOLOGY
DE: 3640 Igneous petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting