HR: 0800h
AN: V11B-0591    [Abstracts]
TI: (226Ra)/(230Th) Disequilibrium and Amphibole Crystallization in Rhyodacite From Ilopango Caldera, El Salvador
AU: * Garrison, J M
EM: jengarrison@gmail.com
AF: California State University, Los Angeles, Department of Geological Sciences 5151 State University Dr, Los Angeles, CA 90032, United States
AU: Reagan, M K
EM: mark-reagan@uiowa.edu
AF: University of Iowa Department of Geoscience, 121 Trowbridge Hall, Iowa City, IA 52242, United States
AU: Sims, K W
EM: ksmis@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics 266 Woods Hole Road, Woods Hole, MA 02543, United States
AU: Patino, L C
EM: patinoL@msu.edu
AF: AB: ²²⁶Ra and ²³⁰Th isotope data from Ilopango caldera, El Salvador suggest that crystallization of amphibole in this system could be responsible for creating ²²⁶Ra deficits from initially ²²⁶Ra-enriched basalt. Assuming that the ²²⁶Ra excess in the basalt represents the initial ²²⁶Ra value of the parental magma, then the time that was required for amphibole crystallization was on the order of less than 1000 years. These data are part of a larger study involving long-lived and short-lived isotopes from calderas in El Salvador and Nicaragua. We analyzed rhyodacite whole rocks and mineral separates from the 1600BP eruption, and we also collected samples of a younger dacite dome within the caldera. Olivine-bearing basalt enclaves within the dacite dome were also collected for analysis. Our results indicate that the rhyodacites erupting in Central America have trace element and Sr, Nd, and Pb isotopic compositions that are similar to other local, mafic magmas, consistent with evidence that the rhyodacites are produced by differentiation of magmas that involve little or no older crust in their genesis. The rhyodacites from Ilopango have ²³⁰Th excesses or are in equilibrium, which has been explained by lower crustal melting and assimilation in other arcs. The moderate crust thickness beneath this arc makes that scenario unlikely, and it is more likely that the ²³⁰Th excess is due partly to ²³⁰Th excesses in parental basalts caused by melting of the subduction-modified mantle beneath Central America. This could also be due, in part, to mineral fractionation during crystallization of the rhyodacite, which is supported by U-series data from the basaltic enclave from Ilopango that shows it to have the smallest ²³⁰Th excess (3%), and one of the rhyodacites to have the largest (30%). Mineral separates from the Ilopango samples show that the magnetite and, in some cases, amphibole have 238U excesses (up to 270%), and crystallization of amphibole could create moderate ²³⁰Th excesses within 1000 years before eruption. This would also explain the observed correlation between (²²⁶Ra/230Th) and (²³⁸U/²³⁰Th) in these samples. With respect to the whole rock ²²⁶Ra data, the mafic enclave has the highest (²²⁶Ra/230Th) excess (22%) that is consistent with the recent addition of ²²⁶Ra-bearing fluid from the subducting slab to the mantle wedge. The dacite sample has equilibrium (²²⁶Ra/²³⁰Th), and the rhyodacite samples have (²²⁶Ra/²³⁰Th) deficits from 4-20%. These data are consistent with recent crystallization of amphibole during petrogenesis.
DE: 1000 GEOCHEMISTRY
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1120 Isotopic disequilibrium dating
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting