HR: 1340h
AN: V33B-1466    [Abstracts]
TI: The Times Scale of Andesite Differentiation at Arenal Volcano, Costa Rica (1968-2003), Indicated by U-Th-Ra Disequilibria.
AU: * Tepley, F J
EM: ftepley@es.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Department, Santa Cruz, CA 95064 United States
AU: Lundstrom, C C
EM: lundstro@uiuc.edu
AF: University of Illinois, Urbana-Champaign, Department of Geology, Urbana, IL 61801 United States
AU: Williams, R W
EM: williams141@llnl.gov
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Science Division, L-231, Livermore, CA 94551 United States
AU: Murrell, M T
EM: mmurrell@lanl.gov
AF: Los Alamos National Laboratory, C Division, MS J514, Los Alamos, NM 87545 United States
AU: Goldstein, S J
EM: goldstein@lanl.gov
AF: Los Alamos National Laboratory, C Division, MS J514, Los Alamos, NM 87545 United States
AB: Arenal Volcano in Costa Rica has continuously erupted since 1968 evolving in a complex fashion involving crystal fractionation, magma mixing, degassing, and wall-rock interaction (Reagan et al., 1987; Cigolini, 1998). We have measured trace-element concentrations and U-series disequilibria in whole rocks and mineral separates (pyroxene, plagioclase, magnetite) over the course of the eruption from 1968 to 2003 by ICP-MS, TIMS and PIMMS techniques. Whole rock and mineral separate analyses (n$>$20) show only minor variation in ($^{230}$Th)/($^{232}$Th) (1.10 to 1.18). In contrast, ($^{230}$Th)/($^{238}$U) range from 0.91 to 1.04 reflecting the moderate spread in Th/U. Th/U consistently change from low values in the early samples (2.4) to a constant, higher value from 1986 to present (2.65). The observed U-Th disequilibria are consistent with a time scale of mineral formation being much less than that of $^{230}$Th decay ($<$10$^{4}$ years -they are not old xenocrysts) for all samples in the study. A further implication is that mantle-derived recharge magmas entering the Arenal magma chamber either have changed through time from more U-enriched to less U-enriched, or that assimilation has changed the Th/U of the system but not ($^{230}$Th)/($^{232}$Th) (but the assimilant has to be very young Arenal cumulates or fortuitously have the same ($^{230}$Th)/($^{232}$Th)). Ra isotope data are being analyzed to evaluate whether differentiation and mineral formation in this system operate on a time scale closer to the half life of Ra (1600 years).
DE: 1749 Volcanology, geochemistry, and petrology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting