HR: 1340h
AN: V43D-1629    [Abstracts]
TI: U-Th Disequilibria in Island Arc Lavas from the Western Aleutians
AU: * Wyatt, C
EM: wyatt@sc.edu
AF: Department of Geological Sciences, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, SC 29208, United States
AU: Yogodzinski, G M
EM: gyogodzin@geol.sc.edu
AF: Department of Geological Sciences, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, SC 29208, United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: Woods Hole Oceanographic Institution, Clark 247B, MS22, Woods Hole, MA 02543, United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 58 Geochemistry, P.O.Box 1000, 61 Route 9W, Palisades, NY 10964, United States
AB: Th-excesses observed in MORB and OIB, where (²³⁰Th/²³⁸U) > 1, are commonly interpreted to result from melting in the presence of garnet. In contrast, U-excesses typical of island arc lavas, where (²³⁰Th/²³⁸U) < 1, are widely believed to originate when hydrous fluids from the subducting plate preferentially mobilize U. Analyses of island arc lavas from four sites in the western Aleutians reveal along-arc trends in trace element composition, including decreasing Y, increasing Sr/Y, and (²³⁰Th/²³⁸U) disequilibria, and suggest a possible westward increasing role for residual garnet in generating some of these subduction related melts. The lavas at Korovin Volcano, a subaerial volcano and the easternmost site in this study (174.1 °W), include basalts, andesites, and dacites with trace element characteristics typical of island arc lavas, including Sr/Y = 6- 30. Y increases with increasing SiO2, from 17 - 51 ppm. Three analyses of (²³⁰Th/²³⁸U) in two basalts and one dacite show ~6-17% U-excesses. At Little Sitkin Volcano (178.5 °E), basalts, andesites and dacites have Sr/Y = 13-25, and Y between 17-30 throughout the SiO2 range. U-excesses range from 2-8%. The Ingenstrem Depression is a pull-apart basin at the summit of the Aleutian ridge at 175 °E, within which are more than 90 submarine volcanic features. The basalts from the Ingenstrem Depression have high Mg# (0.64 - 0.72), Sr/Y <35, and low Y (15-20 ppm). Three analyses of basalts show both U- and Th-excesses. Three distinct andesite compositions are observed: one group of Mg# ~0.45 and Sr/Y = 80, another with Mg# ~0.6 and Sr/Y = 130, and a third with Mg# ~0.63 and Sr/Y = 32- 50. A single andesite analysis shows (²³⁰Th/²³⁸U) near secular equilibrium. The Ingenstrem dacites show a positive correlation between Mg# and Sr/Y, with Mg# from 55-64 and elevated Sr/Y between 84-166. One dacite has Th-excess, while another is close to secular equilibrium. Lavas from submarine Piip volcano (167.3 °E) range from the most primitive andesites in this study, with Mg# up to 0.72, to dacites and rhyolites with Mg# down to 0.44. Consistently low Sr and Y (400 and 18 ppm) give Sr/Y from 20-30. In five analyses, four Piip lavas have Th-excesses of 5-17%, and one has U-excess of 2%. The westward increase in Sr/Y, decrease in Y, and transition from exclusively U-excesses to Th-excesses observed in the western Aleutians suggest a westward emerging contribution of melts from a garnet bearing source. Eastward from this study area, where plate convergence is more nearly orthogonal, U-excesses are predominant (George et al., JGR, 2003). The oblique subduction environment of the western Aleutians may be favorable to the generation, transport, and eruption of these magmas with their unique geochemical features intact.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3075 Submarine tectonics and volcanism
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting