HR: 09:00h
AN: B31E-05    [Abstracts]
TI: Trace Element Geochemistry of Matrix Glass from the Bedout Impact Structure,Canning Basin NW Australia
AU: * Poreda, R J
EM: poreda@earth.rochester.edu
AF: University of Rochester, Dept of Earth and Env.Sciences, Rochester, NY 14627 United States
AU: Basu, A R
EM: abasu@earth.rochester.edu
AF: University of Rochester, Dept of Earth and Env.Sciences, Rochester, NY 14627 United States
AU: Chakrabarti, R
EM: ramanandac@yahoo.com
AF: University of Rochester, Dept of Earth and Env.Sciences, Rochester, NY 14627 United States
AU: Becker, L
EM: lbecker@crustal.ucsb.edu
AF: University of California, Santa Barbara, Institute of Crustal Studies, Dept, of Geological Sciences, Santa Barbara, CA 93106 United States
AB: We report on geochemical and petrographic analysis of separated matrix glass from Lagrange-1 and Bedout-1 drill cores that penetrated the Bedout structure offshore NW Australia. The results support the conclusion that the Bedout structure was produced by a a major ET impact at the end-Permian that generated shock melted glass and impact breccias (Becker et al., Science, v.304, p.1469, 2004) The Bedout structure is a 30 km, circular, 1.5 km uplifted basment high that occurs on the passive margin offshore NW Australia. The isolated feature, covered by 3 km of Triassic to Recent sediments,is not consistent with any typical volcanic province (i.e. arc or hotspot volcanism). This hypothesis is supported by the unique mineralogy and chemistry of the matrix glass. At Lagrange, major elements crudely resemble low-K, Fe-Ti basalts while the trace element patterns have two distinct signatures. The lower 250 m of Lagrange (3260 - 3010 m depth) have essentially flat REE and "spider" patterns that superficially resemble some E-MORB; a signal not typically found in arc, hotspot or continental margin settings. The upper 150 meters (3000 - 2850m) of Lagrange and the entire Bedout core (3030 - 3070m) have similar light REE-enriched patterns but low levels of alkalis, alkaline-earths and high field strength elements. Again, the chemistry is not consistent with an arc or hotspot setting, based on the low Ba and extremely low Sr (30-110 ppm) concentrations. Based on the geophysical, chemical and petrologic evidence, we hypothesize that the Bedout structure formed as the result` of an ET impact with Permian age rift margin basalts and continental sediment. The basalts did not completely melt as evidenced by the abundance of large (1 mm) An$_{50}$ plagioclase,that exist as both crystalline plag and shock melted maskelynite. Plagioclase is the major repository of Sr in basalts and the lack of a plagioclase contribution to the melt glass is reflected in the low Sr abundance. Shock-transformed plagioclase glass that still retains the lath shape and chemistry of An$_{50}$ are one of the major indicators of the impact origin for Bedout.
DE: 5420 Impact phenomena (includes cratering)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Trace elements (3670)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting