HR: 1340h
AN: B33C-0268    [Abstracts]
TI: Osmium-isotope Evidence for a Projectile Component in Impact-melt Rocks, Chesapeake Bay Impact Structure, Virginia, USA
AU: * Lee, S
EM: leesr@geol.umd.edu
AF: Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Horton, J W
EM: whorton@usgs.gov
AF: U.S. Geological Survey, 926A National Center, 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742 United States
AB: The late Eocene Chesapeake Bay impact structure (CBIS) is preserved beneath post-impact sediments on the Atlantic margin of Virginia. This 85-km-diameter complex crater formed on the continental shelf of a passive margin in a layered target consisting of ocean water, Cretaceous and Tertiary sediments (mainly siliciclastic), and crystalline basement rocks. The basement rocks include Neoproterozoic granitoids and felsite as well as gneiss of undetermined age. In May, 2004, the USGS drilled an 823-m test hole in the central uplift of the CBIS at Cape Charles, Va., providing drill cuttings and limited core. The core from 744 to 823 m depth contains crystalline-clast breccia and brecciated gneiss that are distinct from sediment-clast breccias recovered from coreholes in the annular trough of the CBIS. Rocks interpreted to be impact-melt clasts and dikes in the crystalline-clast breccia were sampled for analyses of osmium (Os) concentrations and $^{187}$Os/$^{188}$Os ratios to test for evidence of the projectile. These analyses were conducted on samples from a dike (aphanitic to partly hyaline, ST2440.8C) within a gneissic block, from a block of holocrystalline mafic rock (aphanitic, ST2453.3C), and from a flow-laminated bomb (aphanitic to partly hyaline, ST2570.0C). The Os concentrations and $^{187}$Os/$^{188}$Os ratios for samples ST2440.8C, ST2453.3C and ST2570.0C are 0.928, 0.711 and 0.312 ppb, and 0.15205, 0.15545 and 0.22345, respectively. These values are much higher (Os) or lower ($^{187}$Os/$^{188}$Os) than those reported for rocks of the upper continental crust, suggesting a significant contribution of osmium from the projectile in these impact-melt rocks. Moreover, a strong negative correlation between $^{187}$Os/$^{188}$Os and Os for these samples suggests that it may be possible to use mixing curves to calculate the proportions of projectile and target-rock components. Our results from the CBIS contrast with those from the Chicxulub crater, where there is little or no evidence for the incorporation of meteoritic material in samples of impact melt. The CBIS and the Popigai impact structure in Siberia have similar 35-36 Ma ages, leading to suggestions that they were produced by projectiles of similar composition (comet or asteroid?) from a late Eocene meteor shower. The CBIS is also proposed, based on age and location, to be the source of tektites in the North American strewn field. Analyses of additional impact melts and target rocks from the CBIS, and comparisons with data from Popigai melt rocks and North American tektites, could test these hypotheses.
DE: 9350 North America
DE: 9604 Cenozoic
DE: 5420 Impact phenomena (includes cratering)
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting