HR: 16:15h
AN: U24C-02    [Abstracts]
TI: Osmium Isotopic Evidence Against an Impact at the Frasnian-Famennian Boundary
AU: Gordon, G W
EM: gwyneth.gordon@asu.edu
AF: Arizona State University, School of Earth & Space Exploration MC 871404, Tempe, AZ 85287-1404, United States
AU: Gordon, G W
EM: gwyneth.gordon@asu.edu
AF: Yale University, Department of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520-8109, United States
AU: * Turekian, K K
EM: Karl.turekian@yale.edu
AF: Yale University, Department of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520-8109, United States
AU: Rockman, M
EM: mrockman@Princeton.EDU
AF: Yale University, Department of Geology & Geophysics P.O. Box 208109, New Haven, CT 06520-8109, United States
AU: Rockman, M
EM: mrockman@Princeton.EDU
AF: Princeton University, Lewis-Sigler Institute for Integrative Genomics, Princeton, NJ 08544, United States
AU: Over, J
EM: over@geneseo.edu
AF: SUNY College at Geneseo, Department of Geological Sciences, Geneseo, NY 14454, United States
AB: Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and ¹⁸⁷Os/¹⁸⁸Os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial ¹⁸⁷Os/¹⁸⁸Os of ~0.49, does not show the characteristic meteoritic Os imprint with a ¹⁸⁷Os/¹⁸⁸Os value of about 0.13. Both Re and Os are retained in this section, as indicated by the construction of an isochron with an age of 388 ±41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (¹⁸⁷Os/¹⁸⁸Os ranges from 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. This open system behavior prevents the reconstruction of an initial ¹⁸⁷Os/¹⁸⁸Os value for the boundary at La Serre. Assuming reasonable Re concentrations prior to loss, however, the Os isotopic value is inconsistent with a large meteoritic component. In addition, this study reinforces the need for Os isotopic evidence, not only enriched PGE concentrations, as substantiation for a meteoritic impact.
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1115 Radioisotope geochronology
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 9617 Devonian
SC: Union [U]
MN: 2007 Fall Meeting