HR: 1340h
AN: U23A-0855    [Abstracts]
TI: High-precision 40Ar/39Ar Age of the Janisjärvi Impact Structure (Russia)
AU: * Jourdan, F
EM: f.jourdan@curtin.edu.au
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AU: * Jourdan, F
EM: f.jourdan@curtin.edu.au
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94709, United States
AU: * Jourdan, F
EM: f.jourdan@curtin.edu.au
AF: Western Australian Argon Isotope Facility, Curtin university of Technology, Perth, WA 6845, Australia
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94709, United States
AU: Reimold, U W
EM: uwe.reimold@museum.hu-berlin.de
AF: Museum f. Natural History (Mineralogy), Humboldt-University, Invalidenstrasse 43, Berlin, 10115, Germany
AB: The ~14 km Jänisjärvi impact structure occurs within the Svecofennian Proterozoic terrains, in the southeastern part of the Baltic shield, Karelia, Russia. Previous K/Ar and ⁴⁰Ar/³⁹Ar studies were interpreted to give ages of 700 ± 5 Ma and 698 ± 22 Ma respectively, both results being difficult to interpret. Recent paleomagnetic results challenged those ages and propose instead ages of either 500 Ma or 850-900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new ⁴⁰Ar/³⁹Ar data for melt rocks from the crater. We obtained five concordant isochron ages (based on a total decay constant of 5.543 x 10^-10/y and an age of 28.03 Ma for the FCs standard) that yield a combined isochron age of 682 ± 4 Ma (2 sigma) with a MSWD of 1.2, P = 0.14 and ⁴⁰Ar/³⁶Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to refine the position of the Baltica paleocontinent at this time. Argon isotopic results imply that melt homogenization has been achieved at the hundred-micron scale certainly because of the low-silica content of the molten target rock that allows fast 40Ar* diffusion in the melt. However, the large range of F(⁴⁰Ar*_inherited) (3 to 8 percents) observed for seven grains show that complete isotopic homogenization was not reached at the centimeter and perhaps millimeter scale. This result is in good agreement with previous Rb and Sr isotopic data.
DE: 1100 GEOCHRONOLOGY
DE: 1115 Radioisotope geochronology
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
SC: Union [U]
MN: 2007 Fall Meeting