40Ar-39Ar Ages of the Large Impact Structures Kara and Manicouagan and their Relevance to the Cretaceous-Tertiary and the Triassic-Jurassic Boundary

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Since the discovery of the iridium enrichment in Cretaceous- Tertiary boundary clays by Alvarez et al. (1980) the search for the crater of the K/T impactor is in progress. Petrographic evidence at the K/T boundary material points towards an impact into an ocean as well as onto the continental crust, multiple K/T impacts are now being considered (Alvarez and Asaro, 1990). One candidate is the Kara crater in northern Siberia of which Kolesnikov et al. (1988) determined a K-Ar isochrone age of 65.6 +- 0.5 Ma, regarding this as indicating that the Kara bolide is at least one of the K/T impactors. Koeberl et al. (1990) determined ^40Ar-^39Ar ages of six impact melts ranging from 70 to 82 Ma and suggested rather an association to the Campanian- Maastrichtian boundary, another important extinction horizon 73 Ma ago (Harland et al., 1982). We dated with the ^40Ar-^39Ar technique four impact melts, KA2- 306, KA2-305, SA1-302 and AN9-182. The spectra have rather well- defined plateaus, shown with highly extended age scales (Fig. 1). The plateau ages range from 69.3 to 71.7 Ma. Our data do not support an association either with the Cretaceous-Tertiary or with the Campanian-Maastrichtian boundary. We deduce an age of 69-71 Ma for the Kara impact structure. Nazarov et al. (1991) have demonstrated by isotopic hydrogen studies that the Kara bolide impacted on dry land, while the last regression at the target area before the end of the Cretaceous occurred 69-70 Ma ago. Our data are consistent with an impact shortly after the regression. We further dated impact metamorphic anorthosite samples (10BD5 and 10BD3C) of the Manicouagan crater, Canada, which may be related to the Triassic-Jurassic boundary (McLaren and Goodfellow, 1990). The samples consist of two different phases, one degassing at low temperatures yielding a plateau age of 212 Ma and another phase which was degassed during the cratering event to varying degrees with apparent ages increasing up to 950 Ma, the age of the target rocks (Wolfe, 1971). The low temperature plateaux are in agreement with the crater age of 212 Ma (Grieve, 1991) and do not improve the age of the impact structure. Anyway, while the crater age is quite accurate the ages of the adjacent geologic boundaries seem to be not. The last revision of the Triassic-Jurassic boundary (Harland et al., 1982) approved an age of 213 Ma, while later an age of 208 Ma was determined (Palmer, 1983). We think as far as ages are concerned it is not yet possible to conclude or exclude an association of the impact with the boundary until the age of the boundary is determined more precisely. References: Alvarez, L.W. , Alvarez, W., Asaro, F. and Michel, H.V. (1980) Science, 208, 1095-1108. Alvarez, L.W. and Asaro, F. (1990) Scient. Amer., 362. Grieve, R.A.F. (1991) Met., 26, 175- 194. Harland, W.B., Cox, A.V., Llewellyn, P.G., Pickton, C.A.G., Smith, A.G., and Walters, R. (1982) A geologic time scale. Cambridge Univ. Press. Koeberl, C., Sharpton, V.L., Murali, A.V. and Burke, K. (1990) Geology, 18, 50-53. Kolesnikov, E.M., Nazarov, M.A., Badjukov, D.D. and Shukolyukov, Y.A. (1988) Conf. on Glob. Catastr. in Earth Hist. LPI, Houston, Texas (abstract), 99-100. McLaren, D.J., and Goodfellow, W.D. (1990) Ann. Rev. Earth Planet. Sci., 18, 123-171. Nazarov, M.A., Devirts, A.L., Lagutina, E.P., Alekseev, A.S., Badjukov, D.D. and Shukolyukov, Y.A. (1991) Lunar Planet. Sci. (abstract) 22, 961. Palmer, A.R. (1983) Geology, 11, 503-504. Wolfe, S.H. (1971) J. Geophys. Res., 76, 5424-5436.

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