^1,2Martin Schmieder,³Winfried H. Schwarz,³Mario Trieloff,¹Eric Tohver,⁴Elmar Buchner, ⁵Jens Hopp,¹Gordon R. Osinski
¹School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
²Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
⁴HNU-Neu-Ulm University, Wileystraße 1, D-89231 Neu-Ulm, Germany
⁵Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, London, ON, N6A 5B7, Canada

The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario.New 40Ar/39Ar dating of two virtually fresh impact melt rock samples from the ⩾36 km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2 ± 2.2 (2.6) Ma (2σ; MSWD = 0.33; P = 0.57). In contrast, 40Ar/39Ar results for two chloritized melt rocks from the ∼26 km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around ∼460–470 Ma. No Permian signal was found in either of these melt aliquots. Our new 40Ar/39Ar results reproduce earlier 40Ar/39Ar plateau ages (∼283 Ma and ∼465 Ma, respectively) for the two impact structures by Bottomley et al. (1990) and are in conflict with a previous, statistically non-robust Rb-Sr age of 287 [293] ± 26 Ma for East Clearwater. The combined cluster of apparent ages of ∼460–470 Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a ‘false age effect’ due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-generated hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that clasts of Ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the East Clearwater Lake impact structure, which is overlain by >100 m of post-impact sandstones, shales and carbonates. No resolvable impactor contamination has so far been detected in the West Clearwater impact melt rocks, whereas East Clearwater carries a distinct ordinary (possibly L-) chondritic impactor signature in its melt rocks. East Clearwater Lake might thus represent one among a long list of Ordovician impact structures in North America and northern Europe that were presumably generated in response to the L-chondrite asteroid breakup event ∼470 Ma ago. Paleogeographic reconstructions show that the Ordovician East Clearwater impact probably occurred in a near-coastal to shallow marine setting, while the Permian West Clearwater impact hit continental Pangaea. Along with the new 40Ar/39Ar data, the paleomagnetic, sedimentologic, and paleogeographic findings suggest that the close spatial arrangement of the two Clearwater lakes is probably pure coincidence. The two impact structures seem to represent a ‘false doublet’ struck by impacts separated by ∼180 million years in time. The new results for the Clearwater Lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on Earth and on other planetary bodies in the inner Solar System.

Reference
Schmieder M, Schwarz WH, Trieloff M, Tohver E,Buchner E, Hopp J,Osinski GR (2014) New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) – Not the binary asteroid impact it seems? Geochimica et Cosmoschimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.037]

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