New Ar-Ar Dating of the East and West Clearwater Lake Impact Structures, Québec, Canada – Evidence for Two Separate Impact Events
French Title: Nouvelle Datation Ar-Ar des Structures d’Impact du Lac à l’Eau Claire Est et Ouest, Québec, Canada – Mise en Évidence de Deux Impacts Distincts
Martin Schmieder
1, Winfried H. Schwarz
2, Mario Trieloff
3, Eric Tohver
4, Elmar Buchner
5, Jens Hopp
2,
Gordon R Osinski6 and Richard A F Grieve
7, (1)Lunar and Planetary Institute, Houston, TX, United States, (2)University of Heidelberg, Heidelberg, Germany, (3)Heidelberg University, Institute of Earth Sciences, Heidelberg, Germany, (4)University of Western Australia, Crawley, WA, Australia, (5)HNU Neu-Ulm University of Applied Sciences, Neu-Ulm, Germany, (6)University of Western Ontario, Earth Sciences, London, ON, Canada, (7)Natural Resources Canada, Ottawa, ON, Canada
Contact First Author: Martin Schmieder; martin@suevite.com
Previously Published Material: Schmieder M. et al. (2015) New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) – Not the binary asteroid impact it seems? Geochimica et Cosmochimica Acta 148, 304–324.http://www.sciencedirect.com/science/article/pii/S001670371400595X Phys.org/ScienceNetwork WA: http://phys.org/news/2014-10-scientists-debunk-clearwater-lakes-formation.html; http://www.sciencewa.net.au/topics/technology-a-innovation/item/3156-scientists-debunk-clearwater-lakes-formation-theory/3156-scientists-debunk-clearwater-lakes-formation-theory Spiegel Online, Germany: http://www.spiegel.de/wissenschaft/natur/meteoriten-ort-in-kanada-wurde-zweimal-getroffen-a-996562.html (in German) Der Standard, Austria: http://derstandard.at/2000007389514/Ein-und-dieselbe-Stelle-zweimal-von-kilometergrossen-Asteroiden-getroffen (in German) University of Heidelberg News: http://www.uni-heidelberg.de/presse/news2014/pm20141027_ein-kosmischer-doppelschlag-der-keiner-war.html (in German)
English Abstract:
For 50 years, the two Clearwater Lake impact structures in Québec have been considered as a typical crater doublet formed by the impact of a binary asteroid. New 40Ar/39Ar dating of melt rocks from the ≥36 km West Clearwater Lake (WCL) impact structure yielded two Early Permian plateau ages with a weighted mean age of 286.2 ± 2.2 (2.6) Ma (2σ; MSWD = 0.33; P = 0.57). 40Ar/39Ar results for two chloritized melt rocks from the ~26 km East Clearwater Lake (ECL) impact structure produced age spectra suggestive of extraneous argon. The age spectra corrected for the trapped argon component and inverse isochron plots consistently yielded ages around ~460–470 Ma for ECL, reproducing the 40Ar/39Ar results by Bottomley et al. (1990) and contradicting an earlier Rb–Sr age of 287 ± 26 Ma. The Ar-Ar dates obtained from four different melt samples across the melt sheet favor an Ordovician age for the ECL impact and impact-induced hydrothermal overprint. WCL and ECL, moreover, show different natural remanent magnetizations indicating separate geologic histories. Whereas WCL has no resolvable geochemical impactor traces, the ECL melt rocks carry a strong (possibly L-) chondritic impactor signature. The WCL impact affected a thin layer of Ordovician target carbonates; such rocks are absent in the ECL impact breccia, which is overlain by >100 m of post-impact sediments. Biostratigraphic dating of the fossil-poor post-impact deposits at ECL is currently underway. In the light of the new 40Ar/39Ar dates and in combination with the paleomagnetic and geochemical findings, the close spatial arrangement of WCL and ECL is probably pure coincidence. The two impact structures seem to represent a ‘false doublet’ struck by impacts ~180 million years apart. ECL possibly represents one of several impact structures in North America and Europe potentially related to the Ordovician L-chondrite breakup event ~470 Ma ago.
