40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada

1,2A.E.Pickersgill,2,3D.F.Mark,1M.R.Lee,4,5G.R.Osinski
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.025]
1School of Geographical & Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
2NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride G75 0QF, UK
3Department of Earth & Environmental Science, University of St Andrews, St Andrews, KY16 9AJ, UK
4Institute for Earth and Space Exploration, University of Western Ontario, 1151 Richmond Street, London, ON, Canada
5Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, Canada
Copyright Elsevier

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ 40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ 40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

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