40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada. (1st April 2020) - Pickersgill, A.E.
Abstract: 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 40 Ar/ 39 Ar analyses we have sought to understand the 40 Ar/ 39 Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40 Ar/ 39 Ar 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' 40 Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40 Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ 40 Ar/ 39 Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40 Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40 Ar by using isotope correlation plots to define the initial trapped 40 Ar/ 36 Ar 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.