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AGE FOR GOW LAKE IMPACT STRUCTURE

Annemarie Pickersgill has published new study looking at recovering an accurate and precise age for the Gow Lake impact structure using Ar/Ar geochronology.


Pickersgill et al., 2020, 40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada (https://www.sciencedirect.com/science/article/pii/S0016703720300375)


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 laserin situ40Ar/39Ar analyses we have sought to understand the40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for40Ar/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 extraneous40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laserin situ40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of40Ar inherited from silica-rich melts, data still had to be corrected for extraneous40Ar by using isotope correlation plots to define the initial trapped40Ar/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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