title: 40Ar/39Ar geochronology of impact structures
Creators: name: Pickersgill, Annemarie Elisabeth
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Impact crater, Chicxulub, Boltysh, Gow Lake, 40Ar/39Ar geochronology.

description: Boltysh was of particular interest due to its chronological proximity to the Cretaceous- Palaeogene (K-Pg) boundary, but also due to the silicic melt rock that is known to decrease argon mobility, and therefore make impact melt rocks susceptible to extraneous 40Ar, causing older measured ages and/or discordant age spectra. For the first time, impact melt rocks from the Boltysh impact structure were analysed under the same experimental conditions as sanidine from the K-Pg boundary in Montana (IrZ sanidine), to provide the first high-resolution chronologic comparison of the Boltysh impact event and the K-Pg boundary. Results from Boltysh yielded a refined 40Ar/39Ar age of 65.47 ± 0.21 Ma. This age is three times more precise than the previously accepted 40Ar/39Ar age of 65.80 ± 0.64 Ma. It is also ~0.6 Ma younger than the palynologically derived age that suggested Boltysh predated the K-Pg boundary by 2-5 ka. The new 40Ar/39Ar age of 65.47 ± 0.21 Ma is ii preferred compared to previous ages, as it was reproducible across two samples in this study, is within uncertainty of the previous 40Ar/39Ar age, and is in agreement with the predicted cratering rate of ~1.2 impacts ≥ 20 km in diameter on land every million years. Therefore, the Boltysh impact event occurred 0.573 ± 0.231 Ma after the K-Pg boundary (66.043 ± 0.086 Ma). Chicxulub was an intriguing target for 40Ar/39Ar geochronology due to its temporal association with the K-Pg boundary. Impact melt rocks from the peak-ring, which were recently recovered by IODP-ICDP Expedition 364, were analysed under the same experimental conditions as IrZ sanidine and Chicxulub-related impact melt glass spherules from Beloc, Haiti and Gorgonilla, Colombia. This sample set provides the first high resolution chronologic comparison of material from within the Chicxulub impact structure, associated spherules, and the K-Pg boundary. Step-heating experiments on these samples yielded five age groups: 1) ~66 Ma (K-Pg age), including IrZ sanidine and all impact melt spherules; 2) a spread between ~66 and ~62 Ma; 3) ~62 Ma; 4) ~61 Ma; and 5) ~49 Ma. These age groups are interpreted to represent the impact age (~66 Ma) followed by cooling of the hydrothermal system in at least two stages (~61-62 Ma), and a post-impact thermal event (~49 Ma). The hydrothermal system at Chicxulub therefore lasted approximately 6 Ma, three times longer than previously predicted by models (~2 Ma). The techniques refined in this thesis show that 40Ar/39Ar geochronology is an excellent technique for age determination of terrestrial impact structures, and can be applied with great efficacy even in challenging lithologies. It is therefore of great utility for deconstructing the thermal histories of extraterrestrial samples. Exploring the surfaces of other planets, particularly Earth’s Moon and Mars, is a primary goal of space agencies the world over. Increasing our knowledge of hypervelocity impact processes will not only help us to better understand Earth’s history, but will prepare us to study samples returned from Mars, Earth’s Moon, and asteroids, thereby increasing the scientific return of planetary exploration – which will be one of the most important and rewarding scientific endeavors over the coming decades.