In situ multiphase LA-ICP-MS U-Pb Geochronology of terrestrial impact structures
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Date
2020-08
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University of New Brunswick
Abstract
In an attempt to improve the chronologic record of impact events on Earth, U-Pb geochronology has been conducted on shocked and thermally metamorphosed accessory phases (zircon, titanite and apatite) from several terrestrial impact structures using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). All dated phases occur as inherited grains derived from the underlying target lithologies and now occur within impact melt-bearing breccias and clast-laden melt rocks. This study provides the first application of the apatite U-Pb geochronometer from a terrestrial impact structure, and emphasizes the complexity of dating inherited grains within impact melt-bearing lithologies. Unlike newly-grown (igneous) grains within impact melt sheets, the results presented here highlight the challenges of obtaining precise and accurate impact ages from variably reset grains within complex lithologies. This approach requires an understanding of the relationship between isotopic resetting, extreme pressure-temperature (P-T) conditions and variable temperature-time (T-t) histories realized during impact events, impact-induced deformation microstructures, solid-state recrystallization, and pre-impact radiation damage within inherited grains. The results from this study have contributed to the chronological record of terrestrial impact events and the current understanding of U-Pb isotope systematics within U-bearing accessory phases during hypervelocity impact events. The use of a multiphase, in situ LA-ICP-MS U-Pb geochronological approach has permitted an inter-phase assessment on the comparative reliability of zircon, apatite and titanite as impact chronometers, while also providing insights into the U-Pb isotope systematics of these phases under extreme P-T-t conditions. The results reveal that isotopic resetting in apatite is thermally induced, inferred to be the result of apatite’s lower closure temperature and rapid Pb diffusivities. Consequently, apatite is determined to be more susceptible to isotopic resetting during short-lived temperature excursions compared to zircon and titanite. As such, this study demonstrates that, in the absence of coherent impact melt sheets and where structures remain tectonically undeformed, apatite is the most reliable U-Pb geochronometer for accurately dating terrestrial impact structures.
Under the same P-T-t conditions, zircon and titanite are found to be less useful impact chronometers and may yield geologically unmeaningful ages. Similar to apatite, isotopic resetting in titanite is primarily thermally induced. However, due to numerous factors including its higher closure temperatures and slower Pb diffusivities, titanite is prone to incomplete isotopic resetting, and is considered a particularly complex U-Pb impact chronometer. Unlike apatite and titanite, isotopic resetting in zircon is deformation enhanced. In addition to recrystallization-driven Pb loss, the results presented here demonstrate that, for the first time, radiation damage within pre-impact zircons facilitates isotopic resetting. However, metamict zircons are found to be susceptible to recent Pb loss and common Pb contamination, with lower intercept ages typically yielding anomalously young impact ages that are consistantly unreliable. The application of multiphase in situ LA-ICP-MS U-Pb geochronology has provided the first higher precision age constraints for four terrestrial impact structures in Canada: Nicholson Lake (387 ± 5 Ma), Lac La Moinerie (453 ± 5 Ma), Steen River (141 ± 4 Ma) and Brent (452.8 ± 2.7 Ma). Excluding Nicholson Lake, all structures yield ages that overlap with biological extinction events, with Steen River forming at, or close to, the Jurassic-Cretaceous boundary, and both Lac La Moinerie and Brent forming at, or close to, the Sandian-Katian boundary in the Upper Ordovician.