Investigating Earth's impact history through time using in situ high precision U-Pb dating

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Earth's early history (Hadean Eon) was affected by intense meteorite bombardment recorded on the Moon by impact basins. Dating of lunar impact melt and meteorites indicates ages ranging from ~4.2 to ~3.9 billion years old, with a preponderance towards the young end of the age spectrum. Evidence of the impacts have not survived on Earth's dynamic surface. Instead, we rely on mission-returned samples and meteorites. The precise time sequence of formation of such astro-materials can answer fundamental questions about Earth's beginnings. Did bombardment begin as a consequence of planet formation in the inner Solar System and continue throughout the Hadean? Or was it restricted to a late, intense `terminal cataclysm', as commonly cited? Earth's subsequent meteorite impact history is documented by ~190 known impact structures around the world, but precise age information for these events is severely lacking. This makes it difficult to assess their influence on Earth's evolutionary past. The proposed research will investigate the meteorite impact history of Earth using uranium-lead (U-Pb) dating in combination with an innovative extraction technique to isolate minerals within their geological context. For the first time, in situ ages on Lunar and Martian meteorites, and asteroids, will be determined using the latest advances in high precision U-Pb geochronology to establish precise ages to help clarify the time span of our early bombardment history. This is fundamental in understanding how the Earth and other planets evolved, and to how and why the impact flux rate changed through time. It has implications to understanding the emergence of life, which may have been negatively affected by early intense bombardment. In the terrestrial record, meteorite impacts have left traces of their effects in the geological and biological records. The time of impact cratering can be determined on minerals that crystallized, or were isotopically reset, during the impact process. Ages are important for assessing effects on the ecosphere, and to establishing flux rates, which can have implications for human populations. At least one meteorite impact is known to have played a role in a major mass extinction at the end of the Cretaceous. Samples from impacts (e.g., Brent, Carswell, Vredefort, Chesapeake Bay, and Sudbury) are in hand and will be investigated for their timing, age of target rocks, and effects of post-impact thermal overprinting. The proposed research is a new approach and will position Canada as unique in the world. High precision in situ dating on impacts will build upon my history of impact dating. Results will advance the field of planetary evolution and help evaluate past environmental response to impacts through correlations in the stratigraphic record. It is a uniquely Canadian effort in uniting the birthplace of modern U-Pb dating and nanobeam mineral investigation with the only available instrument for micro-milling minerals.