Determining the ages of single craters is critically important for trying to constrain the bolide flux to Earth, identifying clusters of impacts in Earth history, and understanding the links between impacts, large flood basalt eruptions, mass extinction events and stratigraphic boundaries. Obtaining accurate and precise ages for the Earths impact structures will help identify the links between these processes, therefore, this project aims to significantly improve the accuracy of isotopic ages of 20, large to small, impact structures on Earth. 81% of the known terrestrial impact structures have either never been dated, or have poorly constrained ages (i.e., >10% errors). The reason for these poor age constraints, and the main challenge in dating impact structures, is that the isotope systematics in 90% of the target rocks and minerals are not completely reset during a hypervelocity impact event. Commonly used geochronometers (e.g., U-Pb, Rb- Sr, K-Ar, 40Ar/39Ar and fission track) often yield a scatter of ages with poor accuracy and precision, which are hard to interpret. This project will apply the (U-Th)/He geochronological technique to date terrestrial impact events because this low-temperature geochronometer has many obvious advantages over traditionally-used dating techniques. Because of the unique properties of the radiogenic daughter product (4He) with respect to its low closure temperature and fast diffusivity, the (U-Th)/He dating technique should be capable of dating both newly crystallised and shocked crystals that are likely to have been reset by the brief, but very intense, thermal and shock metamorphism events associated with the formation of impact structures. A recent pilot (U-Th)/He study undertaken by the PIs has yielded exciting results, which demonstrate the potential of the (U-Th)/He method for the dating of impact structures. The results include: (a) agreement with existing well constrained geochronometer ages for Manicouagan and Bosumtwi, (b) more accurate isotopic ages for Bosumtwi and Lake Saint Martin than were available previously, and (c) new ages for two impact structures (Lake Saint Martin and Charlevoix). Based on these results, this project proposes to further explore the applicability and limits of the (U-Th)/He method to the dating of impact structures.
|Effective start/end date||7/1/10 → 6/30/15|
- National Science Foundation (NSF): $306,564.00
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