Isotopic dating of impact craters: Identification of impact metamorphosed zircon grains suitable for U-Pb dating

• Applicant: Dr. Winfried Schwarz
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 466718947

Dating terrestrial impact craters is important to constrain the flux of (large) extraterrestrial bodies to Earth over time. Precise crater age data also allow to evaluate the influence of individual impacts on the terrestrial geosphere and biosphere and to assess the risk of possible future impacts. Moreover, a comparison of the terrestrial cratering record with other bodies in the solar system allow to understand dynamic processes controlling the impactor flux in the history of the solar system. Precise chronological data furthermore constrain impact cratering as process itself, e.g., timing of the impact and post-impact cooling history by applying different isotopic age dating systems. Any isotopic dating method faces the problem that previously accumulated radiogenic isotopes may not be totally removed or equilibrated during the impact cratering process, leading to an incomplete reset and incorrect ages due to inherited radiogenic isotopes. This problem is even more pronounced for more refractory age dating systems like U-Pb dating of zircons. In order to find suitable, completely reset zircon grains during impact processes, some recent studies applied electron backscatter diffraction (EBSD) to identify so called “former reidite in granular neoblastic” (FRIGN) zircon grains, suitable for U-Pb dating of the meteorite impact age. These zircons evolved through a high p-T path leading to formation of the high-pressure mineral reidite, followed by recrystallization of neoblastic zircon domains. Such zircons were apparently completely reset and successfully used to constrain some impact crater ages. However, due to the heterogeneous nature of impact cratering processes, not all zircons experience the p,T conditions necessary to form FRIGN zircon morphologies. Our aim is a systematic survey of impact metamorphic zircon grains (which was not done before) applying different analytical methods. Our preliminary results on impact metamorphosed zircons indicate that not only FRIGN zircon grains (identified via EBSD) can show the correct impact age, partly with newly crystallised minerals, like baddeleyite, but also zircon grains showing no typical signs of recrystallisation. We plan to analyse zircons systematically via EMP, XRD, EBSD and RAMAN analyses (performed at the Heidelberg University and University of Frankfurt) for different impact craters. We also follow up other preliminary results that demonstrated that rare earth elements of impact zircon grains rather resemble hydrothermally overprinted than magmatic zircons, indicating post-impact fluid activity. We also found a correlation between the degree of reset of the U-Pb age system and the U (and also Th) content. This may indicate that zircon crystal structures more damaged by internal radiation were more easily reset, which will also be investigated in the proposed study. U-Pb age dating, REE and U,Th analyses, will be performed using the Cameca 1280 HR SIMS at Heidelberg University.

• DFG Programme: Research Grants