Isotopic dating of impact craters: Identification of impact metamorphosed zircon grains suitable for U-Pb dating
Mineralogy, Petrology and Geochemistry
Final Report Abstract
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. Different isotopic systems were used for impact cratering dating and were more or less successful. The aim of this study was to evaluate a certain mineral, which usually is very stable during geological processes, but is also affected by the high temperature/pressure regime during an impact – zircon. Usually zircon grains can preserve the mother and daughter nuclides over a long time giving precise ages for the U decay series down to Pb, but sometimes the interpretation of the gained isotopic data is difficult, leading to not precise or even incorrect impact ages. Thus it is important to evaluate, which zircon grains are suitable for U-Pb dating and which are not. To evaluate these zircon grains, grains from 12 impact sites were analysed using different optical, geochemical, microstructural and isotopic methods. In a first step 6 of these impact samples were chosen for further analysis having a wide spread of different zircon grains in the specific fields mentioned before. The impact metamorphic grains were analysed with optical ((laser)microscopy), SEM (BSE- and CL-images), structural EBSD- and RAMAN as well as isotopic (U/Pb, U/Th, REE) methods. More than 150 zircon grains were analysed using these methods resulting in a large dataset for zircon grains in different stages of decomposition and recrystallization, leading to the most complete dataset for impact metamorphic zircon grains available so far. As a result it seem to be necessary but not sufficient, that the zircon grains were recrystallized showing in EBSD pole figures a typical pattern for so called “former reidite in granular neoblastic” (FRIGN) zircon grains created during a certain process of heat and pressure during the impact. Not all FRIGN zircon grains are usable for U-Pb dating leading to an age which is not compatible with the literature age, most of the FRIGN grains show too high ages, indicating radiogenic Pb accumulated so far is not released during impact. The U and Th content seem to play a role in decomposition of the zircon grains, because there is a clear trend between U- Pb ages an the U/Th content for all analysed samples, which could be due to the fact, that during U/Th decay there is radiation damage in the zircon grains up to metamictization. This may have in influence on the temperature and pressure, when Pb starts diffusion from the zircon and makes it easier to be completely reset, leading in a suitable U-Pb age. As a result of the analyses there is no clear rule for identifying impact metamorphic zircon grains usable for U-Pb dating, so it is still necessary to put all data together (including statistics) for the evaluation leading to a precise and accurate age for the impact event.
Publications
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U-Pb Dating of the Suvasvesi Impact Structures — Double Impact or Not. 55th LPSC, Abstract #1441.
W. H. Schwarz, M. Schmieder, E. Buchner, L. J. Pesonen & J. Moilanen
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U-Pb Dating of Zircon Grains from the Paasselkä Impact Structure. 55th LPSC, Abstract #1422.
W. H. Schwarz, M. Schmieder, E. Buchner, J. Moilanen & T. Öhman
