154-6 Discordance and Destruction: A look into the Applications of Discordance Dating in Zircons for Bolide Impacts

Session: Asteroid Observations, Return Missions, and Meteoritics: Interweaving Perspectives and Data (Posters)

Presenting Author:

Authors:

Abstract:

     Small impact chronology can improve our understanding of surface processes and evolution. The key to associating an impact with an event in the rock record is by accurately and precisely determining the crater’s age of formation. While large, melt-sheet producing impacts are relatively easy to date with traditional geochronological tools, smaller impacts are more difficult. Historically, relative dating techniques use local stratigraphy and index fossils, as well as absolute dating with ⁴⁰Ar/³⁹Ar, to date post-impact materials. However, these methods rely on the presence of fossils or K-bearing mineral phases that have not experienced subsequent alteration events. Therefore, dating techniques using abundant recorders of the event are critical to accurately determine the age of an impact. Zircon is one such geochronometer, present in almost all siliciclastic sediments, capable of recording bolide impacts.

     Zircons can be dated using the U-Pb isotope system. However, the U-Pb systematics of zircon can be disturbed by post-crystallization processes such as metamorphism, fluid flow, and grain deformation. Several of which occur in small meteorite impacts. Hydrothermal fluids promote U-Pb isotopic disturbance in old, radiation-damaged zircon, leading to discordance in the U-Pb system. This effect can be leveraged for dating when the disturbance occurred. Discordance dating is a growing method for dating alteration events, like regional metamorphism, fluid events, and impact events (e.g., Reimink et al., 2024).

     Here we present results from a test case, using altered zircon to date the Middlesboro Crater located on the Cumberland Gap in Kentucky, a previously undated, relatively small meteorite impact crater (~5.5 km wide). We have extracted detrital zircon grains from Middlesboro impact target sandstones and analyzed them by laser ablation split-stream (LASS) ICP-MS combined with SEM imaging. We characterize the age, chemistry, and textural evidence of impact-induced alteration in the detrital zircon grains from the Middlesboro Crater and attempt to use discordance dating of zircon to assess the age of an impact event. We will also discuss the utility of this approach for small impact chronology across other target rocks globally.

Reimink, J. R., Beckman, R., Schoonover, E., Lloyd, M., Garber, J., Davies, J. H. F. L.,

        Cerminaro, A., Perrot, M. G., and Smye, A. J.: Discordance Dating: A New Approach for

        Dating Sedimentary Alteration Events. Geochronology Discuss.

        [preprint], https://doi.org/10.5194/gchron-2024-27, in review, 2024.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9110

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