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Timescales of impact melt sheet crystallization and the precise age of the Morokweng impact structure, South Africa
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0001-8683-3860
Boise State Universiry.
Boise State University.
Boise State University.
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2021 (English)In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 567, article id 117013Article in journal (Refereed) Published
Abstract [en]

Impact cratering was a fundamental geological process in the early Solar System and, thus, constraining the timescales over which large impact structures cool is critical to understanding the thermal evolution and habitability of early planetary crusts. Additionally, impacts can induce mass extinctions and establishing the precise timing of the largest impacts on Earth can shed light on their role in such events. Here we report a high-precision zircon U–Pb geochronology study of the Morokweng impact structure, South Africa, which appears to have a maximum present-day diameter of ∼80 km. Our work provides (i) constraints on the cooling of large impact melt sheets, and (ii) a high-precision age for one of Earth's largest impact events, previously proposed to have overlapped the ca. 145 Ma Jurassic–Cretaceous (J–K) boundary. High-precision U–Pb geochronology was performed on unshocked, melt-grown zircon from five samples from a borehole through approximately 800 m of preserved impact melt rock. Weighted mean 206Pb/238U dates for the upper four samples are indistinguishable, with relative uncertainties (internal errors) of better than 20 ka, whereas the lowermost sample is distinguishably younger than the others. Thermal modeling suggests that the four indistinguishable dates are consistent with in situ conductive cooling of melt at this location within 30 kyr of the impact. The younger date from the lowest sample cannot be explained by in situ conductive cooling in line with the overlying samples, but the date is within the ∼65 kyr timeframe for melt-present conditions in footwall rocks below the impact melt sheet that is indicated by our thermal model. The Morokweng impact event is here constrained to 146.06 ± 0.16 Ma (2σ; full external uncertainty), which precedes current estimates of the age of the J–K boundary by several million years.

Place, publisher, year, edition, pages
2021. Vol. 567, article id 117013
Keywords [en]
impact melting, zircon, baddeleyite, high-precision geochronology, thermal modeling, Jurassic–Cretaceous boundary
National Category
Geology Geochemistry Geosciences, Multidisciplinary
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-4212DOI: 10.1016/j.epsl.2021.117013OAI: oai:DiVA.org:nrm-4212DiVA, id: diva2:1559103
Funder
EU, Horizon 2020, 792030Available from: 2021-06-01 Created: 2021-06-01 Last updated: 2021-11-19Bibliographically approved

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