Geochemistry of a confirmed Precambrian impact ejecta deposit: The Grænsesø spherule layer, South Greenland

1,2Matthew S. Huber,2,3Christian Koeberl,4 Frank C. Smith,4Billy P. Glass,5Roland Mundil,6Iain McDonald
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13271]
1Department of Geology, University of the Free State, 205 Nelson Mandela Dr., Bloemfontein, South Africa
2Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A‐1090 Vienna, Austria
3Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
4Department of Geological Sciences, University of Delaware, Penny Hall, 255 Academy Street, Newark, Delaware, 19716 USA
5Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California, 94709 USA
6School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff, CF10 3AT UK
Published by arrangement with John Wiley & Sons

Samples from a single outcrop of the Graenseso spherule layer, Midternaes, South Greenland, consist of a spherule‐bearing dolomixtite with matrix‐supported intraclasts up to 1 m in size. In addition to field observations, we performed mineralogical and whole rock geochemical analysis, including electron microprobe, neutron activation analysis, X‐ray fluorescence, and mass spectrometry of the horizon and the overlying and underlying strata. We show that the spherules are petrographically similar to those in the Zaonega spherule layer, Karelia, Russia. Our petrographic and chemical results are consistent with the previous suggestion that the Grænsesø layer correlates with the Zaonega layer, and it is possible that both layers are related to the Vredefort impact event. The samples containing spherules, as well as the overlying rocks, have elevated REEs compared to the underlying pre‐impact layer, suggestive of a new continental source of sediment that may be coincident with the impact event. Zircons separated from the lower part of the Grænsesø spherule layer display complex age patterns suggesting that they have genetically different origins based on distinctly different Th/U ratios. Crystallization ages of all groups are ≥ 2.8 Ga, with ~2.8 Ga representing a time of major crustal growth globally. Therefore, we cannot conclusively determine in this study if the zircons are locally derived or if they are transported with the ejecta. The spherule layer was deposited by a high‐energy, subaqueous debris flow, an origin that is consistent with the hypothesis that the layer was deposited by impact‐induced waves and/or currents.

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