Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa

1,2Jörg Fritz,3Roald Tagle,4Luisa Ashworth,2Ralf Thomas Schmitt,5Axel Hofmann,6Béatrice Luais,4Phillip D. Harris,2,7Desirée Hoehnel,8Seda Özdemir,2,9Tanja Mohr-Westheide,9,10Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12736]
1Saalbau Weltraum Projekt, Heppenheim, Germany
2Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
3Bruker-Nano GmbH, Berlin, Germany
4GeoSpectral Imaging, Johannesburg, South Africa
5Department of Geology, University of Johannesburg, Johannesburg, South Africa
6Centre de Recherches Pétrographiques et Géochimiques, CRPG UMR 7358 CNRS-UL, France
7Institut für Erd und Umweltwissenschaften, Universität Potsdam, Potsdam-Golm, Germany
8Department of Lithospheric Research, University of Vienna, Vienna, Austria
9Institut für Geologische Wissenschaften, Freie Universität Berlin (FU Berlin), Berlin, Germany
10Natural History Museum, Vienna, Austria
Published by arrangement with John Wiley & Sons

A Paleoarchean impact spherule-bearing interval of the 763 m long International Continental Scientific Drilling Program (ICDP) drill core BARB5 from the lower Mapepe Formation of the Fig Tree Group, Barberton Mountain Land (South Africa) was investigated using nondestructive analytical techniques. The results of visual observation, infrared (IR) spectroscopic imaging, and micro-X-ray fluorescence (μXRF) of drill cores are presented. Petrographic and sedimentary features, as well as major and trace element compositions of lithologies from the micrometer to kilometer-scale, assisted in the localization and characterization of eight spherule-bearing intervals between 512.6 and 510.5 m depth. The spherule layers occur in a strongly deformed section between 517 and 503 m, and the rocks in the core above and below are clearly less disturbed. The μXRF element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in (1) sorting of two types of spherules and (2) occurrence of primary minerals (Ni-Cr spinel and zircon). We favor a single impact scenario followed by postimpact reworking, and subsequent alteration. The spherule layers are Al2O3-rich and can be distinguished from the Al2O3-poor marine sediments by distinct Al-OH absorption features in the short wave infrared (SWIR) region of the electromagnetic spectrum. Infrared images can cover tens to hundreds of square meters of lithologies and, thus, may be used to search for Al-OH-rich spherule layers in Al2O3-poor sediments, such as Eoarchean metasediments, where the textural characteristics of the spherule layers are obscured by metamorphism.

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