Meteoritic highly siderophile element and Re‐Os isotope signatures of Archean spherule layers from the CT3 drill core, Barberton Greenstone Belt, South Africa

1Seda Ozdemir, 1,2Toni Schulz, 3David van Acken, 4Ambre Luguet, 5W. Uwe Reimold, 1,6Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13234]
1Department of Lithospheric Research, University Vienna, , 1090 Vienna, Austria
2Institut für Geologie und Mineralogie, University Cologne, , 50674 Cologne, Germany
3Irish Centre for Research in Applied Geosciences (iCRAG), UCD School of Earth Sciences, University College Dublin, , Dublin 4, Ireland
4Steinmann‐Institut of Geology, Mineralogy and Palaeontology, University of Bonn, , 53115 Bonn, Germany
5Geochronology Laboratory, Instituto de Geociências, Universidade de Brasília, , 70910‐900 Brasília, DF, Brazil
6Natural History Museum, , A‐1010 Vienna, Austria
Published by arrangement with John Wiley & Sons

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day 187Os/188Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.

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