Abstract
The chromium isotopic compositions of samples from an early Archean (3.22 Ga) spherule bed (S4) from the Barberton Greenstone Belt, South Africa, are distinct from that in background rocks and other terrestrial samples. This positively confirms the presence of an extraterrestrial component in this bed and supports hypotheses of an impact origin. The source of the extraterrestrial Cr is most likely to be a carbonaceous chondrite, probably of the CV variety. The estimated extraterrestrial component in this spherule bed is about 15% based on Cr isotopes and platinum group elements. Since this is similar in many regards to the global fallout from the Cretaceous-Tertiary impact, we infer that they have a similar origin. However, the total amount of meteoritic material in the Archean spherule bed is about 30 times greater than in Cretaceous-Tertiary deposits, implying a much larger projectile, possibly 20 km or more in diameter.
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Shukolyukov, A., Kyte, F.T., Lugmair, G.W., Lowe, D.R., Byerly, G.R. (2000). The oldest impact deposits on earth — First confirmation of an extraterrestrial component. In: Gilmour, I., Koeberl, C. (eds) Impacts and the Early Earth. Lecture Notes in Earth Sciences, vol 91. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0027758
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DOI: https://doi.org/10.1007/BFb0027758
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