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Meteoritic traces in Archean spherule layers in the Barberton Greenstone Belt, South Africa

Research questions

Archean spherule layers (SL) in the Barberton Greenstone Belt (BGB) are amongst the oldest known impact deposits on Earth. Spherules could be molten impact ejecta and condensation products from impact plumes or crater ejecta melted during atmospheric reentry. Barberton SL were originally identified by their excessive PGE contents. The search for phases hosting this extraterrestrial PGE signature lead to discovery of primary sub-µm sized platinum-group minerals (PGMs). The formation of such heterogeneous sub- and micrometer sized PGE micronuggets that have been discovered by our group for the first time in a recent pilot project on spherule layers in the ICDP BARB 5 drill core is controversial. Primary particles from the projectile, the product of impact melting, exsolution from the host or condensation in the impact plume have all been proposed as origin of these alloys and scientific knowledge still remains limited. A second drill core with plenty spherule layers from the northern BGB is now analyzed. This study is aimed at clarification whether the new observations from the recent BARB 5 project are symptomatic for other (all?) spherule layer intersections with respect to the composition of PGE micronuggets and the occurrence of such nuggets within and between individual spherules. The study of Archean spherule layers can provide key information regarding type and frequency of projectiles that impacted the Archean Earth and, thus, for deciphering the Earth’s ancient impact history and the impact history of the early solar system.

Methodology

High-resolution FE-SEM/EDX, TEM, EBSD and TKD techniques; Ru isotope systematics

Partners

  • Natural History Museum, London
  • Helmholtz-Zentrum Potsdam – Deutsches GeoForschungsZentrum (GFZ)
  • Bruker Nano Berlin
  • Bundesanstalt für Materialforschung und –prüfung (BAM)
  • University of Vienna 
  • Natural histoy museum of Vienna
  • University of Münster
  • Federal University of Campinas, Brasil
  • University of Brasilia, Brasil

Funding and duration

  • DFG funding: 15.07.2013-15.07.2015
  • EUROPLANET project funding: 09.11.-23.11.2016 

Selected publications

  • MOHR-WESTHEIDE, T., GRESHAKE, A., WIRTH, R. and REIMOLD, W. U. 2017.Transmission Electron Microscopy of Impact-Generated Platinum-Group Element Alloys from Barberton  Spherule Layers: New Clues to their Formation (submitted to Meteoritics & Planetary Science in September 2017
  • HOEHNEL, D., REIMOLD, W.U., ALTENBERGER, U., HOFMANN, A., MOHR-WESTHEIDE, T., OEZDEMIR, S., KOEBERL, C. 2017.Petrographic and Micro-XRF Analysis of Multiple Archean Impact-Derived Spherule Layers in Drill Core CT3 from the northern Barberton Greenstone Belt (South Africa). Journal of African Earth Sciences 138: 264-288.
  • OEZDEMIR, S., SCHULZ, T., KOEBERL, C., REIMOLD, W.U., MOHR-WESTHEIDE, T., HOEHNEL, D., SCHMITT, R.T. 2017.Early Archean Spherule Layers from the Barberton Greenstone Belt, South Africa: Mineralogy and Geochemistry of the Spherule Beds in the CT3 Drill Core. Meteoritics & Planetary Science 52:2586-2631. doi:10.1111/maps.12998
  • SCHULZ, T., KOEBERL, C., LUGUET, A., VAN ACKEN, D., MOHR-WESTHEIDE, T., OEZDEMIR, S. and REIMOLD, W.U. 2017. New constraints on the Paleoarchean meteorite bombardment of the Earth - Geochemistry and Re-Os isotope signatures of spherule layers in the BARB5 ICDP drill core from the Barberton Greenstone Belt, South Africa. Geochimica et Cosmochimica Acta 211:322-340. 
  • FRITZ, J., ROALD, T., ASHWORTH, L., SCHMITT, R.T., HOFMANN, A., LUAIS, B., HARRIS, P.D., HOEHNEL, D., OZDEMIR, S., MOHR-WESTHEIDE, T. and  KOEBERL, C. 2016. Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa. Meteoritics & Planetary Science 1–18 doi:10.1111/maps.12736.
  • MOHR-WESTHEIDE, T., REIMOLD, W. U., FRITZ, J., KOEBERL, C., SALGE, T., HOFMANN, A., SCHMITT, R.T. 2015. Discovery of extraterrestrial component carrier phases in Archean spherule layers: Implications for estimation of Archean bolide sizes. GEOLOGY, 43:299-302.