Use of γ-γ coincidence spectrometry in the geochemical study of diamictites from South Africa

γ-γ coincidence spectrometry was used to investigate the possible presence of a meteoritical component in 27 samples of South African diamictites. Recently, several studies have suggested that some tillites/diamictites may represent impact breccias, but a petrographical study by our group found no evidence for the presence of impact-characteristic shocked minerals. The siderophile elements, such as Cr, Co, Ni, and, especially, the platinum group elements, have high abundances in meteorites, but low abundances in terrestrial crustal rocks. The Ir content of the diamictites was measured with the new iridium coincidence spectrometer (ICS) at the University of Vienna, with detection limits of around 0.02 ppb. No enrichments in the contents of Ir and other siderophile elements compared to average crustal concentrations were found; thus, no unequivocal evidence for an impact origin of these diamictites of the South African Dwyka Group can be documented. (author)