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

We used gamma-gamma coincidence spectrometry to investigate the possible pr esence of a meteoritical component in 27 samples of South African diamictit es. Recently, several studies have suggested that some tillites/diamictites may represent impact breccias, but a petrographical study by our group fou nd 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 w ith the new iridium coincidence spectrometer (ICS) at the University of Vie nna, with detection limits of around 0.02 ppb. No enrichments in the conten ts of Ir and other siderophile elements compared to average crustal concent rations were found; thus, no unequivocal evidence for an impact origin of t hese diamictites of the South African Dwyka Group can be documented.