Eclogitic diamonds of Proterozoic age from Cretaceous kimberlites

The existence of a link between eclogitic diamond formation and kimberlite or lamproite magmatism is tested by examining eclogitic diamonds from younger southern African kimberlites. Eclogitic garnet and clinopyroxene inclusions in Orapa and Finsch diamonds yield Sm-Nd isochron ages of 990 and 1580 Myr respectively, compared with host kimberlite emplacement ages of about 100 Myr. These ecologitic diamond ages are Proterozoic and distinct from the about 3200 Myr age of Finsch peridotitic diamonds. Peridotitic and eclogitic diamonds from the same locality are evidently both xenocrysts, having been stored in the deep continental mantle beneath the Kaapvaal craton for extended periods before being sampled by kimberlite. Such mantle storage may not be detected by the Ar-40 - Ar-39 laser-probe dating technique as applied to Premier diamonds.