ECLOGITIC DIAMONDS OF PROTEROZOIC AGE FROM CRETACEOUS KIMBERLITES

DIAMONDS with eclogitic inclusions ('eclogitic diamonds') from the Premier kimberlite and Argyle lamproite were previously shown to be of Proterozoic age1-3 and distinct in origin from peridotitic diamonds of Archaean age4 and deep-continental-mantle (tectospheric5) provenance. The Premier and Argyle host rocks are themselves of Proterozoic age, suggesting a link between eclogitic diamond formation and kimberlite or lamproite magmatism1. Here we test this link 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 1,580 Myr respectively, compared with host kimberlite emplacement ages of ∼ 100 Myr. These eclogitic diamond ages are again Proterozoic, and distinct from the ∼3,200-Myr age of Finsch peridotitic diamonds4. Thus 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 40Ar-39Ar laser-probe dating technique as applied to Premier diamonds2,3. © 1990 Nature Publishing Group.