3 GENERATIONS OF DIAMONDS FROM OLD CONTINENTAL MANTLE

INCLUSION-BEARING diamonds erupted by kimberlites are time capsules from the sub-continental mantle. Diamonds with peridotitic mineral inclusions ('peridotitic diamonds') from Cretaceous kimberlites in southern Africa have resided in stable mantle beneath the Kaapvaal craton since the Archaean1. Diamonds with eclogitic inclusions ('eclogitic diamonds') reflect episodic mantle events during the Proterozoic2,3. Here we discuss and present isotope data for a further category of diamonds from the Proterozoic Premier kimberlite, in which peridotitic inclusion minerals have compositions reflecting an origin in both harzburgitic (clinopyroxene-free) and lherzolitic (clinopyroxene-bearing) assemblages. The harzburgitic garnet inclusions and their counterparts found as isolated minerals ('macrocrysts') in the kimberlite host rock4 have neodymium and strontium isotope signatures consistent with an Archaean age (>3,000 Myr) and metasomatized mantle source. Lherzolitic garnet and clinopyroxene inclusions yield a preferred Sm-Nd isochron age of 1,930 Myr, which is approximately 100 Myr less than that of the adjacent Bushveld Complex5, suggesting a link analogous to that between harzburgitic diamond formation and komatiitic magmatism in the Archaean1. The final generation of diamonds, those with eclogitic inclusions, were formed approximately 1,150 Myr ago, just before kimberlite emplacement6.