Three generations of diamonds from old continental mantle

Abstract

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 Proterozoic^2–3. 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 Iherzolitic (clinopyroxene-bearing) assemblages. The harzburgitic garnet inclusions and their counterparts found as isolated minerals ('macrocrysts') in the kimberlite host rock⁴ have neodym-ium 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 ∼100 Myr less than that of the adjacent Bushveld Complex⁵, suggesting a link analogous to that between harzburgitic diamond formation and komatiitic magma tism in the Archaean¹. The final generation of diamonds, those with eclogitic inclusions, were formed ∼1,150 Myr ago, just before kimberlite emplacement⁶.