Diamond formation episodes at the southern margin of the Kaapvaal Craton: Re–Os systematics of sulfide inclusions from the Jagersfontein Mine

Abstract

Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have δ13C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial 187Os/188Os (187Os/188Osi) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with 187Os/188Osi of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower 187Os/188Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.