Proterozoic diamond formation at the Kaapvaal craton edge: Re-Os of Jagersfontein sulfide inclusions

The mantle sampled by the ~90 Ma Jagersfontein kimberlite, near the southern edge of the Kaapvaal craton block in South Africa, is unusual as the diamonds contain asthenospheric, transition zone and a preponderance of eclogitic inclusions. This unusual depth range of diamond provenance may indicate that the multiple episodes of diamond formation seen in other Kaapvaal kimberlites (3.4 to 1 Ga; Richardson et al. 1993; Shirey et al. 2004) are distinct at Jagersfontein.

Sixteen diamonds, examined for N levels and aggregation states, hosted Fe-sulfide inclusions that were analyzed for composition and Re-Os isotopes. Eleven stones have low N contents (15 – 57 ppm) similar to, but not exclusive of, sublithospheric diamonds (Tappert et al. 2005), N contents in
four stones range from 62 to 94 ppm, one stone contains 756 ppm. Based on N aggregation state, two diamond groups exist (<20 % vs. >33 % N_total as B centers) suggesting different mantle residence temperatures and/or ages. The majority of the sulfide Re/Os ratios lie between >0.5-46, typical of eclogite with komatiitic to basaltic precursors.

Sulfides from crack-free diamonds fall on two arrays in the Re-Os isotope diagram: one with a 1.7 +/- 0.11 Ga age and a highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os (0.45 +/- 0.07), the other with an overlapping age of 1.42 +/- 0.40 Ga and a less radiogenic initial ¹⁸⁷Os/¹⁸⁸Os of 0.11 +/- 0.13. The former may indicate Proterozoic diamond growth by remobilisation of Archaean mafic components in the lithosphere, whilst the latter may represent growth from new fluids added to the lithosphere. N aggregation states and inferred temperature of provenance suggest diamond crystallization at multiple depths. In stark contrast to all other Kaapvaal eclogitic sulfide suites studied to date, Archaean ages are absent in the present suite. This result may relate to unique aspects of the Jagersfontein kimberlite, which has transported and preserved very deep diamonds, or be a consequence of its location on the craton edge.