SULFIDE INCLUSIONS IN DIAMONDS FROM THE KOFFIEFONTEIN KIMBERLITE, SOUTH-AFRICA - CONSTRAINTS ON DIAMOND AGES AND MANTLE RE-OS SYSTEMATICS

Re-Os isotope compositions of syngenetic sulphide inclusions in both e clogite suite (E-type) and peridotite suite (P-type) parageneses in di amonds from the Koffiefontein mine, South Africa have been analysed us ing a technique capable of analysing single inclusion grains, or, in s ome cases multiple inclusions from the same diamonds. Sulphide inclusi on Ni contents broadly correlate with Os abundances in that low-Ni (6. 8-8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two hi gh-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type su lphides from the same diamond define the first mineral isochron obtain ed for a single diamond which has an age of 69 +/- 30 Ma with chondrit ic initial Os-187/Os-188. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplac ement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measu rements of a fragment from one of the sulphides, together with the abs ence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compos itions, Os-187/Os-188 0.346 To 2.28, and Re-Os model ages from 1.1 to 2.9 Ga. They define an array on a Re-Os isochron diagram that may be i nterpreted as defining a single period of E-type sulphide growth at 1. 05 +/- 0.12 Ga, with an elevated initial Os-187/Os-188, Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir , may be invoked in the Archaean and in the Proterozoic. The results f or both P- and E-type diamonds point to a spectrum of diamond crystall isation ages. High contents of both Re and Os, and the similarity of R e/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Rece nt addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the vari able, sometimes young Os model ages seen in whole rock xenolith Re-Os data. (C) 1998 Elsevier Science B.V. All rights reserved.