GRAPHITE-BEARING AND DIAMOND-BEARING ECLOGITE XENOLITHS FROM THE BELLSBANK KIMBERLITES, NORTHERN CAPE, SOUTH-AFRICA

One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bells-bank kimberlites, Cape Province, South Africa . Graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms. Diamond is octahedral. Both Group I and Group II eclogite varieties are represented by the graphite-bearing specimen s, while the single diamond-bearing eclogite is of the Group I variety . The carbon isotopic composition of the graphite varies from delta(13 )C = -7 parts per thousand to delta(13)C = -2.8 parts per thousand. Th is is within the range of carbon isotopic compositions for inclusion-f ree diamonds in kimberlite from this locality, suggesting that the car bon for the eclogites as well as some of the kimberlite diamonds are d erived from the same source. The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly hi gher than the bulk earth estimate. Sr isotopic compositions of the cli nopyroxene in these xenoliths vary from Sr-87/Sr-86 = 0.703 to Sr-87/S r-86 = 0.706. This could be due to derivation of the xenoliths from a protolith with variable Sr-87/Sr-86 isotopic composition or could be t he result of mixing between a low-Sr, high Sr-87/Sr-86 component and a high Sr, low Sr-87/Sr-86 component.