Graphite- and diamond-bearing eclogite xenoliths from the Bellsbank kimberlites, Northern Cape, South Africa

Abstract

One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bellsbank 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 specimens, while the single diamond-bearing eclogite is of the Group I variety. The carbon isotopic composition of the graphite varies from δ13C=−7‰ to δ13C=−2.8‰. This is within the range of carbon isotopic compositions for inclusion-free diamonds in kimberlite from this locality, suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source. The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly higher than the bulk earth estimate. Sr isotopic compositions of the clinopyroxene in these xenoliths vary from 87Sr/86Sr=0.703 to 87Sr/86Sr=0.706. This could be due to derivation of the xenoliths from a protolith with variable 87Sr/86Sr isotopic composition or could be the result of mixing between a low-Sr, high 87Sr/86Sr component and a high Sr, low 87Sr/86Sr component.