DIAMONDIFEROUS GARNET HARZBURGITES FROM THE FINSCH KIMBERLITE, NORTHERN CAPE, SOUTH-AFRICA

Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has m ineral compositions typical of low-T coarse textured garned peridoties , whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low- CaO content of garnet. Geothermobarometric calculations show both xeno liths equilibrated at temperatures above 1,100-degrees-C and pressures > 55 kbar, which is near the low-pressure end of the range of equilib ration conditions for diamond-free garnet lherzolites and garnet harzb urgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch d iamonds. This, as well as the different delta-C-13 compositions betwee n xenolith diamonds (-2.8 to -4.6 parts-per-thousand) and diamonds in the kimberlite (generally < -4.3 parts-per-thousand) suggest different origins or sources for the diamonds.