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 con- tain diamond. One of the xenoliths has mineral composi- tions typical of low- T coarse textured garned peridotites, whereas minerals in the other are similar but not identi- cal to most peridotite-suite minerals included in dia- monds, especially in the low-CaO content of garnet. Geothermobaromet ric calculations show both xenoliths equilibrated at temperatures above 1,100~ C and pres- sures > 55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different 613C compositions between xenolith diamonds (-2.8 to -4.6%0) and diamonds in the kimberlite (generally < - 4.3%0) suggest different origins or sources for the dia- monds.