Diamond eclogite and graphite eclogite xenoliths from Orapa, Botswana

Four eclogites with both diamond and graphite, and 20 graphite eclogites are described. All the diamond-bearing rocks and most of the graphite eclogites contain minor amounts of Na₂O in their garnet and K₂O in their clinopyroxenes, and are classed as Group I eclogites (after MacGregor & Carter, 1970). Four of the graphite-bearing rocks are classed as Group II eclogites. The diamond eclogites tend to contain relatively magnesian garnets and to give higher estimated T of equilibration than the Group I graphite eclogites. A relatively deep origin is postulated for the diamond eclogites. The Group II graphite eclogites are geochemically distinct from those of Group I. They give relatively low estimated T of equilibration and appear to be derived from shallower depths. There is no reason to link the Group II eclogites with diamond-forming processes. Geochemical evidence suggests that many of the diamond-bearing eclogites are not directly related to the eclogitic inclusions in diamond and the silicates intergrown with diamond aggregates which have been studied from the Orapa kimberlite. Some fundamental differences between the xenolith diamonds and those from the kimberlite indicate that the former are not dominant in the Orapa mine diamond population. This is notwithstanding that the eclogitic paragenesis predominates amongst the Orapa diamonds and that the xenoliths studied are many thousands of times richer in diamonds than the kimberlite.-J.M.H.