Silicate and oxide inclusions in diamonds from the Orapa mine, Botswana

In approximate order of decreasing abundance, garnet, sulphides, clinopyroxene, chromite, olivine and orthopyroxene occur as inclusions in Orapa diamonds. One coesite has been identified. Except for the olivines (Fo_92-94), the common minerals show a wide range in chemistry; related to a minor low-Ca, high-Mg, harzburgitic assemblage, an inferred lherzolite field, a websterite field and an eclogite field terminating with high-Ca garnets and jadeitic clinopyroxenes typical of kyanite eclogite. Mineral compositions in single diamonds and tie-lines for co-existing phases indicate that inclusions approximate to equilibrium assemblages. Calculations suggest formation of garnet-clinopyroxene pairs and a single garnet-olivine pair within an interval of 1135o-1212oC, averaging 1165oC, and equilibration P of 50-60 kbar for garnet-orthopyroxene. The websterite field is expanded in the diamonds relative to Orapa xenoliths and this and other geochemical features are interpreted as probably due to an igneous process, involving volatile-induced partial melting of pre-existing eclogite or garnet lherzolite. The inclusion suite is unusual for southern Africa because: 1) eclogitic and websteritic mineral compositions predominate; 2) there is no readily apparent compositional gap between the peridotitic and non-peridotitic associations; 3) the orthopyroxenes have a wide range in Mg/Fe; and 4) chromites are relatively common. (Following abstracts)-J.M.H.