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 (Fo92-94), 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.
|Number of pages||7|
|Publication status||Published - 1984|