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

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Authors:Gurney, J. J.; Harris, J. W.; Rickard, R. S.
Author Affiliations:Primary:
Univ. Cape Town, Dep. Geochem., Cape Town, South Africa
Other:
Univ. Strathclyde, Dep. Appl. Geol., United Kingdom
Volume Title:Kimberlites; II, The mantle and crust-mantle relationships
Volume Authors:Kornprobst, J., editor
Source:Vol.(11B), p.3-9; Third international kimberlite conference, Clermont-Ferrand, France, Sept. 1982, edited by J. Kornprobst. Publisher: Elsevier Sci. Publ., Amsterdam, Netherlands. ISBN: 0-444-42274-9
Publication Date:1984
Note:In English; illus. incl. 3 tables
Summary: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 1135°-1212°C, averaging 1165°C, 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. [J.M.H.]
Subjects:Diamond; Diatremes; Electron probe data; Geochemistry; Igneous rocks; Inclusions; Intrusions; Kimberlite; Mineral composition; Mineral inclusions; Native elements; Oxides; P-T conditions; Partial melting; Petrology; Silicates; Ultramafic composition; Xenoliths; Africa; Botswana; Southern Africa; Northeastern Botswana; Orapa Mine
Abstract Numbers:85M/4672
Record ID:1984046713
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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