Silicate and oxide inclusion characteristics and infra-red absorption analysis of diamonds from the Klipspringer kimberlites, South Africa

Abstract

The diamonds associated with the 148 Ma Group II Klipspringer kimberlite dyke system emplaced on the Thabazimbi-Murchison Lineament are predominantly of eclogitic origin, and in a parallel study have been demonstrated to have a late Archean origin. Fourier Transform Infra-Red (FTIR) analysis of diamond plates demonstrates complex intergrowth of N-rich and N-poor diamond. Two groups of diamonds occurring in both the Main Fissure and the Sugarbird Blow have been recognised with time averaged mantle residence temperatures (MRT) based on nitrogen aggregation of approximately 1090°C (low-T) and 1170°C (high-T) respectively. In some cases a core of high-T diamond is enclosed within an envelope of low-T diamond. At Marsfontein, a third diamond population with an MRT < 1070°C is present that has not been recorded in diamonds from the Main Fissure or the Sugarbird Blow. Observed lamination lines attest to a deformation event having affected most of the high-T diamonds. A correlation between hydrogen and the ratio between nitrogen present in B aggregates and platelet peak intensity suggests that the presence of hydrogen affects the formation of platelets. Mineral inclusions in the diamonds are predominantly eclogitic (sulfide, garnet, clinopyroxene, kyanite, coesite and rutile). The garnets and clinopyroxenes have a wide range in compositions, extending the worldwide fields for diamond inclusions. The garnets define four groups, one of which is grospyditic and the individual groups display inter-element correlations, which are consistent with magmatic fractionation. The clinopyroxenes include a high aluminium group exhibiting cation site deficiencies (7 to 28% pseudojadeite). Garnets from all four groups, the high aluminium clinopyroxenes and other clinopyroxenes of widely different compositions occur in both the high-T and low-T diamonds. Estimated bulk compositions for diamond bearing eclogite are akin to MOR cumulates from the S.W. Indian Ridge. Thermobarometric estimates for four non-touching garnet-clinopyroxene inclusion pairs in low-T diamonds are within the range 1152 to 1233°C at 50kb. It is considered that the high-T Klipspringer diamonds formed in the Archean and underwent deformation followed by the low-T diamond formation in the host-rock to the high-T diamonds. The deformation event might have been associated with reactivation of the Thabazimbi-Murchison Lineament. The most likely protolith for the diamonds is subducted oceanic crust in which the inclusions of the low temperature diamonds formed by re-crystallisation of pre-existing minerals. At or shortly after the low-T diamond formation, a cool (37 to 39mW/m?) geotherm was established within this part of the Kaapvaal craton. The diamonds survived the emplacement of the Bushveld Igneous Complex and were subsequently sampled and transported by their host kimberlites in the late Jurassic.