Browsing by Author "Westerlund, K J"

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    A metasomatic origin for late Archean eclogitic diamonds: Implications from internal morphology of diamonds and Re-Os and S isotope characteristics of their sulfide inclusions from the late Jurassic Klipspringer kimberlites
    (2004) Westerlund, K J; Gurney, J J; Carlson, R W; Shirey, S; Hauri, E H; Richardson, S H
    The majority of the Klipspringer eclogitic diamonds display complex crystal zonation with zones of N- and H-rich cubic diamond alternating with zones of N-poor octahedral diamond. These growth features were likely related to different levels of carbon supersaturation in the diamond forming fluid(s). The diamonds are younger than their host rock and formed through differential flux of fluids into the eclogite. Re-Os isotope systematics of sulfide inclusions yield an isochron age of 2554 ± 150 Ma. The age overlaps the break-up of the Vaalbara supercontinent, the formation of the Ventersdorp lavas and a major re-activation of the Thabazimbi-Murchinson lineament into which the Klipspringer kimberlites intruded 148 ± 4 Ma ago. While a separate, relatively older generation of Klipspringer diamonds (Westerlund and Gurney, 2004) may be related to a proposed ~2.9 Ga craton-wide eclogitic diamond formation event (Shirey et al., 2001; 2002), a temporal connection between the current diamonds and the Ventersdorp lavas is inferred. Sulfur isotope compositions (δ34S = −1.8 to +2.4 ‰) of the sulfide inclusions overlap mantle values and large S isotope excursions indicative of altered MORB and associated sediment protoliths are absent. The data are compatible with slightly altered oceanic basalt or sub-lithospheric mantle sources for the S in these inclusions. The radiogenic initial Os isotope composition of the sulfide inclusions (187Os/188Os of 0.187 ± 0.046; γOs = +69) is not compatible with an origin of the PGE in the sulfides from the Ventersdorp lavas. The diamond growth characteristics along with the S and Os isotopic composition of their sulfide inclusions could be accounted for by the remobilization of local fluids in or proximal to the pre-existing eclogite host rock. We propose that the event that generated the Ventersdorp lavas may have raised the ambient lithospheric temperature and triggered fluid remobilization and the formation of the diamonds.
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    Silicate and oxide inclusion characteristics and infra-red absorption analysis of diamonds from the Klipspringer kimberlites, South Africa
    (2004) Westerlund, K J; Gurney, J J
    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.