A detailed investigation into diamond-bearing xenoliths from Newlands Kimberlite, South Africa

Menzies, Andrew

A detailed investigation into diamond-bearing xenoliths from Newlands Kimberlite, South Africa

Thesis / Dissertation

2001

Publisher

University of Cape Town

Department

Department of Geological Sciences

Faculty

Faculty of Science

Abstract

This study involves a detailed investigation of diamond-bearing peridotite and eclogite from Newlands kimberlite, located on the Kaapvaal craton, South Africa. Eighteen peridotitic garnet macrocrysts (1 to 3 cm in size) containing diamond, and one with graphite, consist predominantly of lilac garnet with diamond (or graphite) ± chromite and altered silicates. The garnets are predominantly high-Cr(> 8 wt¾), Ti-depleted, and extremely sub-calcic {< 3 wt¾). However, three specimens have high-Cr calcic compositions. The chromites are Cr-rich, Ti-depleted, whilst the solitary olivine is Fo 93 .4. The sub-calcic garnets display a narrow range of trace element signatures, depleted in Zr, Y, Ga, and Ti, whilst Sr and LREE's are enriched. The [REE] 0 patterns are all very similar and display a sinusoidal shape. The most calcic diamondiferous garnet analysed at Newlands yields a [REE] 0 pattern similar to "fertile" mantle garnets. The major and trace element geochemistry of the sub-calcic garnets is similar to inclusions in diamonds and diamond-bearing peridotites world-wide. Geothennometry yields temperatures ranging from 900 to 1050 °C, equivalent to pressures of 40 to 55 kbar (based on a 37-38 mW/m2 geothenn determined from Newlands coarse peridotites). This P-T range is within the diamond stability field. Four diamond-bearing peridotitic garnet macrocrysts are Re depleted and yield a range of ages spanning from the Proterozoic (minimum TRD 1.77 Ga) to the mid-Archrean (maximum TMA 3.52 Ga). Seventeen eclogites (2 to 6 cm in size) containing diamond are dominated by coarse-grained garnet and clinopyroxene, with primary trace phases including sulphides and rutile. Alteration is pervasive and ranges from minor grain boundary effects to massive mineral (primarily clinopyroxene) replacement. The garnets have Na2O concentrations greater than 0.07 wt%, whilst the clinopyroxenes have K2O concentrations greater than 0.08 wt%. Both phases are Fe-rich and Ca- and Mg- poor relative to other eclogites from Newlands. The garnets display a very restricted range of trace element compositions and are relatively enriched in Ga, Zn, Mn and Ti, and depleted in Cr and Ni relative to most diamond-free eclogites from Newlands. The garnets have similar [REE] 0 patterns that are [HREE]n enriched. The clinopyroxenes also display a restricted range of trace element compositions (with one exception), and are relatively enriched in Ga, Zn, Zr, Mn and Ti, and depleted in Ni relative to diamond-free eclogites from Newlands. The clinopyroxenes have similar [REE]n patterns that are [LREE]n enriched. Bulle rock major element compositions (calculated assuming a 50:50 gamet-clinopyroxene ratio) are compositionally similar to ancient (Proterozoic or Archrean?) magnesian basalts. A statistical analysis of eclogites at Newlands indicates that those associated with diamonds have a discernible geochemistry. The diamond-bearing eclogites, with one exception, have textures and mineral geochemistry equivalent to Group I (and Group B) eclogites (depending on the classification scheme used). Geothermometry yields temperatures between 920 to 1080 °C, equivalent to pressures between 42 and 58 kbar (based on a 37-38 mW/m 2 geotherm), which lies within the diamond stability field. Re-Os systematics for the diamond-bearing eclogites are indicative of formation ages in the Archrean. These eclogites had 187 Os/1 88 Os substantially higher than chondritic mantle at circa 3 Ga. The Re-Os systematics, major and trace element mineral chemistry, and stable isotopes (albeit preliminary) of the diamond-bearing eclogites from Newlands are consistent with a protolith that has interacted within surficial environments. This implies the operation of plate tectonics since the early part of the Earths history. The peridotitic diamonds occur in two primary forms, namely (< I mm) single octahedra or octahedral aggregates. The eclogitic diamonds range in size from approximately 100 μm within diamond aggregates to large single crystals up to 2 mm. The dominant morphology is octahedral but there are also significant numbers of cubes and dodecahedrons, and some cubo-octahedrons. Cathodoluminescence indicates that the eclogitic diamonds grew in at least three distinct periods. FTIR spectra indicate that the peridotitic and eclogitic diamonds display different nitrogen concentrations and platelet peak positions. Specifically, the vast majority of harzburgitic diamonds are Type II (or near Type II) whilst all eclogitic diamonds are Type I, with significantly higher N concentrations. Based on these criteria, the majority of diamonds at Newlands are of eclogitic origin. Furthermore, this difference implies that the peridotitic and eclogitic diamonds at Newlands are derived from separate sources. Both sets of diamonds have type IaAB aggregation states ranging from 0 to 20 %, consistent with diamond formation in the Archrean for both parageneses. Time-averaged temperatures imply that the diamonds formed at higher temperatures than their ambient levels at the time ofkimberlite eruption. The early Proterozoic to mid-Archrean Re-Os ages obtained for both the diamond-bearing garnet macrocryst and diamond-bearing eclogites at Newlands overlap the major crustal building periods of the Kaapvaal craton ( de Wit et al., 1992). This implies that cratonic root stabilisation beneath the Kaapvaal craton is potentially coeval with crustal formation (Kramers, 1979; Richardson et al., 1984; Richardson et al., 1990; Richardson and Harris, 1997). The presence of diamond implies that the SCLM may have extended to depths of approximately 200 km by the end of the Archrean. In addition, the apparent lack of any systematic age variation with calculated equilibrium conditions implies that the SCLM and overlying continental crust nuclei have been coupled since formation. Once formed, these assemblages remain coupled and, under favourable circumstances, can survive the destructive forces of plate tectonics.

Keywords

Geological Sciences

Minerology

Reference:

Collections

PhD / Doctoral

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