The composition of the lithospheric mantle beneath the Karowe Mine and its associated diamond sources in north-eastern Botswana

• Author / Creator

  Motsamai, Theetso

• This study presents the first comprehensive data on the recently developed Karowe diamond mine from the Orapa kimberlite cluster, which hosts mines such as Orapa, Damtshaa, and Letlhakane. The objectives of the study were to establish the compositional characteristics of the lithospheric mantle beneath the Karowe mine at the time of kimberlite eruption by studying the major- and trace-element characteristics of 24 mantle xenoliths and 106 clinopyroxene xenocrysts. In addition, the possible diamond sources beneath Karowe were evaluated through examination of major- and trace-element compositions of mineral inclusions in 120 diamonds. Finally, the physical characteristics of the diamonds themselves as well as their total nitrogen contents and carbon isotope compositions were investigated.The mantle xenoliths are predominantly melt-depleted garnet-free spinel peridotites (n = 14), with a small proportion of pervasively refertilised garnet-spinel lherzolites (n = 4), and variably metasomatised garnet-bearing pyroxenites (n = 3). The remaining three xenoliths are a garnet lherzolite, an eclogite, and a megacrystic olivine. The mineralogical evidence indicates that all these come from relatively low-pressure regions of the mantle outside of the diamond stability field. The predominant spinel lherzolites are characterised by coarse-grained and granolublastic textures, exsolution lamellae in clinopyroxenes and orthopyroxenes, and low equilibration temperatures (630 to 800 oC). They have high Mg# in olivines (median: 92.1) and orthopyroxenes (median: 92.4) as well as variable Cr# in spinels (13 to 47; median: 37). Clinopyroxenes show variable LREEN enrichment (LaN/SmN=0.24 to 3.8) which is consistent with secondary re-enrichement processes. Trace element compositions of the garnets and clinopyroxenes in the garnet-bearing xenoliths indicate cryptic melt metasomatism as well as modal metasomatism associated with the introduction of phlogopite. Clinopyroxene-based geothermobarometry on clinopyroxene xenocrysts (25 out of the106) and one garnet lherzolite xenolith indicate a 39 to 40 mW/m2 model paleogeotherm, which overlaps at greater depths with two non-touching garnet-orthopyroxene inclusion pairs recovered from Karowe diamonds. These data imply that the lithospheric mantle beneath Karowe is 210 km thick with a diamond window of approximately 90 km thickness, which is consistent with other estimates of lithospheric depths in this region of the Zimbabwe Craton derived from petrology and geophysics. The major- and trace-element compositions of mineral inclusions in the diamonds indicate that 53 % are derived from eclogitic sources, 44 % are peridotitic, 2% have a sublithospheric origin, and 1 % are websteritic. The sublithospheric inclusion suite consists of three eclogitic garnets containing a majorite component (>6.12 to 6.46 apfu Si; with [O] = 24). This new observation of superdeep mineral inclusions in Karowe diamonds is unique within the Orapa cluster and may provide a key link to the presence of exceptionally large diamonds from this mine.From the present diamond suite, Karowe diamonds have variable morphologies with a dominance of octahedra (30 %), macles (20 %), and moderately resorbed morphologies (transitional between octahedra and dodecahedra; 18 %). The remaining 32 % are made up of 10 % rounded dodecahedra, 4 % cubo-octahedra, 2 % pseudo-hemimorphic and 16 % aggregated crystals. Overall, diamonds have FTIR nitrogen contents that range from below the limit of detection (≤15 at.ppm) to 1217 at.ppm with variable nitrogen aggregation states (0 to 100 %B) and δ13C values from -34.5 to -2.0 ‰. Eclogitic diamonds contain 24 to 1217 at.ppm nitrogen with a median of 513 at.ppm and their carbon isotope compositions range between -21.5 and -2.5 ‰ with a median at -4.9 ‰. Peridotitic diamonds contain up to 937 at.ppm nitrogen with a median of 129 at.ppm. The majority of peridotitic diamonds fall into a typical mantle-like δ13C range (-8.6 to -2.0 ‰), with the exception of two diamonds that display unusual strongly 13C-depleted isotope compositions (-34.5 and -14.9 ‰). The mode in δ13C values for peridotitic diamonds (in class -4.0 to -3.5 ‰) is offset by about +1 ‰ relative to eclogitic diamonds (mode in class -5.0 to -4.5 ‰). These results could reflect derivation of mantle-like carbon from different sources through time for the two main parageneses or relate to the speciation of carbon fluids with constant δ13C (e.g., a minor increase in the CO2/CH4 ratio in the diamond-forming fluid from peridotitic to eclogitic diamonds).

• Subjects / Keywords

  • Orapa kimberlite cluster
  • mantle xenoliths
  • sublithospheric inclusions
  • Karowe diamonds

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-j6ws-k877

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.