Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?
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Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4–16 wt%) of garnets and the Mg# (69–92) and jadeite contents (14–48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39–40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere–transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.
KeywordsZimbabwe Craton Orapa kimberlite cluster Sublithospheric Majorite
Lucara Diamond Corporation is sincerely thanked for the provision of production diamonds for this study and JWH also extents gratitude to the Company for the hospitality and resources provided, which enabled the diamonds to be collected. John Gurney (Cape Town) is thanked for bringing the project to the Lucara board and getting us started. Andrew J. Locock is thanked for his assistance and advice related to microprobe analyses. T.M. thanks Janina Czas and Nicole Meyer (University of Alberta) for comments, discussions and advice that helped to improve this manuscript. Thoughtful reviews by Sonja Aulbach (Frankfurt) and Ben Harte (Edinburgh) are gratefully acknowledged. T.M. received a bursary from the Government of Botswana as part of Pre-University Academic Programmes under Botswana International University of Science and Technology (BIUST). T.S. acknowledges research funding through an Natural Sciences and Engineering Research Council (NSERC) Discovery Grant and the Canada Research Chairs program.
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