Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?

Abstract

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 northeast 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/m 2 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.

Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana

“…Deep formation/storage near the base of the lithosphere is supported by the inferred T MR of 1195 °C in LK058 (66 at. ppm N with 71%B) that overlaps with equilibration temperatures calculated from olivine-garnet diamond inclusion thermometry from Orapa (1230 ± 80 °C; Stachel et al (2004b)), Karowe (1200 °C, 5.2 GPa; Motsamai et al (2018)) and the inferred geophysical structure of the SCLM beneath Letlhakane (Griffin et al 2003;Shirey et al 2002).…”

“…Application of the Cr/Ca-in-garnet barometer of Grütter et al (2006) using a 40 mW/m 2 geotherm yields 5.9-6.1 GPa (~ 200 km depth) for LK058 gnt A, B and 7.8-8.0 GPa for the high-Cr LK101 gnt A, B (~ 260 km depth). There are potentially increased uncertainties from microprobe measurements when using unpolished surfaces of the inclusions; however, these pressure estimates are comparable to those recorded in a harzburgitic garnet from the nearby Karowe mine (7 GPa; Motsamai et al (2018)). Deep formation/storage near the base of the lithosphere is supported by the inferred T MR of 1195 °C in LK058 (66 at.…”

“…4a) indicate metasomatic enrichment of a depleted protolith, as the garnets have low Zr (24-48 ppm) and Y contents (1.8-3.5 ppm). They are characterised by high LREE and low HREE ((La/Gd) N 295-467) as has been observed in garnet inclusions from Orapa and Karowe (Motsamai et al 2018;Stachel et al 2004b). Overall, harzburgitic diamonds (86% of peridotitic diamonds) are widespread in the SCLM (Stachel and Harris Fig.…”

Evaluating the Formation Pressure of Diamond‐Hosted Majoritic Garnets: A Machine Learning Majorite Barometer

“…All 221 available inclusion compositions were divided into regional datasets, consisting of diamonds sourced from Southern Africa, South America, Western Africa, China, North America, and Russia. Southern African diamonds originate from major South African mines including Jagersfontein, Monastery and Cullinan and also incorporate inclusion compositions from Botswana, Tanzania, and Zimbabwe (Kaminsky et al, 1997;Korolev et al, 2018;Moore et al, 1991;Moore & Gurney, 1985, 1989Motsamai et al, 2018;Pokhilenko et al, 2004Pokhilenko et al, , 2001Shatskii et al, 2010;Smith et al, 2009;Tappert et al, 2005;Tsai et al, 1979). Those from South America are predominantly from the kimberlite and alluvial sources in the Juina region (Bulanova et al, 2010;Burnham et al, 2016Burnham et al, , 2015Harte & Cayzer, 2007;Kaminsky et al, 2001;Meyer & Svisero, 1975;Smith et al, 2016;Thomson et al, 2014;Wilding, 1990;Zedgenizov et al, 2014).…”

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

“…The mineral compositions of diamond host rocks such as kimberlites have been extensively studied (e.g., Stachel & Luth, 2015). Diamonds form within subcontinental lithospheric mantle where necessary pressure and temperature conditions are achieved (the diamond stability field) or at greater depths within the asthenosphere (Motsamai et al, 2018). The locations of kimberlite fields in the Slave craton and its surroundings are superimposed on 100-km depth slices as green stars in Figures 6 and 7.…”