Nature and origin of an exceptional Cr-rich kyanite-bearing clinopyroxenite xenolith from Mbuji-Mayi kimberlite (DRC)

Abstract

A xenolith made of exceptionally Cr-rich minerals (mostly omphacite, kyanite and minor rutile) has been found in the diamondiferous Mbuji-Mayi kimberlite province, Democratic Republic of Congo (DRC). Chromium contents (Cr 2 O 3) are heterogeneously distributed in the bulk sample, as well as in a single crystal: 3.34-5.70 wt% in omphacite, 4.45-11.81 wt% in kyanite and 2.68-4.65 wt% in rutile. Such Cr-rich kyanite is extremely rare; only grospydite xenoliths from the Zagadochnaya kimberlite (Yakutia) have been reported with kyanite containing up to 12.86 wt% Cr 2 O 3. However, unlike typical grospydites, no garnet is observed in the Mbuji-Mayi Cr-rich kyanite-bearing clinopyroxenite. The textural features of this rock and the tentative thermobarometric equilibration conditions (25-35 kbar, 700-800 C) argue for an origin related to that of eclogites, which are the predominant xenoliths in the Mbuji-Mayi kimberlites. The calculated whole-rock composition suggests an Al-, Ca-and Cr-rich protolith that resembles Cr-rich leucogabbros or anorthosites commonly found in Archean anorthosite massifs. The peak P-T conditions registered by this unusual rock can be reached either by Archean subduction or by mafic magma underplating at the crust-to-mantle boundary with subsequent delamination into the upper mantle.

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

“…These different mineral species do not belong to a single paragenesis, but they are included in the kimberlite magma or are the fragments of xenoliths. The nodules are mostly eclogite, and rare peridotite and pyroxenite, or exceptional Cr-rich kyanite-bearing clinopyroxenite xenoliths (Pivin et al, 2011). These nodules do not contain corundum.…”

“…In the diamond-bearing Mbuji-Mayi kimberlites from the Democratic Republic of Congo (DRC), centimetre-sized gem-quality ruby and pink sapphire megacrysts occur along with diamond and other minerals (Demaiffe et al, 1991;Pivin et al, 2011). In the sapphire-bearing Changle alkali basalts from the People's Republic of China (PRC), two to three centimetre-sized dark blue or yellow to brown gem-quality sapphires occur with diopside, anorthoclase, magnetite, zircon and garnet megacrysts (Guo et al, 1996;Song and Hu, 2009).…”

Pressure-enforced Cr substitution in Cr1+xAl1−xO(SiO4), synthetic analogues of kyanite

“…Cr 3+ -rich kyanites that are also known to be from upper mantle settings. Sobolev et al (1968) and Pivin et al (2011) reported the presence of kyanites with up to 12.7 and 11.8 wt% Cr 2 O 3 from kimberlite-hosted grosspydite and clinopyroxenite xenoliths. Fe 3+ , too, is a common substituent in natural kyanite, however, with concentrations not exceeding approximately 1 wt% Fe 2 O 3 and usually on the order of a few hundred to a few thousand µg/g.…”

“…Fe 3+ , too, is a common substituent in natural kyanite, however, with concentrations not exceeding approximately 1 wt% Fe 2 O 3 and usually on the order of a few hundred to a few thousand µg/g. Other elements such as Ti, V, Mg, Ca or Na show concentrations ≤ 150-200 µg/g (e.g., Pivin et al 2011;Müller et al 2005Müller et al , 2016Pyka et al 2014).…”