Major and trace element composition of chromian diopsides from the Zagadochnaya kimberlite (Yakutia, Russia): Metasomatic processes, thermobarometry and diamond potential

The major and trace element composition of chromian diopsides and associate minerals from concentrates of the diamond-free Zagadochnaya kimberlite have been studied to gain an insight into the lithological variability of the underlying mantle and explore the possible reasons for the different diamond potential of contiguous portions of the Yakutian lithosphere. A few (Al, Cr, Na)-poor chromian diopsides (group I) have a megacrystic signature and are interpreted as spinel-facies magmatic segregates from melts related to the host kimberlite. Other (Al, Cr, Na)-rich chromian diopsides (groups II and III) originated from variously metasomatized garnet peridotites and, possibly, pyroxenites from relatively shallow mantle depths ( 130-160 km). Two main metasomatic stages are distinguished, which led to strong enrichments in LILE and LREE. The metasomatic processes recorded by group II diopsides are broadly similar to those shown by garnet peridotite xenoliths from the nearby, highly diamondiferous Udachnaya kimberlite. Group III diopsides document a peculiar, subsequent modification after reaction with ultra-alkaline melts, which led to variable enrichments in Na, Al, Cr and Fe and was probably related to the magmatic stage connected with the eruption of the host kimberlite. It is suggested that the barren nature of the Zagadochnaya kimberlite is primarily a consequence of the relatively shallow depth of mantle sampling. Extensive melt-peridotite interaction at shallow levels may have played a significant role in controlling the high-Mg, low-Ti composition of this kimberlite.