Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

Abstract

Olivine is the principal mineral of kimberlite magmas, and is the main contributor to the ultramafic composition of kimberlite rocks. Olivine is partly or completely altered in common kimberlites, and thus unavailable for studies of origin and evolution of kimberlite magmas. The masking effects of alteration, common in kimberlites worldwide, are overcome in this study of exceptionally fresh diamondiferous kimberlites of the Udachnaya-East pipe from the Daldyn-Alakit province, Yakutia, northern Siberia. The serpentine-free kimberlites contain large amount of olivine (~ 50 vol%) in a chloride-carbonate groundmass. Olivine is represented by two populations (olivine-I and groundmass olivine-II) differing in morphology, colour and grain size, and trapped mineral and melt inclusions. The large fragmental olivine-I is compositionally variable in terms of major (Fo85-94) and trace element concentrations, including H2O content (10-136 ppm). Multiple sources of olivine-I, such as convecting and lithospheric mantle, are suggested. The groundmass olivine-II is recognised by smaller grain sizes and perfect crystallographic shapes that indicate crystallisation during magma ascent and emplacement. However, a simple crystallisation history for olivine-II is complicated by complex zoning in terms of Fo values and trace element contents. The cores of olivine-II are compositionally similar to olivine-I, which suggests a genetic link between these two types of olivine. Olivine-I and olivine–II have oxygen isotope values (+5.6 ± 0.1 ‰ VSMOW, 1 std. dev.) that are indistinguishable from one another, but higher than values (+5.18 ± 0.28 ‰) in “typical” mantle olivine. These elevated values most likely reflect equilibrium with the Udachnaya carbonate melt at low temperatures and 18O - enriched mantle source. The volumetrically significant rims of olivine-II have constant Fo values (89.0 ± 0.2 mol%), but variable trace element compositions. Uniform Fo compositions of the rims imply absence of fractionation of the melt’s Fe2+/Mg, which can be possible in the carbonatite melt – olivine system. The kimberlite melt is argued to have originated in the mantle as a chloride-carbonate liquid, devoid of “ultramafic” or “basaltic” aluminosilicate components, but became olivine-laden and olivine-saturated by scavenging olivine crystals from the pathway rocks and dissolving them en route to the surface. During emplacement the kimberlite magma changed progressively towards an original alkali-rich chloride-carbonate melt by extensively crystallising groundmass olivine and gravitational separation of solids in the pipe.