Olivine in the udachnaya-east kimberlite (Yakutia, Russia): Types, compositions and origins
Résumé
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 the origin and evolution of kimberlite magmas. The masking effects of alteration, common in kimberlites worldwide, are overcome in this study of the exceptionally fresh diamondiferous kimberlites of the Udachnaya-East pipe from the DaldynAlakit province, Yakutia, northern Siberia. These serpentine-free kimberlites contain large amounts of olivine (similar to 50 vol.%) in a chloridecarbonate 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 (Fo(8594)) 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 recognized by smaller grain sizes and perfect crystallographic shapes that indicate crystallization during magma ascent and emplacement. However, a simple crystallization 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 parts per thousand VSMOW, 1 SD) that are indistinguishable from one another, but higher than values (+5.18 +/- 0.28 parts per thousand) in typical mantle olivine. These elevated values probably reflect equilibrium with the Udachnaya carbonate melt at low temperatures and O-18-enriched mantle source. The volumetrically significant rims of olivine-II have constant Fo values (89.0 +/- 0.2 mol%), but variable trace element compositions. The uniform Fo compositions of the rims imply an absence of fractionation of the melts Fe2+/Mg, which is possible in the carbonatite meltolivine system. The kimberlite melt is argued to have originated in the mantle as a chloridecarbonate 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 chloridecarbonate melt by extensively crystallizing groundmass olivine and gravitational separation of solids in the pipe.