Contrast compositions of kimberlites - genetic conclusions (Kuoyka field, Yakutia)

The compositions (mineralogy, major- and trace-element chemistry of rocks and minerals, as well as Sr-Nd-Hf isotope systematics) of two kimberlite bodies, the Obnazhennaya pipe and the Velikan dyke from the Kuoyka field, Yakutian kimberlite province (YaKP), located close to each other (the distance is 1 km) and having the same Upper Jurassic age. The kimberlites of the two bodies are contrastingly different in composition. The Obnazhennaya pipe is composed of pyroclastic kimberlite of high-Mg, low-Ti of composition, characterized by high saturation of clastic material of the lithospheric mantle (CMLM). Pyroclastic kimberlite contains rare inclusions of coherent kimberlite from previous intrusion phases. The Velikan dyke is represented by coherent kimberlite of relatively high-Fe, high-Ti composition, having neither mantle xenoliths nor olivine xenocrysts. The similarity of isotope-geochemical characteristics for kimberlites of both bodies, and their spatial and temporal proximity suggests that their formation is associated with the presence of a single primary magmatic source located in the asthenosphere. It is assumed that the asthenospheric melt differentiated into two parts: 1) predominantly substantially carbonate composition and 2) carbonate-silicate compositions, which formed respectively 1) high-Mg and 2) Mg-Fe and Fe-Ti petrochemical types of kimberlites. Both parts of the melt had different disintegrating abilities. The higher ability to destroy and subsequently capture CMLM belongs to the melt, which formed a high-Mg type of kimberlite and which, according to the structural-textural classification, more often corresponded to the pyroclastic kimberlite. It has been suggested that the primary kimberlite melt of asthenospheric origin, having a high-Fe, high-Ti composition, is similar to the coherent kimberlite composition from the Velikan dyke (in wt.%: SiO₂ - 21.8, TiO₂ - 3.5, Al₂O₃ - 4.0, FeO - 10.6, MnO - 0.19, MgO -21.0 , CaO - 17.2, Na₂O - 0.24, K₂O - 0.78, P₂O₅ - 0.99, CO₂ - 12.6). It is concluded that if the pyroclastic kimberlite contains only xenogenic Ol, then a part of the Ol macrocrysts with a high content of FeO in the coherent kimberlite crystallized from the melt. The similarity of the Sr-Nd-Hf isotope systematics and trace element compositions for kimberlites of different ages (from Devonian to Upper Jurassic) in different parts of the YaKP (in Kuoyka, Daldyn and Mirninsky fields) indicates a single long-lived homogeneous magmatic asthenospheric source.