Inhomogeneity of the composition of lithospheric mantle beneath the Yakutian kimberlite province

For the first time, such an indicator as the Ti content in garnets was used as a criterion for the study of heterogeneity of the lithospheric mantle (LM) beneath the Yakutian kimberlite province (YaKP). Comparison of the compositions of garnet from pipes of most fields (18 out of 21) of YaKP was carried out. The study was based on representative collections of garnets from kimberlite concentrates, as well as literature and own data on the composition of garnets from mantle xenoliths from the Upper Muna pipes and northern fields adjacent to the Anabar shield, as well as from the Udachnaya, Dal’nyaya and Obnajennaya pipes. Three groups of YaKP fields with different Ti content (Fig. 1) and Mg# values ​​in garnets have been identified - 1) southern diamondiferous fields - high TiO₂ content (0.26-0.50 wt%) and high Mg# value (80.6-82.6%); an exception is the Mirninsky field (0.13 wt.% TiO₂); 2) the dominant number of northern fields (10 in total) is a low TiO₂ content (0.06-0.26 wt%) (Fig. 2) and a relatively high value of Mg# (78.8-81.7%, middle - 80.2%); 3) three northern fields (Chomurdakh, Ogoner-Yuryakh and Toluopka) - high TiO₂ content (0.53-0.78 wt.%) (Fig. 3) and low Mg# (76.9-78.3%). The trace element composition of garnets from the third group testifies to their mainly equilibrium magmatic crystallization (Fig. 4). It is assumed that the garnet-bearing rocks, due to the relatively low lithospheric mantle (LM) thickness in the marginal part of the Siberian Craton, were subjected to almost complete metasomatic processing by melt-fluids of the asthenospheric mantle. The obtained data on the composition of garnets allowed the authors to clarify the reason for the different compositions of kimberlites in the southern and northern fields of YaKP. The authors believe that the predominantly high-Ti composition of the kimberlites of the northern fields, despite the low-Ti composition of the LM rocks, reflects the primary composition of the kimberlite melt-fluid of asthenospheric origin. The relatively small thickness of the LM beneath the northern fields limited the degree of assimilation by kimberlite melt of high-Mg rocks of LM and initiated an increase in asthenosphere activity, which led to the formation of high-Ti kimberlites, high-Ti alkaline basalts, and alkaline-carbonatite massifs here. Supported by RBRF grant 19-05-00788