Types of Xenogenic Olivine from Siberian Kimberlites

Abstract

This work is devoted to the systematization of the composition of xenogenic olivine from kimberlites as the main mineral composing the lithospheric mantle. Based on data on the composition of olivines from xenoliths and megacrysts from kimberlites, a general division into four types is proposed: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2) and low-temperature (LTP) peridotites, as well as olivines of low-chromium megacryst association (MCA). The separation scheme uses the CaO content as an indicator of the formation temperature and the Mg/(Mg + Fe) ratio as an indicator of the degree of enrichment in olivines. In contrast to Al, the Ca content in olivines from cratonic peridotites is high enough to use only EPMA when applying the proposed scheme. According to this scheme the study of more than 1500 individual olivine xenocrysts from a number of kimberlite bodies of the Siberian platform was made. It revealed three characteristic distributions of olivine types: without high-temperature differences (Obnazhennaya pipe), with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes), and with a significant development of HTP-1 (Dianga pipe). Only the latter type of distribution is characterized by the presence of a noticeable amount of megacryst association olivines. The study of other minor elements (TiO2 and NiO) in olivines allowed us to propose a model for the formation of high-temperature olivines of two different types due to the interaction of megacryst melt of various fractionation stages on depleted rocks of the lithospheric mantle. HTP-2 olivines arose upon exposure to a fractionated melt of the late stages of crystallization, and HTP-1 olivines appeared upon exposure to unfractionated (less enriched with incompatible components) megacryst melt at higher temperatures of the initial stage of crystallization.