Types of Xenogenic Olivine from Siberian Kimberlites

Nikolay S Tychkov,Nikolay V Sobolev,Alexey M Agashev,Nikolay P Pokhilenko,Vladimir A Tsykh

doi:10.3390/min10040302

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.

Highlights

Xenoliths carried to the surface by basaltic or kimberlitic melts bear a message from the subcratonic lithospheric mantle (SLM)
Low-temperature peridotite (LTP) olivines: narrow ranges of Mg# (89.8–93.1), NiO (0.34–0.42 wt.%), and 0.022 wt.% TiO2 in 90% of cases, which correspond to depleted cratonic lithospheric mantle
Xenolithic olivines in kimberlites can be divided into four main genetic types

Summary

Introduction

Xenoliths carried to the surface by basaltic or kimberlitic melts bear a message from the subcratonic lithospheric mantle (SLM). Mantle xenoliths in kimberlites are of quite rare occurrence for several reasons, including disintegration into xenocrysts during the emplacement. The xenocrysts can provide less precise but more complete information on the SLM structure and on processes at the respective depths. The lithospheric mantle is composed of garnet and spinel peridotites, which contain abundant olivine, one of most widespread kimberlite minerals [1,2]. Considerable attention is paid to the study of olivine, which crystallizes from kimberlite, in order to study the processes of kimberlite formation and the evolution of protokimberlite melts [5,6,7,10,11,12]

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Conclusion