Xenogenic olivine from Siberian kimberlites: types and features of origin

Compared to xenoliths, kimberlite xenocrysts provide, although less accurate, more complete information about the deep structure and processes in the subcratonic lithospheric mantle (SCLM). This work is devoted to the study the composition of xenogenic olivine from kimberlites as the main mineral constituting SCLM. Olivine in kimberlites has a different origin, including those not related to depleted rocks of the lithosphere. It can crystallize directly from kimberlite or belong to the so-called Cr-poor megacryst association. In this regard, for the correct interpretation of data on its composition, it is necessary to have sufficiently clear criteria for the separation of olivine xenocrysts from kimberlites into various genetic types. In order to remove olivines crystallizing directly from kimberlite from consideration, in our study we used only central homogeneous parts of crystals larger than 0.5 mm in size [Giuliani, 2018].

Based on unique and literature data on the composition of olivines from 230 xenoliths of peridotites from 12 kimberlites of the North American, South African and Siberian cratons we proposed a general division into 4 genetic types: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2), low-temperature (LTP) peridotites, olivines of low-chromium megacrystal association (MCA). The separation scheme uses the CaO content as an indicator of the temperature of formation and the ratio Mg/Mg+Fe as an indicator of the degree of enrichment.

A study of more than 1,500 olivines from a number of kimberlite bodies of the Siberian platform according to this scheme revealed three characteristic distributions of olivine types in kimberlite bodies: 1) without high-temperature differences (Obnazhennaya pipe), 2) with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes) and 3) with significant development of HTP-1 (Dianga pipe). Only the latter type is characterized by the presence of a noticeable amount of olivines of the megaryst association.

In general, variations in the composition of LTP olivines correspond to granular ones, while HTP-1 and HTP-2 correspond to deformed (shared) peridotites. Interestingly, the enrichment of olivines with incompatible components in these three types does not correlate directly with the formation temperature. Olivines of ultrahigh-temperature peridotites (HTP-1) have unexpectedly small compositional variations and occupy an intermediate position between low-temperature and high-temperature in content of incompatible elements.

A study of the content of impurity elements (TiO2, NiO) in olivines made it possible to propose the way of formation of two different types of high-temperature olivines. It belongs to the model [Harte et al., 1993; Burgess and Harte, 1999; Burgess and Harte, 2004] where megacrystal melt of various stages of fractionation [Moore et al., 1992] effects depleted rocks of lithospheric mantle. According to it, HTP-2 olivines arose upon exposure to a fractionated melt characteristic of late crystallization stages, and HTP-1 olivines due to unfractionated (less enriched with incompatible components) megacrystal melt at higher temperatures characteristic of the initial crystallization stage.

Funded by RFBR grant 18-05-01143, T.V.A. was supported by RSF grant 16-17-10067.