Evidence for evolution of diamond crystallization medium in eclogite xenolith from the Udachnaya kimberlite pipe, Yakutia

Abstract

The recently obtained data on the superimposed character of diamonds in eclogite xenoliths have compelled us to consider the problem of diamond formation in the upper mantle from a new standpoint [1]. In [2], we reported the first results of the study of a unique diamond-bearing xenolith (length18 cm, weight 6900 g) from the Udachnaya kimberlite pipe (Yakutia). The results testify to the superimposed character of diamonds (particularly, the chemistry of minerals) and suggest that diamond crystallized as a product of interaction between the high-K and C-bearing fluid with the ultramafic substrate [2]. The study of diamond-hosted inclusions from xenoliths and the comparison of their chemical composition with minerals of the xenolith groundmass would be of special interest for reconstructing conditions of formation, composition, and evolution of diamond. In this communication, we present results of the study of inclusions along with defects, impurities, and the internal structure of diamond from this xenolith. We selected from the xenolith 44 diamond crystals 1‐5 mm in size. The crystals with crystalline inclusions were polished along axes (110) and (100) until the exposure of inclusions. These inclusions were examined with a Camebax-micro electron microprobe. The internal structure of diamond crystals was investigated with cathode luminescence on a LEO 1430VP scanning electron microscope. The IR absorption spectra were recorded with a Bruker VERTEX 70 IR Fourier spectrometer to study the impurities and to estimate concentration of structural defects in diamonds from this xenolith. The diamonds in xenoliths occur largely in the intergranular space and as inclusions in pyroxene and mica. They have the shape of differently distorted octahedra. Cathode luminescent images distinctly show two zones (core and shell). As a rule, the core is characterized by a weak luminescence, while the shell reveals a more intense luminescence (Fig. 1). The nonuniform zoning and distorted growth layers confirm the previous conclusion on the growth of the xenolith-hosted diamonds in a confined space. According to the results of IR Fourier spectroscopy, diamonds from this xenoliths pertain to the IaA/B type, widespread among natural diamonds; i.e., they contain impurity centers A (a pair of nitrogen atoms that isomorphically replace carbon), B1 (four nitrogen atoms around a vacancy), and in some cases B2 (sheetlike defects oriented parallel to (001)) with linear dimensions varying from a few nanometers to a few micrometers [3]. The total content of nitrogen admixture in the crystals ranges from 40 to 800 ppm. Besides the absorption bands related to the main nitrogen impurity centers, additional lines 3310, 3237, 3150, 3107, 1405,