Crystallographic Orientation and Geochemical Features of Mineral Inclusions in Diamonds

Abstract

Abstract —The orientation of 76 mineral inclusions represented by olivine (25 inclusions), pyrope (13 inclusions), and magnesiochromite (38 inclusions) was measured in 16 diamond samples from the major primary diamond deposits of Yakutia: Mir, Udachnaya, Internatsionalnaya, Aikhal, and Yubileynaya kimberlite pipes. The novelty of the study is that it provides a special purposeful approach to selection of samples containing not only olivine inclusions that have been extensively studied in the most recent years after the publication of the book Carbon in Earth (2013). The present collection accounts for more than 25% of all samples studied across the world and includes the most typical mineral inclusions of the predominant peridotitic paragenesis in almost all known kimberlites. Both this experiment and similar studies conducted by foreign colleagues in 2014–2019 have found no inclusions whose orientation meets the epitaxial criterion. Only single magnesiochromite inclusions in three diamonds demonstrate an orientation close to the regular one. A significant correlation between the carbon isotope composition and the mineral composition of inclusions of peridotitic and eclogitic paragenesis diamonds as well as the lack of a correlation with other properties may be considered one of the geochemical features. However, given the numerous published and proprietary data demonstrating the complex diamond growth history and, in some cases, wide variations in the composition of mineral inclusions in different zones, along with the difference in their morphology, the authors a believe that syngenetic and protogenetic inclusions can coexist in the same diamond. This is also confirmed by the discoveries of diamondiferous peridotite and eclogite xenoliths in kimberlites where diamonds are completely enclosed in garnet or olivine. Of particular note is the constant presence of heavy hydrocarbons (rel.%), from pentane (C5H12) to hexadecane (C16H34), that are predominant in fluid inclusions in kimberlite and placer diamonds as well as in pyrope and olivine of diamondiferous peridotite xenoliths.