Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

Experimental studies modeling diamond crystallization processes in a reduced iron-bearing mantle were conducted at a pressure of 5.5 GPa under cooling conditions using BARS technique. In the Fe-C system, one of the possible scenarios of heterogeneous nucleation and growth of diamond was realized due to an increase in carbon supersaturation of the melt with decreasing temperature. The morphology of the diamonds is determined by the relative development of octahedron, tetrahexahedron, and cube faces in order of decreasing significance. Crystallized diamonds contain nitrogen impurity in the form of C-centers with concentrations of 30-60 ppm. Inclusions in the crystals are represented by cohenite, wüstite, as well as quenched melts of Fe-C and Fe-C-O compositions. Under conditions of relatively rapid temperature decrease (55-41 deg/hour) at the final stage the diamond morphology is determined only by {111} faces. With slow cooling of the melt (16.5-30 deg/hour), the stage of diamond crystallization is succeeded by a stage of dissolution due to the interaction between diamond and the melt with formation of cohenite. The evolution of diamond morphology in the Fe-C system under decreasing temperature is determined by the sequence: {111}, {hkk}, {100} → {111} → {111} with rounded vertices and edges → rounded crystals. Cracks and pseudo-secondary inclusions were found in diamond crystals with contrastingly inhomogeneous growth zones, and the mechanism of their formation was determined. It was established that the primary cause of crack formation is iron-bearing dispersed inclusions ranging in size from 60 to 600 nm. The uneven distribution of inclusions, with a maximum density of up to 1.2×10⁹ cm⁻², creates a mosaic structure in the diamond matrix and generates significant stresses. Relaxation of these stresses leads to brittle deformation and the formation of cracks under cooling conditions. All stages of the transformation of cracks into pseudo-secondary inclusions due to regeneration of crack walls and their overgrowth by growth layers were determined. The experimentally established regularities of diamond crystallization and subsequent evolution in the Fe-C system due to temperature decrease are discussed in relation to natural diamond-forming processes in the reduced iron-bearing mantle.