EXPERIMENTAL STUDIES OF DIAMOND FORMATION AT HIGH PT-PARAMETERS (supplement to the model for natural diamond formation) Available to Purchase

The results of the experimental studies on synthesis, growing, and dissolution of diamonds which were carried out for many years at the Institute of Geology, Novosibirsk, are generalized, and the processes of natural diamond formation are considered. It is supposed that metal-carbon melts were a source for the formation of the majority of natural diamonds. The following stages are recognized: diamond phase formation (diamond synthesis), diamond crystallization (growth of perfect single crystals), postgrowth diamond annealing at the mantle PT-parameters, and partial dissolution of diamond crystals in the Earth’s mantle and during their transportation to the surface. Attention is given to the fluid regime of diamond crystallization. The graphite-synthesized diamond crystals differ sharply in morphology and physical properties from the kimberlite diamonds. The reason is high rates of carbon mass transfer from graphite to diamond. Diamond crystallization in the field of temperature gradient permits a wide range of crystal growth rates, including low rates typical of natural diamond growth. Synthetic diamonds obtained at low growth rates are very similar to the natural ones and differ from them primarily in the form of nitrogen impurity entering their structure and the presence of metal inclusions. Postcrystallization treatment of synthetic diamonds eliminates these differences. The possibility of diamond formation in the Earth’s mantle under the action of reducing fluids has been experimentally demonstrated by diamond synthesis in oxide, silicate, and sulfide-carbon systems by means of their partial reduction by hydrogen at PT-parameters corresponding to diamond stability. It is also shown that interaction of eclogites of different chemical compositions with hydrogen at high PT-parameters can produce clinopyroxenites, disthene- and coesite-bearing eclogites. The molecular composition of fluid which provides stability of diamond and natural iron, or taenite, in the Earth’s mantle is calculated. Diamond dissolution in silicate and sulfide melts at high pressure has been studied. It is suggested that the main factors governing diamond deposit formation are both the conditions of formation of diamondiferous rocks and their subsequent geological history, in particular the conditions of their transport to the surface.