EXPERIMENTAL STUDIES OF DIAMOND FORMATION AT HIGH PT-PARAMETERS (SUPPLEMENT TO THE MODEL FOR NATURAL DIAMOND FORMATION)

The results of the experimental studies on synthesis, growing, and dis solution of diamonds, which were carried out for many years in the Ins titute of Geology (Russian Academy of Sciences, Siberian Branch), are generalized, and the processes of natural diamond formation are consid ered. It is supposed that metal-carbon melts were a source for the for mation of most natural diamonds. Stages of diamond phase formation (di amond synthesis), diamond recrystallization (perfect-single crystal gr owth), postgrowth diamond annealing at the mantle PT-parameters, and p artial dissolution of diamond crystals in the Earth's mantle and on th eir outcropping are presented. The fluid regime of diamond crystalliza tion is considered. The diamond crystals obtained by synthesis from gr aphite sharply differ in morphology and physical properties from the d iamonds from kimberlite tubes, which is the result of the high rates o f carbon mass transfer from graphite to diamond. Diamond recrystalliza tion in the field of temperature gradient permits one to realize a wid e range of the rates of crystal growth, including low rates which are typical of natural diamond growth. Artificial diamonds obtained at low growth rates are very similar to the natural ones and differ from the m primarily by the manner of nitrogen impurity intrusion into their st ructure and the presence of metal intercalates. These differences disa ppear on postcrystallization treatment of artificial diamonds. The pos sibility of diamond formation in the Earth's mantle under the action o f reducing fluids has been experimentally demonstrated by diamond synt hesis in oxide, silicate, and sulfide-carbon systems by means of their partial reduction by hydrogen at PT-parameters corresponding to diamo nd stability. The possibility of Formation of clinopyroxenites, disthe ne- and coesite-containing eclogites under interaction of eclogites of different chemical compositions with hydrogen at high PT-parameters i s also shown. The molecular composition of fluid, which provides stabi lity 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. The main factors of diamond deposit formati on are supposed to be not only the conditions of diamondiferous-rock f ormation but also the subsequent geological history of these rocks, in particular the conditions of their outcropping.