THE EFFECT OF C-O-H-N-S FLUIDS ON DIAMOND CRYSTALLIZATION UNDER REDUCING MANTLE CONDITIONS (A REVIEW OF EXPERIMENTAL DATA)
Yu.N. Palyanov1,2, Yu.M. Borzdov1, I.N. Kupriyanov1, A.F. Khokhryakov1,2, Yu.V. Bataleva1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: diamond, experiment, high pressures, metal-carbon melts, mantle, fluids, diamond genesis.
Abstract
Natural diamonds are polygenic and form over a very wide range of P-T parameters, crystallization medium compositions, and oxygen fugacity. As has been demonstrated in recent years, the genesis of some diamonds is directly related to their crystallization from metal-carbon melts. Since natural mineral-forming processes are highly likely to involve various components characteristic of mantle environments, it is relevant to analyze the results of experiments on the influence of fluids of the C-O-H-N-S system on the crystallization features and indicator properties of diamond. The experimental data presented in this review demonstrate that increasing the concentration of fluid components (N, O, S, H2O, CH4-H2) at constant P-T parameters inhibits diamond crystallization processes in metal-carbon melts and ultimately leads to the formation of metastable graphite instead of diamond. Increasing P and T reduces the inhibitory effect of impurities and expands the diamond crystallization region. The main patterns of specific changes in the morphology, defect-impurity composition, and internal structure of diamond crystals are revealed, depending on the type and concentration of impurity in the crystallization medium. It is demonstrated that impurity-induced specific changes in diamond morphology and trends in nitrogen concentration in diamond are indicative of crystallization conditions and provide a basis for reconstructing diamond formation processes under reducing conditions in the metal-bearing mantle.
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