EFFECT OF C–O–H–N–S FLUIDS ON DIAMOND CRYSTALLIZATION UNDER REDUCING MANTLE CONDITIONS ( review of experimental data )

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 formation is assumed to include various components typical of the mantle media, we consider it relevant to analyze the experimental results of the effect of C–O–H–N–S fluids on the crystallization features and indicator properties of diamond. The experimental data presented in this review show that increasing the concentration of fluid components (N, O, S, H2O, CH4–H2) at constant P–T parameters inhibits diamond crystallization in metal–carbon melts and eventually leads to the formation of metastable graphite instead of diamond. Increasing P and T decreases the inhibitory effect of impurities and expands the diamond crystallization region. We have revealed the basic patterns of specific changes in the morphology, defect–impurity composition, and internal structure of diamond crystals, depending on the type and concentration of impurity in the crystallization medium. We have established 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 under reducing conditions in the metal-bearing mantle.