DIVERSITY OF THE KOKCHETAV METAMORPHIC DIAMONDS AND THEIR FORMATIONS RELATED WITH H2O-RICH FLUID CONDITIONS

The metamorphic diamonds in the Kokchetav Massif show very diverse features in morphologies and grain sizes with other crystal characters, Raman spectra (FWHM, PL), cathodoluminescence spectra, C isotope, abundance, paragenesis with silicates and carbonates. The formations of these diamonds, however, seem to be related to H₂O-fluid conditions.　Dolomite marble has the highest abundance in diamond. The diamond grew at two stages and 2^nd stage growth was from H₂O fluid. Grt-Bt gneiss is 2^nd highest and the diamond shows several morphologies; however, no 2^nd stage growth. In dolomite marble, diamond at 2^nd stage has light carbon isotope, -17 to -27 ‰, whereas 1^st stage diamond has -8 to -15 ‰. The light carbon of 2^nd stage could be organic carbon in gneisses carried by H₂O-fluid; dissolution of diamond in gneisses could have occurred. No 2^nd stage growth in gneisses supports this idea. Carbon-bearing H₂O fluid infiltration into dolomite marble caused the change of carbon solubility in fluid to precipitate abundant fine diamonds (10-20 mm), quickly. Recently discovered sp² graphitic carbon inclusions in 2^nd stage diamond (AGU2014F V13B-4771), which is a relic of metastable intermediate phase for diamond formation, suggest the diamond participation from H₂O fluid. A minor amount of diamond (large-grained, ca. 150 µm at average) occurs in Grt-Cpx rock. Recently, we found the overgrowth of large-grained cubic diamond on small-grained one by multi-layered 2D Raman mappings (JpGU2014 No.02541). This indicates slow-growth in H₂O-fluid having low oversaturation degree of carbon. UHP calcite marble contains a trace amount of small-grained diamond (no 2^nd stage growth) only in diopside; titanite with coesite exsolution does not contain diamond. These suggest that very high H₂O activity for titanite stability makes diamond unstable, and dissolution of diamond was possible. This is a similar relation with UHP dolomitic marble, which Arg-Fo and Arg-Ti-Chum were stable but diamond was unstable. In Tur-Fel-Qtz rock, diamond is included in new mineral “maruyamaite” (K-rich Tur); recent experiments show high-P and fluid-bearing conditions for maruyamaite. Summarizing these diverse features of the Kokchetav diamonds, those formation and their possible dissolutions have strong relationships with H₂O-fluid conditions.