2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 52-7
Presentation Time: 3:10 PM


OGASAWARA, Yoshihide, SHIMIZU, Rentaro and SAKAMAKI, Kunihiko, Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050, Japan, yoshi777@waseda.jp

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 2nd stage growth was from H2O fluid. Grt-Bt gneiss is 2nd highest and the diamond shows several morphologies; however, no 2nd stage growth. In dolomite marble, diamond at 2nd stage has light carbon isotope, -17 to -27 ‰, whereas 1st stage diamond has -8 to -15 ‰. The light carbon of 2nd stage could be organic carbon in gneisses carried by H2O-fluid; dissolution of diamond in gneisses could have occurred. No 2nd 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 2nd 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 2nd 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.
  • GSA2015_Ogasawara-etal_Diversity_of_Kokchetav_metamorphic_diamonds_and_their_formations_with_H2O_fluid.pdf (5.6 MB)