Fluid microinclusions in octahedral diamonds

Microinclusions carrying high-density fluids (HDFs) with silicic, carbonatitic and saline compositions are common in fibrous diamonds and represent the medium in which their host diamond grew. Such inclusions have not been documented previously in monocrystaline octahedral diamonds or in the octahedral cores of coated diamonds, allowing a debate on whether such fluids are responsible for the formation of octahedral diamonds as well. The other notable difference is that all nitrogen in microinclusion-bearing zones resides in A-centers, whereas most octahedral diamonds carry both A and B centers. We report the first finding of HDF microinclusion in an octahedral diamond from Finsch, South Africa and in the core of a coted diamond from Kankan, Guinea. The microinclusions in the Finsch diamond are restricted to two thin layers ( approximately 10 mu m), parallel to the (111) face, approximately 20 and 200 mu m from the rim. Cathodoluminescence (CL) reveals concentric zoning and octahedral growth throughout the diamond. The inclusion-rich layers are easily recognized by their weak fluresence. The diamonds carry approximately 800-1200 ppm nitrogen. Absorbance of B centers is observed in the inner part (A/B=5) and between the inclusion rich layers and the rim (A/B=16). Forty-five inclusions of carbonatitic HDF were analyzed along the inner layer. Their major and trace element compositions and FTIR analyses are highly similar to the ones observed in HDFs from fibrous diamonds. In the octahedral core of the Kankan diamond we found six microinclusions with saline composition. Unlike the case of the Finsch diamond, these inclusions are sporadically scattered, up to approximately 100 mu m away from a sulfide mineral inclusion and were hard to find. Supporting evidence for the involvement of HDF, similar to the ones capsulate in fibrous diamonds, in the formation of monocrystaline diamonds comes from LA-ICP-MS analyses of trace element patterns in monocrystalline diamonds (McNeill et al. 2009) and the sinosuidal REE patterns of garnet inclusions in diamonds (Stachel and Harris, 2008; Weiss et al. 2009). Based on the above observation we have good reasons to believe that microinclusions and HDFs may be found in other octahedral diamonds, extending the role of HDFs to the formation of most natural diamonds.1 page(s