AQUEOUS FLUIDS AND METAL ALLOYS DRIVING GROWTH OF MANTLE-SOURCED TYPE IIB BLUE DIAMONDS

Blue diamonds are among the rarest type of gems, mainly found in the Cullinan Mine, South Africa, and in the Kollur mine, India. They are defined as type IIb diamonds because of the presence of little amount of boron (10 ppm), giving them a blue color, and the near absence of nitrogen, two yet unexplained characteristics. A detailed study of forty-nine specimens from the Cullinan Mine, was carried out. Among the samples, some of them contain multi-phased primary and secondary inclusions which were characterized by Raman and infrared spectroscopies, X-ray diffraction and X-Ray fluorescence. These samples hold inclusions of graphite, graphite+Fe-Ni-Cu-S metallic alloys, and a common H₂O+CH₄ fluid phase. CaSiO₃-walstromite and wüstite had also been observed. The calculated residual trapping pressures of the inclusions suggest that blue diamonds form in a mantle continuum, from lithospheric to sub-lithospheric depths. It is here proposed that blue diamonds may grow in water-dominated supercritical fluids associated with metallic melts, a growth environment characterized by low oxygen fugacities. Because type IIb diamonds contain a few ppm of boron and even less nitrogen, it is proposed that the mantle itself, and not subducted crust as previously postulated, supplies boron in the sufficient amount, while metallic alloys would prevent the incorporation of nitrogen.