ABSTRACT

Microinclusions in cuboid diamonds represent the mantle-derived fluid or melt from which they crystallized. The low degree of N aggregation in diamonds with microinclusions (cuboid or coated diamonds) indicates a short mantle residence and suggests that they crystallized shortly before kimberlite eruption. New data on the composition of microinclusions in cuboid diamonds from V. Grib kimberlite pipe (Arkhangelsk province, NW Russia) reveal two contrasting types: carbonatitic and hydrous-silicic. Significant variations in phase relations and in the ratios between different phases in the microinclusions of different diamonds are consistent with the potentially wider compositional variations expected for local volumes of diamond-forming fluids/melts. Compared to the host kimberlites, diamond microinclusions show a strong enrichment in K, a shallower REE profile, and pronounced depletion in Ti, Zr and Y. All microinclusions in diamonds from the V. Grib kimberlite pipe show enrichment in many incompatible elements, which supports their formation by partial melting of metasomatized mantle peridotites and eclogites. These data suggest that the diamonds have crystallized from carbonate-silicate fluids/melts that were derived from a source also common to kimberlites.