inclusions of diamond crystals in the tourmaline of the schorl-uvite series: problems of genesis

The mineralogical and geochemical features of diamond-bearing tourmaline crystals (schorl-uvite series) from garnet-clinopyroxene rocks of the kumdy-kol deposit (northern kazakhstan) have been studied in detail. the formation of the main rock-forming minerals (garnet + k-bearing clinopyroxene) occurred in the diamond stability field at 4-6 gpa and 950-1000 °c. crystallization of k-bearing clinopyroxene at these parameters is possible in the presence of an ultra-potassic fluid or melt formed because of crustal material melting in subduction zones. tourmaline crystals (up to 1 cm) containing diamond inclusions perform veins crosscutting high-pressure associations. the composition of individual zones varies from schorl to uvite within both a single grain and the sample as a whole. the potassium content in this tourmaline does not exceed 0.1 wt.% k2o, and the isotopic composition of boron  δ11b varies from –10 to –15.5 ‰, which significantly differs from the previously established isotopic composition of boron in maruyamaite crystals (δ11b 7.7 ‰ in the core and –1.2 ‰ in the rim) of the same deposit. analysis of the obtained data on δ11b in the tourmalines from the diamond-grade metamorphic rocks within the kumdy-kol deposit suggests the existence of two boron sources that resulted in crystallization of k-bearing tourmaline crystals (maruyamaite-dravite series) and potassium-free tourmalines of the schorl-uvite series.