The evolution of deeply subducted carbonates and the formation of microdiamonds

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This study revealed the evidence for the aqueous fluid recycling in the continental materials during deep subduction to form UHP metamorphic rocks and for the metasomatism under UHP conditions. Intraslab UHP metasomatism model was proposed to explain these processes occurred in natural UHP metamorphic rocks. Multicomponent aqueous fluid released from subducted silicate rocks had been infiltrated into carbonate layers and had led to strong metasomatism. During such a metasomatism process, abundant microdiamond could form in carbonate rocks, particularly dolomite marble. Several components could be solved in recycled aqueous fluids under UHP conditions ; TiO2 was one of the representative one. In the Kokchetav UHP metamorphic rocks, Ti-clinohumite occurs in diamond-free UHP dolomitic marble, and the origin of TiO2 in clinohumite could be a solved component in aqueous fluid indicating high H2O activity. This suggest that diamond could be unstable under very high H2O activity even in carbo … More nate rocks and carbon could dissolve into fluid phase. Diopside in this rock contains over 850ppm of H2O as hydroxyl on the basis of micro-FT-IR study. This also means diamond-free dolomitic marble could form under very high H2O activity conditions. Diopside in this marble does not contain K2O-bearing exsolved phases contrary to diamond-bearing dolomite marble. Not only the bulk K2O composition, the solubility of K2O in aqueous fluid under UHP conditions might control the presence of exsolved lamellae in diopside, and probably, K2O had been completely leached out into fluid phase in this rock.K2O-bearing tourmaline had been discovered in tourmaline-K-feldspar-quartz rock, and could be a new mineral because no such high-K2O compositions were reported in tourmaline. The occurrence of this tourmaline also indicates the high activity of volatile components in such rocks and the possibility of aqueous fluid as a carrier for these volatile components.All these results described above strongly suggest the importance of the role of aqueous fluid during UHP metamorphism and lead to the proposed model of“Intraslab UHP metasomatism". Less