4:15 PM - 4:45 PM
★ [SMP10-05] Fluid control of deeply subducted continental materials and diamond formation by Intraslab UHP metasomatism
H2O-bearing fluid plays an important role for diamond formation during subduction of continental materials. Diamonds form and dissolve in subducting materials through H2O fluid. In UHP dolomite marble, diamonds formed at two different stages and 2nd stage growth was from H2O fluid. The diamond at 2nd stage growth has light carbon isotope compositions, -17 to -27 ‰, whereas 1st stage diamond has -8 to -15 ‰. The light carbon of 2nd stage could be organic carbon in gneisses carried by fluid; dissolution of diamond in gneisses had occurred. H2O fluid infiltration into dolomite marble caused the change of carbon solubility in fluid itself to precipitate abundant fine-grained (10-20 μm) diamonds quickly. Recently discovered sp2 graphitic carbon inclusions in 2nd stage diamond suggest the fluid participation in diamond growth from H2O fluid. Very large cubic diamond (max. 200 μm) in garnet-clinopyroxene rocks could be different fluid conditions; low oversaturation degree of carbon in fluid and slow crystallization, and led to low abundance of diamond.
In deeply subducting carbonate rocks, the abundant carbonate remains after decarbonations and are carried to the mantle. H2O is stored in NAMs, which become new water carriers to the mantle. The amount of H2O in carbonate rocks carried to the mantle is smaller than calc-silicate rocks because of small modal compositions of silicate minerals. In the case of calc-silicate rocks, for example garnet-clinopyroxene rocks of the Kokchetav, the modal compositions of carbonate is small; therefore, even a small amount of H2O can decompose all amount of carbonate to form garnet and clinopyroxene. These NAMs contain several hundreds to 1,000 ppm order of water (OH and H2O) as new water reservoirs and carriers. The modal compositions of H2O-bearing NAMs control the potential of water transportation. UHP metasomatism with skarn mineral formation brings the swapping of H2O carrier from hydrate minerals in silicate rocks to NAMs in calc-silicate rocks to expand the life of H2O transportation into mantle much longer.
We can regard deep continental subduction as the transportation mechanism of H2O and CO2. CO2 transportation is controlled by H2O behavior in deeply subducted materials and poor amount of H2O expands the volume of CO2 transportation into the deep mantle as carbonate. Summarizing these, Intraslab UHP metasomatism was proposed and will be available for volatile transportation into the mantle. All these ideas were occurred from the research on the Kokchetav UHPM rocks. The author thanks Prof. Shige Maruyama, who gave me a great opportunity to study exciting materials, Kokchetav UHPM rocks and diamonds.