EXPERIMENTAL MODELING OF THE INTERACTION between GARNETS OF MANTLE PARAGENESES and CO2 FLUID AT 6.3 GPa AND 950-1550 °C
I.D. Novoselov1,2, Yu.N. Pal’yanov1,2, Yu.V. Bataleva1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: CO fluid, carbonates, garnet, high-pressure experiment, mantle metasomatism, diamond growth
Abstract
Experimental modeling of the interaction of garnet of eclogite and lherzolite parageneses with CO2 fluid was carried out on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS) in Pt ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa in the temperature range 950-1550 ºC. It has been established that the main interaction processes at 6.3 GPa and 950-1250 °C are partial dissolution, recrystallization, and carbonation of garnet, which result in the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on a seed at ≥1250 °C were observed. In the temperature range 1350-1550 °C, the experimental garnet underwent partial dissolution and recrystallization in the CO2 carbonation fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the peripheral zones of crystals relative to the cores, and the presence of inclusions of carbonates, kyanite, coesite, and CO2. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism involving CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental study, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate-carbonate-CO2 simulating natural diamond-forming media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation involving CO2 fluid.
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