Abstract
Data on compositions of coexisting minerals in the graphite-bearing carbonatites of the Chernigovka massif are reported. Thermodynamic analysis of these results made it possible to establish that the temperature of equilibrium between graphite, dolomite, calcite, magnetite, and olivine for silica activity buff- ered by the (zircon + baddeleyite) assemblage is approximately 600 ° C. The minimal pressure of formation of these mineral assemblages is approximately 0.2 GPa, which is consistent with estimates of the erosion depth for the Chernigovka massif. The oxygen fugacity typical of the graphite-bearing carbonatite is 0.6-0.8 log units below the quartz-magnetite-fayalite buffer. Such values are typical of magmatic systems, e.g., basalts of the mid-ocean ridges (MORB). At 600 ° C, the gas phase in the C-H-O system equilibrated with the mineral assem- blage of the carbonatite studied is dominated by CO 2 and H 2 O, whereas methane-rich fluids appear at lower temperatures.
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