Compositions of co-existing minerals from graphite bearing carbonatites were obtained by the EPMA analysis for beforsites from Chernigovsky complex (Ukraine), Pogranichnoe dolomite-rich carbonatites, Chagatay carbonatites, Uzbekistan, Khibina alkaline igneous complex, which includes carbonatites and graphite-bearing rocks (Kola Peninsula). In all cases graphite-bearing rocks contain magnetite. Sometimes late magnetite and graphite form intimate intergrowths. Thermodynamic analysis of equilibria between magnetite, silicate minerals, carbonates and graphite permitted to estimate temperatures and oxygen fugacities prevailing during the formation of the investigated rocks.
Large number of co-existing minerals in the graphite-bearing beforsites from the Chernigovsky complex permitted to estimate temperature, oxygen fugacity and minimal pressure using several approaches. For this purpose equilibrium constants of the following reactions among the components of co-existing minerals have been used 6Fe2SiO4 + 2CaMg(CO3)2 + 5ZrO2 = 2CaCO3 + Mg2SiO4 + 2C + 4Fe3O4 + 5ZrSiO4; CaMg(CO3)2 + 1\2ZrSiO4 = CaCO3 + 1\2Mg2SiO4 + 1\2ZrO2 + CO2; 0.6Fe2SiO4 + 1.2CaMg(CO3)2 = 1.2CaCO3 + 0.6Mg2SiO4 + 1.2C + 0.4Fe3O4 + O2. Temperatures of equilibria among graphite, dolomite, calcite, magnetite and olivine at silica activity buffered by zircon + baddeleite assemblage in beforsites of the Chernigovsky complex are close to 600°C. Minimum pressure of the formation of these mineral assemblages is close to 0.2 GPa. This is consistent with the estimated depth of erosion for the Chernigovsky complex. Oxygen fugacities characteristic of carbonatites with graphite are 0.6 to 0.8 log units below the level of QMF buffer. This range is typical for magmatic systems. At 600°Cprevailing components of a gas phase in the system C-H-O are CO2 and H2O, whereas at lower temperatures methane-rich fluids may appear.
Pogranichnoe carbonatite is characterized by lower temperatures of the formation of dolomite-bearing mineral assemblages by comparison with the Chernigovsky complex: close to 450°C. fO2 values for this mineral deposit are close to QMF buffer. Possible reason for the appearance of graphite in this carbonatite is decrease in silica activity, which may be caused by partial decarbonation. On the other hand, stability field of graphite in carbonatitic mineral assemblages extends with the dropping temperature, and, therefore, cooling may result in the formation of graphite. Precipitation of graphite may also be caused by the mixing of CO2-rich gases ascending from the deeper and hotter parts of a magmatic system with gases rich in CH4 in the upper parts.
Doroshkevich, Wall, Ripp, 2007 Magmatic graphite in dolomite carbonatite at Pogranichnoe, North Transbaikalia, Russia. CMP 153, 339-353
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