Regime of carbon compounds in carbonatites in Uzbekistan: Evidence from carbon isotopic composition and thermodynamic simulations

Thermodynamic analysis of equilibria between minerals (with regard for their compositions) in carbonatites of the Chagatai complex, Uzbekistan, provides us with the possibility of estimating the oxygen fugacity at which carbonates could occur in equilibrium with elementary carbon. Isotopic studies and thermodynamic simulations show that graphite started to crystallize at 775A degrees C and an oxygen fugacity value approximately one logarithmic unit below the QFM buffer and continued to crystallize with further cooling, simultaneously with a decrease in the Ti concentration in the equilibrium magnetite. Graphite crystallized from carbonatite melt at higher temperatures and likely precipitated from hydrothermal fluid at lower temperatures. The composition of gas in equilibrium with graphite in the C-H-O system was calculated for oxygen fugacity values evaluated for the Chagatai carbonatites. Inasmuch as the values of oxygen potential are almost identical in graphite- and diamond-bearing carbonatites, the presence of graphite in carbonatite dikes and diatremes can be regarded as a prospecting guide in exploration for diamondiferous carbonatites.