Isotope carbon composition of graphite and calcite in Chagatai carbonatites (Uzbekistan)

Kogarko L.N., Divaev F.K. ,Wall F., Sevastyanov S.V.

The Chagatai carbonatite-trachyte complex is represented by dyke swarms and diatemes present in the basin of Chagatai creek on the northern slopes of the South Nuratau mountains. The carbonatites are mainly calcite carbonatites (sovites) but there are also carbonate-silicate rocks, so called kasenites, ringites and melilite carbonatites (alvikites). Rare dykes of trachyte are also present. The rock-forming minerals in Chagatai carbonatites are predominantly calcite, pyroxene, biotite, garnet, melilite, apatite, magnetite with secondary amphibole, chlorite, albite, K-feldspar, hematite, quartz, and montmorillonite.‰ A very interesting feature of the Chagatai carbonatites is the previous finding of diamonds. The diamonds are very rare and are mainly represented by intergrowths of octahedral crystals and intergrowths of grains with non-defined crystallographic forms and skeletal forms 0.01-0.05 mm across. Monocrystals of octahedral shape with flat faces and acute vertices and edges are found occasionally; they are 0.02-0.05 mm diameter. Graphite is present in some varieties of carbonatites, forming separate black flakes with a dull luster up to 2 mm across and also as cement in interstices between gangue minerals. The carbon isotope composition of graphite and calcite was measured on a DELTA plus mass-spectrometer. The d¹³C of graphite is -8.21, whereas d¹³C of calcite is -4.98. This yields an equilibrium temperature of 775^oC. Assuming a mantle source for Chagatai carbonatites and equilibrium between calcite, graphite, CO² and methane, we have calculated the composition of fluid phase. The estimated mole fraction of CO₂ is 0.7 and that of CH₄ is 0.3.