Carbon isotope fractionation during experimental crystallisation of diamond from carbonate fluid at mantle conditions

We report first results of a systematic study of carbon isotope fractionation in a carbonate fluid system under mantle PT conditions. The system models a diamond-forming alkaline carbonate fluid using pure sodium oxalate (Na2C2O4) as the starting material, which decomposes to carbonate, CO2 and elementary carbon (graphite and diamond) involving a single source of carbon following the reaction 2Na(2)C(2)O(4) -> 2Na(2)CO(3) + CO2 + C. Near-liquidus behaviour of carbonate was observed at 1300 degrees C and 6.3 GPa. The experimentally determined isotope fractionation between the components of the system in the temperature range from 1300 to 1700 degrees C at 6.3 and 7.5 GPa fit the theoretical expectations well. Carbon isotope fractionation associated with diamond crystallisation from the carbonate fluid at 7.5 GPa decreases with an increase in temperature from 2.7 to 1.6 parts per thousand. This trend corresponds to the function Delta(Carbonate fluid-Diamond) = 7.38 x 10(6) T-2.