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 2Na₂C₂O₄ → 2Na₂CO₃ + CO₂ + C. Near-liquidus behaviour of carbonate was observed at 1300 °C and 6.3 GPa. The experimentally determined isotope fractionation between the components of the system in the temperature range from 1300 to 1700 °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 ‰. This trend corresponds to the function Δ_{Carbonate fluid–Diamond} = 7.38 × 10⁶ T⁻².