An almost universal CO₂- CO₃^2- carbon isotope fractionation function for high temperatures

High -temperature experiments on the CO2 - carbonate melt and carbonatite - nephelinite melt pairs are used to quantify equilibrium carbon isotope fractionation at conditions of 1 atm, 750-950 C-degrees and 0.3-0.8 GPa, 1170 C-degrees, respectively. Together with available experimental data, all results are within error consistent with a universal CO2 - CO32 fractionation function 10(3)ln alpha CO2/carbonate = 6.41(11).10(6) / T(2)2.54(8).10(12) /T-4 _ (T in K, valid for >650( degrees)C) that encompasses solid and molten carbonates and also carbonate components in silicate melts. The experiments hint towards a small fractionation of carbonatite vs. silicate melt of +0.39 +/- 0.35 %o (1 sigma), which however remains within error indistinguishable from zero. A generalized single fractionation function suggests that the nearest neighbours of carbon (i.e., O) dominate isotope fractionation while the next nearest neighbours only have a minor role, which greatly facilitates the understanding of carbon transfer in the deep Earth. If there is small to negligible C-13/C-12-fractionation between solid carbonate, molten (ionic) carbonate liquid and covalent carbonated silicate melt, then H2CO3- and HCO(3)(-)species in fluids or gases should also have low C-13/C-12 fractionation factors relative to the carbonate-group in depolymerized silicate melts or carbonates. Speciation models predict that the H(2)CO(3)(- )and HCO3-species dominate over the CO2 species in COH-fluids at high pressures or high temperatures, i.e. during MOR basalt degassing or subduction zone devolatilization. A consequent small C-13/C-12 fractionation (of 0.1-0.3 %o) applies to (i) continuous degassing of COH-fluids from mid-ocean ridge basalts, consistent with an observed difference of delta C-13(MORB) and delta C-13(mantle )<= 1 %o and (ii) Ctransfer of a carbonatite or COH-fluid from the subducted crust to the mantle, which then has a near-identical delta C-13 as the source. Finally, a combination of our results with available experimental values allows calculation of a consistent set of high -temperature (>650 C) carbon fractionation factors for CO2, CO32(carbonate, carbonatite or carbonated silicate melt), CH4, CO, and graphite/diamond.