CARBON ISOTOPIC FRACTIONATION BETWEEN CO2 VAPOR, SILICATE AND CARBONATE MELTS - AN EXPERIMENTAL-STUDY TO 30 KBAR

The carbon isotopic fractionation between CO2 vapour and sodamelilite (NaCaAlSi2O7) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag2C2O4 was the source of CO2 and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the δ13C of coexisting vapour was determined directly by capsule piercing. CO2 solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO2 dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO2 reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO2 solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO2 vapour and carbonate ions dissoived in sodamelilite melt at 1200°-1400° C and 5-30 kbars average 2.4±0.2‰, favouring13C enrichment in CO2 vapour. The results are maxima and are independent of pressure and temperature. Similar values of ≈2‰ are obtained for the carbon isotopic fractionation between CO2 vapour and carbonate melts at 1300°-1400° C and 20-30 kbars. © 1990 Springer-Verlag.