Carbon isotope fractionation during high pressure and high temperature crystallization of Fe-C melt

•Crystallization of Fe-C melt at high pressure and high temperature accompanied by carbon isotope fractionation•Fe3C is 2‰ heavier in C isotopes than its parent Fe-C melt at 6.3 GPa and 1400 °C•Diamond works as a trap for 13C isotope in Fe-C system•Peritectic reaction of diamond with liquid provides increase of δ13C in the melt•MORB and OIB mantle source should be distinct in carbon isotope signature

There is a growing body of experimental evidence that iron carbides can play an important role in the mantle both as a host of carbon and as a redox couple (Fe-C) determining the nature of reduced phases. If carbides are significant in the mantle, it could be of interest to know if any carbon isotope fractionation accompanies carbide crystallization. A series of high-pressure and high-temperature experiments were performed on carbide crystallization from a Fe-C melt. An offset of 2‰ was observed between the δ13C values of Fe3C and Fe-C melt at 6.3 GPa and 1400 °C. The carbon isotopic compositions of Fe3C and diamond crystallizing from a single carbon source near the peritectic region at 6.3 GPa differ by 2.5‰. Fe7C3 was detected as a quench phase during Fe-C melt quenching. Our results have important implications for understanding carbon isotope distributions in iron meteorites and indicate that iron carbide crystallization may be a significant mechanism for carbon isotope heterogeneity in the Earth's mantle.