2017 年 27 巻 4 号 p. 278-283
Carbon, the fourth most abundant element in the solar system, is believed to be an important light element constituent in the Earth's core. The high carbon content of carbonaceous chondrites (3.2 wt.%) compared to bulk earth estimates, the presence of graphite/diamond and metal carbides in Iron meteorites, the high solubility of carbon into iron melts in the Fe-C system, all suggests the plausible presence of carbon in the Earth's core. However, the distribution of carbon isotopes in the core and deep mantle remains elusive. Newly reported experimental data and theoretical estimates on equilibrium carbon isotope fractionation between graphite/diamond and carbide phases suggests that iron carbide melt will preferentially gather 12C than 13C. These results are consistent with the carbon isotope distribution between graphite and cohenite (Fe3C) observed in iron meteorites. The temperature dependent fractionation of carbon isotopes between carbide phases and elemental carbon can be an effective mechanism that might have created a 12C-enriched core. If the Earth's core is a large reservoir of 12C-enriched carbon, then it can result in large perturbations in surface carbon cycle caused by the flux of isotopically lighter carbon from the core-mantle boundary.