ISOTOPE PERSPECTIVES ON THE DEEP CARBON CYCLE

Our current understanding of the deep carbon cycle is very limited. First, the accretion, magma ocean formation, mantle-core differentiation, and the Moon-forming impact must have had profound impacts on the distribution of carbon within the Earth. Carbon also exists within the deep Earth as many different, accessory and trace components such as C-O-H fluids, carbonates, graphite-diamond, carbides, and dissolved in Fe-alloy. Given these different C-bearing phases under varying redox conditions and an estimated long-residence time of carbon in the mantle (>1 to 5 Gy) especially during the Hadean-Archean, it is expected that the distribution of carbon within the mantle is very heterogeneous. Isotopic studies in the past decades on mantle-derived rocks (MORB, OIB, diamond, carbonatite, carbonates from kimberlite, carbides), including those from the lower mantle, have shown that in addition to the major component of mantle carbon, which has δ¹³C values of about -5±3‰, some carbon compounds (diamond, carbide) have a wide range of δ¹³C values as low as -40‰. Several models have been put forward to explain these low δ¹³C values, including heterogeneity within the mantle since the formation-differentiation of the Earth, subduction of oceanic crusts containing organic matter, and high-temperature Rayleigh-type fractionation processes. In the past decade, Fe-C carbides have been increasingly recognized as a dominant C-bearing phase in the reduced mantle and the core. Recent experimental and our theoretical isotope studies also showed that carbides (Fe₃C and SiC) can be significantly depleted in ¹³C relative to carbonates and diamond even under mantle conditions (>1000 °C). Here, we present our updated overview on distributions and fractionation processes for carbon isotopes within the deep Earth.