Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

The carbon budget and dynamics of the Earth's interior, including the core, are currently very poorly understood. Diamond-bearing, mantle-derived rocks show a very well defined peak at delta C-13 approximate to -5 +/- 3 parts per thousand with a very broad distribution to lower values (similar to-40%). The processes that have produced the wide delta C-13 distributions to the observed low delta C-13 values in the deep Earth have been extensively debated, but few viable models have been proposed. Here, we present a model for understanding carbon isotope distributions within the deep Earth, involving Fe-C phases (Fe carbides and C dissolved in Fe-Ni metal). Our theoretical calculations show that Fe and Si carbides can be significantly depleted in C-13 relative to other C-bearing materials even at mantle temperatures. Thus, the redox freezing and melting cycles of lithosphere via subduction upwelling in the deep Earth that involve the Fe-C phases can readily produce diamond with the observed low delta C-13 values. The sharp contrast in the delta C-13 distributions of peridotitic and eclogitic diamonds may reflect differences in their carbon cycles, controlled by the evolution of geodynamical processes around 2.5-3 Ga. Our model also predicts that the core contains C with low delta C-13 values and that an average delta C-13 value of the bulk Earth could be much lower than similar to-5%, consistent with those of chondrites and other planetary body. The heterogeneous and depleted delta C-13 values of the deep Earth have implications, not only for its accretion-differentiation history but also for carbon isotope biosignatures for early life on the Earth.