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 δ(13)C ≈ -5 ± 3‰ with a very broad distribution to lower values (∼-40‰). The processes that have produced the wide δ(13)C distributions to the observed low δ(13)C 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 (13)C 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 δ(13)C values. The sharp contrast in the δ(13)C 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 δ(13)C values and that an average δ(13)C value of the bulk Earth could be much lower than ∼-5‰, consistent with those of chondrites and other planetary body. The heterogeneous and depleted δ(13)C 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.