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

Abstract

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 δ13C ≈ −5 ± 3‰ with a very broad distribution to lower values (∼−40‰). The processes that have produced the wide δ13C distributions to the observed low δ13C 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 13C 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 δ13C values. The sharp contrast in the δ13C 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 δ13C values and that an average δ13C value of the bulk Earth could be much lower than ∼−5‰, consistent with those of chondrites and other planetary body. The heterogeneous and depleted δ13C 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.

Nitrogen isotope fractionation during terrestrial core-mantle separation

“…In each step the metal equilibrates with the silicate magma ocean, and it remains chemically isolated once it segregates in the core. The fractionation of light element isotopes in this model can best be described by the Rayleigh distillation model (Wood et al, 2013;Horita and Polyakov, 2015) (Fig. 3).…”

High-Pressure and High-Temperature Carbon Isotope Fractionation in the Fe-C System: Implications for Carbon Dynamics in the Deep Earth

“…For comparison the theoretical computation of fractionation between diamond and iron carbide is also shown (after Ref. [15]). The vertical line represent a possible temperature of crystallization of iron melt, which will correspond to a fractionation of 2.…”

“…Thus far, the heterogeneities in carbon isotopic composition in the upper mantle have been explained mainly by recycling process. However, recent studies have suggested the possible presence of 12 C enriched carbon in the core by fractionation processes during early accretion and segregation of metallic core [14,15]. Studies on other planetary materials in the solar system, such as howardite-eucrite-diogenite (HED) meteorites, shergottite-nakhlite-chassignite (SNC) meteorites and chondrites suggest that there is a remarkable difference in isotopic composition between these bodies and bulk silicate Earth values, the later being heavier than the former in the order of about 10 to 15 [16][17][18].…”

“…Studies on other planetary materials in the solar system, such as howardite-eucrite-diogenite (HED) meteorites, shergottite-nakhlite-chassignite (SNC) meteorites and chondrites suggest that there is a remarkable difference in isotopic composition between these bodies and bulk silicate Earth values, the later being heavier than the former in the order of about 10 to 15 [16][17][18]. Thus, the validity of the assumption that the bulk Earth carbon isotope composition is equivalent to the carbon isotopic composition of silicate mantle is being debated recently [1,14,15]. It is therefore essential to clearly understand how far the fractionation of carbon isotopes has occurred during the segregation of core, what is going on in the present day at the core-mantle boundary and whether the carbon in the core has an impact on Earth's carbon cycle and surface processes.…”

Penciling in details of the Hadean

“…For example, the Earth's core could contain significant C, resulting in a fractionated mantle (20). In any case, I think it is worth considering that at its beginning, the Earth was highly reducing and had significant 13 C-depleted carbon in its upper mantle.…”