Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth

One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of f_O2 from ΔIW−0.4 to ΔIW−3.5 at 1.2 to 3 GPa, and 1400 °C or 1600 °C, where ΔIW is the logarithmic difference between experimental f_O2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning (D_N^{metal/silicate}) depends chiefly on f_O2, decreasing from 24±3 to 0.3±0.1 with decreasing f_O2. D_N^{metal/silicate} also decreases with increasing temperature and pressure at similar f_O2, though the effect is subordinate. In contrast, C partition coefficients (D_C^{metal/silicate}) show no evidence of a pressure dependence but diminish with temperature. At 1400 °C, D_C^{metal/silicate} partition coefficients increase linearly with decreasing f_O2 from 300±30 to 670±50_. At 1600 °C, however, they increase from ΔIW−0.7 to ΔIW−2 (87±3 to 240±50) and decrease from ΔIW−2 to ΔIW−3.3 (99±6). Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH₄). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (ΔIW−0.4 to −2.2), N is more compatible in core-forming metal than in molten silicate (1≤D_N^{metal/silicate}≤24), while at more reduced conditions (ΔIW−2.2 to ΔIW−3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated (100≤D_C^{metal/silicate}≤700). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.