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 Delta IW-0.4 to Delta IW-3.5 at 1.2 to 3 GPa, and 1400 degrees C or 1600 degrees C, where Delta IW is the logarithmic difference between experimental 102 and that imposed by the coexistence of crystalline Fe and wustite. N partitioning (D-N(metal/silicate)) depends chiefly on f(O2), decreasing from 24 +/- 3 to 0.3 +/- 0.1 with decreasing f(O2)center dot 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 degrees C, D-C(metal/silicate) partition coefficients increase linearly with decreasing f(O2) from 300 +/- 30 to 670 +/- 50. At 1600 degrees C, however, they increase from Delta IW-0.7 to Delta IW-2 (87 +/- 3 to 240 +/- 50) and decrease from Delta IW-2 to Delta IW-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 (Delta IW-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 (Delta IW-2.2 to Delta IW-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. (C) 2016 Elsevier B.V. All rights reserved.