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 fO2 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 fO2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning (DNmetal/silicate) depends chiefly on fO2, decreasing from 24±3 to 0.3±0.1 with decreasing fO2. DNmetal/silicate also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients (DCmetal/silicate) show no evidence of a pressure dependence but diminish with temperature. At 1400 °C, DCmetal/silicate partition coefficients increase linearly with decreasing fO2 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≤DNmetal/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≤DCmetal/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.
Bibliographical noteFunding Information:
This work was supported by NSF AST1344133 to M.M.H. SIMS analyses were obtained with the help of Richard Hervig and Lynda Williams at the ASU National SIMS facility, supported by NSF EAR0948848 . We also acknowledge helpful comments by A. Sarafian and two anonymous reviewers and editorial handling of the manuscript by B. Marty.
© 2016 Elsevier B.V.
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- partition coefficients