Fig. 4.

Modeled δ¹⁵N values of the silicate proto-mantle during core formation as a function of core–mantle equilibration and fO₂ conditions of the magma ocean. We explored 2 different ${\text{δ}}^{15}{\text{N}}_{\text{BE}}$ values: the lowest (−45‰) and highest (−15‰) ${\text{δ}}^{15}\text{N}$ values measured in enstatite chondrites (13). The respective values used for D_N^{metal-silicate} and ∆¹⁵N^{metal-silicate} were 7 and −49‰ at IW, 3.02 and −89‰ at ∆IW −1, and 0.2 and −257‰ at ∆IW −3.1 (Table 1). The present-day ${\text{δ}}^{15}\text{N}$ value of the mantle is −5 ± 2‰ (gray bar; the thickness of the bar represents the 2σ SD), based on a compilation from diamonds and MORBs (3). This model assumes that ∆¹⁵N^{metal-silicate} was not affected by Rayleigh distillation during the experiments, and thus only considers that the mantle’s ${\text{δ}}^{15}\text{N}$ value was modified by core–mantle fractionation without accounting for the effect of degassing. After 30% core–mantle (metal–silicate) equilibration, the modeled fO₂ variations account for a difference in the mantle δ¹⁵N value of up to 27‰. The vertical bar between −51 and −25‰ represents the plausible range of δ¹⁵N values of Earth’s building blocks necessary to produce the present-day δ¹⁵N_mantle value of −5 ± 2‰ at fO₂ conditions between ∆IW −3 and −0.5, assuming that core formation is the only process that fractionated N isotopes during the Earth’s early history. At ∆IW −3.1, assuming a ${\text{δ}}^{15}{\text{N}}_{\text{BE}}$ value of −25‰ for Earth’s building blocks, complete metal–silicate equilibration is required to reach the present-day δ¹⁵N_mantle value. In contrast, at IW, conditions under which N is siderophile, the present-day δ¹⁵N_mantle value is attained at 60% metal–silicate equilibration from any source of N with a ${\text{δ}}^{15}{\text{N}}_{\text{BE}}$ ≥ −45‰.