a–c The N abundance and (d–f) δ15N in the bulk silicate reservoir as a function of the residual N fraction after evaporation. (a, d) Under relatively reducing conditions; (b, e) Relatively oxidizing conditions. (c, f) The modeled N concentration and δ15N in the bulk silicate part at the oxygen fugacity (log fO2) of Earth’s accreting materials (~IW-3 to IW-1). Earth’s core/mantle mass ratio was also used for models in (a) and (b). The fO2 affects the N partition coefficient between metal and silicate (Dmetal/silicateN) and the N species in silicate melts21,22,24,31,33, which consequently controls the equilibrium N isotope fractionation between vapor (N2) and silicate melt (103lnαN2-silicate) (Fig. 1c). The green areas represent the values of the bulk silicate Earth. The red and blue lines in upper panels represent the initial N concentrations (Cinit) of 500 and 1000 ppm in planetesimals before evaporation, respectively. The dash lines refer to the modeling results at different Dmetal/silicateN values. The yellow, blue, and red shadow regions in the lower panels represent the modeled δ15N in the bulk silicate reservoir with an initial δ15N (δ15Ninit) of +20‰, 0‰, and −20‰, respectively. — Nature Communications
The relative roles of protoplanetary differentiation versus late accretion in establishing Earth’s life-essential volatile element inventory are being hotly debated.
To address this issue, we employ first-principles calculations to investigate nitrogen (N) isotope fractionation during Earth’s accretion and differentiation. We find that segregation of an iron core would enrich heavy N isotopes in the residual silicate, while evaporation within a H2-dominated nebular gas produces an enrichment of light N isotope in the planetesimals.
The combined effect of early planetesimal evaporation followed by core formation enriches the bulk silicate Earth in light N isotopes. If Earth is comprised primarily of enstatite-chondrite-like material, as indicated by other isotope systems, then late accretion of carbonaceous-chondrite-like material must contribute ~ 30–100% of the N budget in present-day bulk silicate Earth.
However, mass balance using N isotope constraints shows that the late veneer contributes only a limited amount of other volatile elements (e.g., H, S, and C) to Earth.
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