Limited nitrogen isotopic fractionation during core-mantle differentiation in rocky protoplanets and planets

N-15/N-14 ratios of meteorites are a powerful tool for tracing the journey of life-essential volatiles like nitrogen (N), carbon and water from nebular solids to the present-day rocky planets, including Earth. The utility of N-15/N-14 ratios of samples originating from differentiated protoplanets (e.g., iron meteorites) and planets (e.g., Earth's mantle) for tracing this journey could be affected by the fractionation of N isotopes during core-mantle differentiation, which would overprint their primitive compositions. The extent of N isotopic fractionation during core-mantle differentiation and its effect on the N-15/N-14 ratios of resulting metallic and silicate reservoirs is, however, poorly understood. Using high pressure-temperature experiments, here we show that equilibrium N isotopic fractionation between metallic and silicate melts (Delta N-15(alloy-silicate) = delta N-15(alloy) - delta N-15(silicate) = -3.3 parts per thousand to -1.0 parts per thousand) is limited across a wide range of oxygen fugacity and is much smaller than previous estimates. Also, we present ab initio calculations based on the relevant N speciation in metallic and silicate melts confirming both the magnitude and direction of equilibrium N isotopic fractionation predicted by our experimental results. Limited N isotopic fractionation during core-mantle differentiation suggests that the core and mantle relicts largely preserve the N isotopic compositions of their bulk bodies. Based on the delta N-15 values of non-carbonaceous iron meteorites (as low as -95 parts per thousand), we predict that the extent of variations in the N isotopic compositions of inner solar system protoplanets was larger than that recorded by enstatite chondrites (delta N-15 = -29 parts per thousand to -6 parts per thousand). As most of the Earth grew primarily via the accretion of similar inner solar system protoplanets, a relatively high delta N-15 value of present-day Earth's primitive mantle (-5 parts per thousand) cannot be explained by the accretion of enstatite chondrite-like materials alone and necessitates a significant contribution of N-15-rich materials to the Earth's interior. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The N-15/N-14 ratios of meteorites are important in tracing the origins of life-essential volatiles on rocky planets. The study reveals limited N isotopic fractionation during core-mantle differentiation, suggesting that the core and mantle can preserve their primitive isotopic compositions. This indicates a significant contribution of N-15-rich materials to the Earth's interior, beyond the accretion of enstatite chondrites.