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

Abstract

¹⁵N/¹⁴N 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 ¹⁵N/¹⁴N 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 ¹⁵N/¹⁴N 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 (Δ¹⁵N^(alloy–silicate) = δ¹⁵N^(alloy) – δ¹⁵N^(silicate) = –3.3 ‰ to –1.0 ‰) 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 δ¹⁵N values of non-carbonaceous iron meteorites (as low as –95 ‰), 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 (δ¹⁵N = –29 ‰ to –6‰). As most of the Earth grew primarily via the accretion of similar inner solar system protoplanets, a relatively high δ¹⁵N value of present-day Earth's primitive mantle (–5‰) cannot be explained by the accretion of enstatite chondrite-like materials alone and necessitates a significant contribution of 15N-rich materials to the Earth's interior.

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