Limited nitrogen isotopic fractionation during core-mantle differentiation in rocky protoplanets and planets
Damanveer S. Grewal, Tao Sun, Sanath Aithala, Taylor Hough, Rajdeep Dasgupta, Laurence Y. Yeung, Edwin Schauble
Geochimica et Cosmochimica Acta
Available online 19 October 2022
“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 N isotopic fractionation between metallic and silicate melts (Δ15Nalloy–silicate = δ15Nalloy – δ15Nsilicate = –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 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.”
