Limited nitrogen isotopic fractionation during core-mantle differentiation in rocky protoplanets and planets. (1st December 2022)

• Record Type: Journal Article
• Authors: Grewal, Damanveer S.; Sun, Tao; Aithala, Sanath; Hough, Taylor; Dasgupta, Rajdeep; Yeung, Laurence Y.; Schauble, Edwin A.
• Is Part Of: Geochimica et cosmochimica acta. Volume 338(2022)
• Page Start: 347
• Page End: 364
• Publication Date: 2022-12-01
• Subjects: Nitrogen -- Core formation -- Core-mantle differentiation -- Isotope fractionation -- Iron meteorites -- Chondrites; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2022.10.025
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: 15 N/ 14 N 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 15 N/ 14 N 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 15 N/ 14 N 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 (Δ 15 N alloy–silicate = δ 15 N alloy – δ 15 N 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 δ 15 N 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 (δ 15 N = –29 ‰ to –6‰). As most of the Earth grew primarily via the accretion of similar inner solar system protoplanets, a relatively high δ 15 N 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 15 N-rich materials to the Earth's interior.