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

Grewal, Damanveer S. and Sun, Tao and Aithala, Sanath and Hough, Taylor and Dasgupta, Rajdeep and Yeung, Laurence Y. and Schauble, Edwin A. (2022) Limited nitrogen isotopic fractionation during core-mantle differentiation in rocky protoplanets and planets. Geochimica et Cosmochimica Acta, 338 . pp. 347-364. ISSN 0016-7037. doi:10.1016/j.gca.2022.10.025.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Grewal, Damanveer S.0000-0002-5653-1543
Sun, Tao0000-0001-6266-3350
Aithala, Sanath0000-0002-8757-5370
Dasgupta, Rajdeep0000-0001-5392-415X
Yeung, Laurence Y.0000-0001-9901-2607
Schauble, Edwin A.0000-0002-1266-1282
Additional Information:Amrita P. Vyas is thanked for helping improve the clarity of our communication. Gelu Costin is thanked for his assistance during the microprobe analysis. We are grateful to have received thorough and constructive reviews by Mathieu Roskosz and two anonymous reviewers which significantly helped us improved the manuscript. We also thank Mathieu Roskosz for his efficient handling of the manuscript as an Associate Editor. D.S.G. received support from a Barr Foundation Postdoctoral Fellowship by California Institute of Technology, a NASA FINESST grant 80NSSC19K1538, and a Lodieska Stockbridge Vaughn Fellowship by Rice University. T.S. received support from NASA grant 80NSSC19K0784. R.D. received support from NASA grants 80NSSC18K0828 and 80NSSC18K1314. E.S. received support from NSF grants EAR1524811 and EAR1530306.
Funding AgencyGrant Number
Barr FoundationUNSPECIFIED
NASA Earth and Space Science Fellowship80NSSC19K1538
Rice UniversityUNSPECIFIED
Record Number:CaltechAUTHORS:20221220-736506000.5
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ID Code:118573
Deposited By: Research Services Depository
Deposited On:25 Jan 2023 15:13
Last Modified:25 Jan 2023 18:43

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