Redox control on nitrogen isotope fractionation during planetary core formation

• Published in: Proceedings of the National Academy of Sciences - PNAS 2019-07, Vol.116 (29), p.14485-14494
• Main Authors: Dalou, Celia, Füri, Evelyn, Deligny, Cécile, Piani, Laurette, Caumon, Marie-Camille, Laumonier, Mickael, Boulliung, Julien, Edén, Mattias
• Format: Article
• Language: English
• Subjects: Alloys; Astrochemistry; Coexistence; Composition; core formation; Degassing; Fractionation; Fugacity; ion probe; Iron; Isotope fractionation; Isotopes; Mantle; Metals; Nitrogen; Nitrogen isotopes; Oxygen; Physical Sciences; Planet formation; Planetary cores; PNAS Plus; Sciences of the Universe; speciation; Subduction (geology); Wustite
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1820719116

The present-day nitrogen isotopic compositions of Earth’s surficial (15N-enriched) and deep reservoirs (15N-depleted) differ significantly. This distribution can neither be explained by modern mantle degassing nor recycling via subduction zones. As the effect of planetary differentiation on the behavior of N isotopes is poorly understood, we experimentally determined N-isotopic fractionations during metal–silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW −3.1 < logfO₂ < ΔIW −0.5, where ΔIW is the logarithmic difference between experimental oxygen fugacity [fO₂] conditions and that imposed by the coexistence of iron and wüstite) at 1 GPa and 1,400 °C. We developed an in situ analytical method to measure the N-elemental and -isotopic compositions of experimental run products composed of Fe–C–N metal alloys and basaltic melts. Our results show substantial N-isotopic fractionations between metal alloys and silicate glasses, i.e., from −257 ± 22‰ to −49 ± 1‰ over 3 log units of fO₂. These large fractionations under reduced conditions can be explained by the large difference between N bonding in metal alloys (Fe–N) and in silicate glasses (as molecular N₂ and NH complexes). We show that the δ15N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core.