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Redox control on nitrogen isotope fractionation during planetary core formation

Abstract : The present-day nitrogen isotopic compositions of Earth's surficial (15 N-enriched) and deep reservoirs (15 N-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 fraction-ations during metal-silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW −3.1 < logfO 2 < ΔIW −0.5, where ΔIW is the logarithmic difference between experimental oxygen fugacity [fO 2 ] 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 fO 2. 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 N 2 and NH complexes). We show that the δ 15 N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core. nitrogen isotopes | fractionation | core formation | speciation | ion probe
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https://hal.archives-ouvertes.fr/hal-02172788
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Submitted on : Thursday, December 3, 2020 - 10:55:08 AM
Last modification on : Tuesday, November 23, 2021 - 9:34:19 AM
Long-term archiving on: : Thursday, March 4, 2021 - 6:07:27 PM

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Célia Dalou, Evelyn Füri, Cécile Deligny, Laurette Piani, Marie-Camille Caumon, et al.. Redox control on nitrogen isotope fractionation during planetary core formation. Proceedings of the National Academy of Sciences of the United States of America , National Academy of Sciences, 2019, 116 (29), pp.14485-14494. ⟨10.1073/pnas.1820719116⟩. ⟨hal-02172788v2⟩

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