• Füri, Evelyn

Abstract

<jats:p>The present-day nitrogen isotopic compositions of Earth’s surficial (<jats:sup>15</jats:sup>N-enriched) and deep reservoirs (<jats:sup>15</jats:sup>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 fractionations during metal–silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW −3.1 &lt; log<jats:italic>f</jats:italic>O<jats:sub>2</jats:sub> &lt; ΔIW −0.5, where ΔIW is the logarithmic difference between experimental oxygen fugacity [<jats:italic>f</jats:italic>O<jats:sub>2</jats:sub>] 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 <jats:italic>f</jats:italic>O<jats:sub>2</jats:sub>. 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<jats:sub>2</jats:sub> and NH complexes). We show that the δ<jats:sup>15</jats:sup>N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core.</jats:p>

Topics

• impedance spectroscopy
• Oxygen
• melt
• glass
• glass
• Nitrogen
• iron
• degassing
• fractionation