Nitrogen isotope fractionation during terrestrial core-mantle separation

Abstract

The origin and evolution of the terrestrial nitrogen remains largely unresolved. In order to understand the potential influence of core-mantle separation on terrestrial nitrogen evolution, experiments were performed at 1.5 to 7.0 GPa and 1600 to 1800 °C to study nitrogen isotope fractionation between coexisting liquid Fe-rich metal and silicate melt. The results show that the metal/silicate partition coefficient of nitrogen DNmetal/silicate ranges from 1 to 150 and the nitrogen isotope fractionation δ15Nmetal/silicate is -3.5 ± 1.7 ‰. Calculations show that the bulk Earth is more depleted in δ15N than the present-day mantle, and that the present-day mantle δ15N of -5 ‰ could be derived from an enstatite chondrite composition via terrestrial core-mantle separation, with or without the addition of carbonaceous chondrites. These results strongly support the notion that enstatite chondrites may be a main component from which the Earth formed and a main source of the terrestrial nitrogen. Moreover, in the deep reduced mantle, the Fe-rich metal phase may store most of the nitrogen, and partial melting of the coexisting silicates may generate oceanic island basalts (OIBs) with slightly positive δ15N values.