Nitrogen isotope fractionation during terrestrial core-mantle separation

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 degrees 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 D-N(metal/silicate) ranges from 1 to 150 and the nitrogen isotope fractionation delta N-15(metal-silicate) is -3.5 +/- 1.7 parts per thousand. Calculations show that the bulk Earth is more depleted in delta N-15 than the present-day mantle, and that the present-day mantle delta N-15 of -5 parts per thousand 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 delta N-15 values.