Subduction-related diamonds? — The evidence for a mantle-derived origin from coupled δ13C–δ15N determinations

For the first time, δ13C–δ15N of more than 150 diamonds of known paragenesis, originating from different localities are reported. These coupled δ13C–δ15N determinations argue against a direct formation from subducted biogenic carbon for most of these diamonds. Our data confirm the apparent isotopic disequilibrium of nitrogen between the internal (mantle) and external (atmosphere+crust) reservoirs of the Earth, and thus support the heterogeneous accretion model of the Earth inferred from earlier fibrous diamond studies. We suggest that non-fibrous diamonds have recorded the evolution of the nitrogen isotopic composition starting from a δ15N-value lower than −25‰ to the present δ15N-value around −5‰. This isotopic composition change of mantle nitrogen was built up by large recycling of nitrogen during the early history of the Earth.