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

For the first time, δ¹³C-δ¹⁵N more than 150 diamonds of known paragenesis, originating from different localities are reported. These coupled δ¹³C-δ¹⁵ 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.