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

For the first time, delta(13)C-delta(15)N of more than 150 diamonds of known paragenesis, originating from different localities are reported. These coupled delta(13)C-delta(15)N 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 delta(15)N-value lower than -25%. to the present delta(15)N-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. (C) 1998 Elsevier Science B.V. All rights reserved.