We report new δ13C data and N content and aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray-Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 to 589 atomic ppm ± 20%) with a consistently low aggregation state and show identical infrared spectra dominated by strong absorption between 1130 cm-1 and 1344 cm-1, and therefore characterize type Ib diamond. Microdiamonds from the Luobusa peridotites have δ13C (PDB) values ranging from -28.7‰ to -16.9‰, and N contents from 151 to 589 atomic ppm. The δ13C and N content values for diamonds from the Luobusa chromitites are -29‰ to -15.5‰ and 152–428 atomic ppm, respectively. Microdiamonds from the Ray-Iz chromitites show δ13C values varying from -27.6‰ to -21.6‰ and N contents from 108 to 499 atomic ppm. The carbon isotopes values have features similar to previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds we analyzed, in that they show enhanced δ13C variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N aggregation state and N content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e., <950 °C), and in a short residence time (i.e., within several million years) at high temperatures in the deep mantle.

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