Fourier transform infrared spectroscopy data and carbon isotope characteristics of the ophiolite-hosted diamonds from the Luobusa ophiolite, Tibet, and Ray-Iz ophiolite, Polar Urals

We report new delta C-13 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 delta C-13 (PDB) values ranging from -28.7% to -16.9%, and N contents from 151 to 589 atomic ppm. The delta C-13 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 delta C-13 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 delta C-13 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 degrees C), and in a short residence time (i.e., within several million years) at high temperatures in the deep mantle.