The Formation of Micro-Diamonds in Decompression-Cracks During the Uplift of the Kimberlite Controlled by the C:O:H Ratio in NAMS

We show new evidence that natural micro-diamonds can be formed in decompression-cracks by C:O:H bearing volatiles in a bimineralic eclogite. The investigated rock sample is a heterogeneous kyanite- bearing and bimineralic eclogite from the Roberts Victor mine, South Africa. Kyanite reacts out in the kyanite bearing part of the sample, but metastable relics are still present within the bimineralic part of the rock. The presence of these metastable kyanite relicts, suggest very low fH2O during the phase transition from the kyanite- bearing into the bimineralic eclogite. High-spatial-resolution synchrotron based FT-IR and RAMAN spectroscopy have been used to detect C:O:H-bearing volatiles around micro diamonds in planar defect structures in garnet in the bimineralic parts of the sample and N concentration has been analyzed within the micro-diamonds. In micro-diamond-bearing planar defect structures, a correlation between C:O:H-bearing volatiles can be identified whereas in micro-diamond -ree planar defect structures no correlation of the different C:O:H containing volatiles was detected. We suggest that the micro-diamond forming reaction was triggered by water released by the breakdown of water-bearing kyanite. We propose that the C:O:H bearing volatiles acted as a catalyst, changing in composition with changing P-T conditions in the rock during metamorphism. This catalytic process leads to permanent modification of C:O:H ratios and under favourable thermodynamic, stoichiometric and kinetic conditions micro-diamonds can be formed. Nitrogen concentrations in the analyzed micro-diamonds suggest that the formation of the micro-diamonds took place shortly before the uplift of the eclogite from the Earth mantle to the surface. The conclusions from our study proves that C:O:H-bearing volatiles, and their distribution pattern around the investigated micro-cracks, are indicative of the formation mechanisms of micro-diamonds controlled by C:O:H bearing fluids rather than by the solid-solid transformation from graphite into diamond.