The Formation of Micro Diamonds in Dislocation Creep Controlled by the C:O:H Ratio of NAMS

Abstract:

Diamonds are expected to be formed in the diamond window under high-pressure conditions due to the polymorph graphite/diamond phase transition but they can also be formed by C:O:H bearing volatiles. In this study we present the formation of micro-diamonds in micro-cracks by C:O:H bearing fluids in a bimineralic eclogite. The studied rock sample is a kyanite- bearing and bimineralic eclocite from the Roberts Victor mine, South Africa. Relicts of the former kyanite bearing eclogite are still present within the bimineralic eclogite. The presence of these metastable relicts suggest very low fH₂O during the phase transition from the kyanite- bearing into the bimineralic eclogite. The micro-diamond forming reaction was triggered by fluids during the breakdown of kyanite in the kyanite bearing eclogite. Our results show that the C:O:H ratio in the fluid controls the diamond-forming reaction as the ratio changes due to the movement of planar defect structures in garnet in the bimineralic eclogite. This process leads to the formation of isotopically light carbon molecules and under favorable thermodynamic, stoichiometric and kinetic conditions micro diamonds can be formed. High-spatial-resolution synchrotron based FT-IR and RAMAN spectrometry have been used to map the distribution of C:O:H-bearing volatiles around micro diamonds in planar defect structures in garnet. In micro diamond bearing planar defect structures, a correlation between C:O:H-bearing volatiles can be identified whereas in micro diamond free planar defect structures no correlation of the different C:O:H containing volatiles is visible. The conclusions from our study shows that C:O:H-bearing volatiles, and their distribution pattern around the studied micro cracks provide insight into the formation process of micro diamonds in natural bimineralic eclogites.