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Diamond dissolution rates in kimberlitic melts at 1300–1500 °C in the graphite stability field

¹ Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan
² Asahi Diamond Industrial Co. Ltd., Chiyoda-ku, Tokyo, Japan

^* Corresponding author, e-mail: arima{at}ed.ynu.ac.jp

A series of diamond dissolution experiments were conducted in the graphite stability field under the metal iron-wüstite (IW) oxygen buffer to evaluate the rate of diamond dissolution reactions into two sets of kimberlitic solvents having different CO₂ and H₂O contents. The solvents used were a natural kimberlite from Wesselton Mine, South Africa and a synthetic H₂O-free and CO₂-rich kimberlite. Experiments with the Wesselton kimberlite were carried out at 1400 and 1500 °C under 1.0 GPa and those with the synthetic kimberlite were conducted at 1500 °C under 2.5 GPa. In both series of experiments, the diamond crystal form changed from that of a sharp octahedron through a combination of octahedron and hexoctahedroidal forms to a spherical trioctahedroidal with increasing run duration. The natural and synthetic kimberlitic solvents produced contrasting etch characteristics on the diamond crystal surfaces. Negatively-oriented trigons were formed on the octahedral {111} faces in the runs with the natural kimberlite agent; while diamond surfaces were covered by both negatively and positively oriented smaller trigons and larger hexagonal etch pits in the runs with the synthetic kimberlite solvent. The degree of dissolution was highly sensitive to temperature and carbonate solubility of the solvent. The diamond dissolution rate (radius loss of the diamond crystal) was 0.012 mm/h at 1400 °C and 0.066 mm/h at 1500 °C in the Wesselton kimberlite solvent, and 0.024 mm/h at 1500 °C in the synthetic kimberlite solvent. The data give an activation energy of ~342 kJ/mol for the diamond dissolution reaction in the natural kimberlite solvent at 1.0 GPa under the IW buffer.

Key-words: diamond dissolution, crystal morphology, etch pits, kinetics, high pressure experiment.