Conditions of diamond and graphite crystallization in the reduced metal-bearing mantle

According to modern concepts, some natural diamonds, including CLIPPIR diamonds, were formed via crystallization from metal‑carbon melts under reducing conditions of the mantle, but many questions regarding their genesis remain debatable. In the present work, the influence of methane‑hydrogen fluid on diamond crystallization was studied in the Ni₇Fe₃-C system with anthracene (C₁₄H₁₀) additives at pressure 5.5 GPa and temperature 1400 °C using multi-anvil split-sphere high-pressure apparatus BARS. It has been found that an increase in the anthracene content from 0 to 2 wt% leads to a decrease in the degree of graphite-to-diamond conversion from 100 % to zero indicating the inhibitory role of the additive. With increasing anthracene content, the growth of diamond single crystals ([C₁₄H₁₀] ≤ 0.72 wt%) is replaced by spontaneous crystallization ([C₁₄H₁₀] = 0.9 wt%), then antiskeletal diamond crystals and metastable graphite are formed ([C₁₄H₁₀] = 1.17–1.61 wt%), at higher anthracene contents ([C₁₄H₁₀] = 2–2.69 wt%) only metastable graphite crystallizes. Inclusions of metal, graphite, methane and hydrogen are found in crystallized diamonds. With increasing anthracene addition in the system, the nitrogen impurity in the diamonds decreases from 190 to 225 ppm to ≤20 ppm. To elucidate the influence of pressure and temperature on diamond crystallization, the second series of experiments was carried out at an anthracene content of 2 wt%, pressure of 7.5 GPa, in the temperature range of 1400–1700 °C. It has been established that at temperatures of 1400 and 1500 °C diamond and metastable graphite crystallize, and at 1600 and 1700 °C only diamond crystallization is realized. The degree of graphite-to-diamond transformation increases from 0 to 5 to 100 % with increasing temperature in the range of 1400–1700 °C. Thus, three main factors controlling the processes of diamond and metastable graphite crystallization in metal‑carbon systems have been experimentally established: the concentration of CH₄-H₂ fluid, which has an inhibitory effect on the processes of diamond formation, as well as pressure and temperature, the increase of which significantly expands the region of diamond crystallization. It is substantiated that the inclusions of metal, graphite, methane and hydrogen, antiskeletal structure of crystal faces and reduced nitrogen concentrations in diamonds can be indicative features of diamonds formed in the reduced metal-bearing mantle with an increased content of methane‑hydrogen fluid.