Sulfide melts–graphite interaction at HPHT conditions: Implications for diamond genesis

The interaction of graphite with (Fe,Ni)9S8 and FeS sulfide melts has been studied at P = 6.3–7.5 GPa and T = 1450–2200 °C in experiments with a duration from 2 to 65 h. It has been found that both pentlandite and pyrrhotite melts interacting with graphite show similar behavior. At < 7.5 GPa and 1450–1800 °C and at 7.5 GPa and < 1600 °C carbon crystallizes predominantly as metastable graphite and to the minor extent as diamond on the seed crystals. Only at a pressure of 7.5 GPa and a temperature of 1600 °C and higher, sulfide melts provide spontaneous diamond nucleation at the melt–graphite interface and directly within carbon-saturated sulfide melt. In this case no metastable graphite was found in the products. Diamond crystals synthesized in the (Fe,Ni)9S8–C system were found to contain nitrogen impurity with concentrations of an order of 1000 ppm and exhibited IR absorption peaks due to hydrogen. The luminescence measurements revealed specific optical centers related to nickel impurity in the crystals. It is concluded that sulfide melts are less efficient diamond forming media as compared to carbonate, carbonate–silicate–fluid and fluid systems. The results of the study suggest that diamond nucleation from carbon-supersaturated sulfide melt as supposed in the sulfide model of diamond genesis seems improbable at the UHP conditions of the formation of most natural diamonds.