The paper is concerned with experimental studies aimed at assessing the possibility of diamond crystallization during interactions of alkaline-earth and transition metal carbonates with sulfur and at characterizing the obtained diamond crystals and inclusions in them. The experiments were carried out on a multi-anvil high-pressure “split sphere” apparatus (BARS) at constant P–T–t parameters: 6. 3 GPa, 1550 ºC, and duration of 20 h, in the Mg, Ca-carbonate–sulfur system with different CaO/MgO ratios and in the Fe, Mg, Ca-carbonate–sulfur system with variable FeO/CaO/MgO proportions. It has been experimentally established that in the alkaline-earth carbonate–sulfur systems, a Ca, Mg-carbonate melt with dissolved sulfur (1. 2–6. 0 wt. %) forms, which is a diamond growth medium. An octahedron is the stable growth form of diamond crystals, and an increase in the Ca# value of the system is accompanied by an increase in the growth rate of the 111 faces. In the Fe, Mg, Ca-carbonate–sulfur system, Fe, Mg, Ca-carbonate melt with dissolved sulfur is shown to be the crystallization medium and the source of diamond carbon, and diamond growth occurs as a result of the redox interaction of the sulfide and carbonate melts. An increase in the Fe# value of the system from 0. 41 to 0. 78 is accompanied by: (1) an increase in the growth rate of the 100 faces; (2) a change in the predominant elements of the octahedron face microrelief from triangular growth layers to hexagonal vicinals; and (3) an increase in the number and size of inclusions and the evolution of their phase composition. Inclusions in diamond form as a result of the melt preservation on the 100 faces during the transformation of a cuboctahedron into an octahedron. The inclusions contain quenched carbonate melt (quenching phases are carbonate, magnetite, and graphite), sulfur melt, and Fe–S–O melt. The revealed carbonate + magnetite + graphite assemblage in the inclusions testifies to a siderite redox dissociation reaction at the P–T parameters of the lithospheric mantle. These results shed light on the possible origin of magnetite inclusions in upper-mantle diamonds and suggest that the presence of magnetite in syngenetic inclusions is an indicator of the participation of iron-bearing carbonates in natural diamond formation processes.
Furman et al. (Sun,) studied this question.