Role of Volatiles in the Earth
Diamond genesis can be clarified by estimating the common parental media for diamond and its syngenetic inclusions. Formation of diamond and diamondite in carbonatitic melts with garnets, clinopyroxenes, carbonates, iron-chromium alloys, and other minerals was confirmed in experiments using diamond-bearing Kokchetav (Kazakhstan) and Chagatai (Uzbekistan) carbonatitic rocks as the starting materials. Experiments on melting equilibrium of an eclogitic garnet-pyrrhotite join at 7 GPa revealed the existence of a nearly complete silicate-sulfide liquid immiscibility. Very low solubility of silicate components in the sulfide melt implies that the melt is not so efficient for syngenesis of diamonds and silicate inclusions, whereas carbonatitic (carbonate-silicate) parental melts can provide syngenesis of diamond and their primary inclusions more viably. The major components of the parental media for diamond syngenesis are carbonates and silicates, and the minor components are oxides, sulfides, phosphates, haloids, carbon dioxide, water, etc. These media are partially or completely molten during diamond formation, and they have compositionally variable major and minor component contents. It is obvious that the parental media for diamond is closely related to the genesis of carbonatitic magmas in the Earth's mantle.