Phase relations in the peridotite-carbonate-chloride system at 7.0-16.5Â GPa and the role of chlorides in the origin of kimberlite and diamond

Melting phase relations have been determined in a model chloride-carbonate-bearing peridotite at 7.0-16.5 GPa and 1200-1800 °C. The typical assemblage coexisting with partial melts is forsterite/wadsleyite-enstatite/clinoenstatite-garnet. In contrast to carbonated peridotite, clinopyroxene was absent in chloride-carbonate system. Also, carbonatite melt formed at low-degree of partial melting are enriched by CaO. We observed Ca- and Mg-bearing carbonatite melts in all experiments. The solidus temperature was estimated using the stability of magnesite and appearance of alkali-poor carbonatite melt at 1300 °C at 10.5 GPa and 1400 °C at 16.5 GPa. Halides are stable 100-200° C above this 'apparent' solidus. This fact indicates that melt composition in chloride-carbonate peridotite can be effectively controlled by the presence of water. In the anhydrous environments a low-fraction of 'dry' Cl- and alkali-poor Ca-Mg-rich carbonatite melts can be formed and migrate from the source, whereas under the hydrous conditions, chloride can be dissolved into water to form alkali-rich chloride-carbonate brine-like liquids. At higher temperatures (1400-1600 °C) two immiscible carbonatite and chloride-carbonate liquids may coexist. The composition of carbonatite and chloride-carbonate melts is consistent with that of melt inclusions in fibrous/cloudy diamonds worldwide. Typical trends of melt evolution upon cooling and formation the melts included in diamonds may involve formation of immiscible Si-poor carbonatite and Si-poor chloride-carbonate melts from homogenous Cl-bearing kimberlite- or carbonatite-like melt.