Thermodynamic analysis of equilibria involving minerals of the lower mantle of pyrolite composition and crystalline carbon-bearing compounds indicates that the range of oxygen fugacity values at which diamond can be formed is separated from the region in which Fe-rich metallic alloy is generated by a field in which Fe carbides are stable. This implies that diamond can be formed in the lower mantle under more oxidizing conditions than those thought to be dominant in this geosphere. The absence of a metallic phase from the lower-mantle diamond-bearing mineral assemblage is consistent with the high (approximately 1%) Ni concentration in the ferropericlase found as inclusions in diamonds (Fe-rich metallic alloy is able to intensely extract Ni). An elevated redox potential also follows from the occurrence of carbonate phases found among mineral inclusions in lower-mantle diamonds. The main reason for a local increase in oxygen fugacity in the lower mantle may be shifts of redox equilibria toward a decrease in the amount, and then the disappearance of the Fe-Ni alloy with increasing temperature. An important role in the formation of diamond may be played by the generation of carbonate-phosphate and silicate melts in high-temperature zones and the migration of these melts and their interaction with wall rocks.
- divalent iron disproportionation
- lower mantle
- oxygen fugacity
- oxygen potential