Crystallization of Diamonds and Phase Composition of the FeNi–Graphite–CaCO3 System at 5.5 GPa: the Role of Subduction in Their Formation

Experimental modeling of diamond crystallization is conducted in a FeNi–graphite–Ca carbonate system at 5.5 GPa and 1400°C using two scenarios of assembly configurations: layer-by-layer and mixing of all components. It is found that a Ca carbonate is decomposed with the formation of Ca, Fe oxides and release of CO2 upon interaction with FeNi melt. Magnetite can be present as an accessory mineral. Due to the formation of solid reaction products (Ca,Fe oxides) in layer-by-layer assembly configuration, the position of Ca carbonate between graphite and FeNi melt hampers the diamond crystallization in a graphite layer and carbon transport on diamond seed crystals. Mixing of components in an assembly configuration leads to diamond synthesis and growth on seed crystals. The phenomenon of segregation of diamond crystals together with Ca carbonate and oxide phases (the reaction products in metal volume) is recognized. The fluid inclusions trapped by growing diamond contain aliphatic, cyclic, and O-bearing hydrocarbons including heavy compounds (С13–С17), СО2, Н2О, and N- and S-bearing compounds. The fluid phase in these diamonds is more oxidized in comparison with that in diamonds synthesized in a carbonate-free FeNi–graphite system. Our data are consistent with data on natural diamonds, which include crystals with “significantly hydrocarbon” compositions of fluid inclusions indicating a possible involvement of crustal carbonate material in diamond formation processes upon subduction to deep mantle.