Genesis of Diamonds and Paragenetic Inclusions under Lower Mantle Conditions: The Liquidus Structure of the Parental System at 26 GPa

Abstract

The regularities of the joint genesis of diamonds and their paragenetic inclusions under lower mantle conditions are controlled by the melting relations at the liquidus of the multicomponent diamond-forming system. The boundary compositions of this system are evident from the generalized data on the analytical mineralogy of paragenetic inclusions in lower-mantle diamonds. The structure of the liquidus of the diamond-forming system was studied in a physicochemical experiment for P–T parameters typical of the depths of 670–800 km. The compositions of parental melts/solutions for diamonds and paragenetic inclusions correspond to the multicomponent system MgO–FeO–СаО–SiO₂–MgCO₃–FeCO₃–CaCO₃–Na₂CO₃–C. Its primary melting is controlled by the peritectic phase relations at the solidus, which was shown as a result of experimental studies of polythermal sections of the system during a study of their phase diagrams. Of key importance is the effect of the “stishovite paradox,” i.e., the peritectic reaction between the ultrabasic bridgmanite phase and melt with the formation of basic oxide associations of periclase–wustite solid solutions and stishovite. The peritectic reaction of bridgmanite is a fundamental feature of the diamond-forming system and determines the major peculiarity in its liquidus structure. Structure of the peritectic liquidus provides the physicochemical basis for the evolution of growth melts of diamonds and their paragenetic minerals. Based on the experimental data, we have constructed a fractional diagram of syngenesis of diamonds and inclusions clearly illustrating the solution–melt mechanism of diamond genesis and sequence of growth trapping of primary inclusions by diamonds under the lower-mantle conditions. The physicochemical factors of the genesis of diamonds and primary inclusions are agreed and generalized in a compositional diagram of lower-mantle diamond-forming media, and they provide a natural basis for the genetic classification of inclusions of rock-forming and accessory minerals in diamonds