Genesis of diamonds and paragenetic inclusions under lower mantle conditions: the liquidus structure of the parental system at 26 GPa

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The patterns of the joint genesis of diamonds, and their paragenetic inclusions under lower-mantle conditions, are controlled by the liquidus melting conditions of the multicomponent diamond-forming system. The boundary compositions of this system are evident from the generalized data produced by the analytical mineralogy of paragenetic inclusions in lower-mantle diamonds. We studied the structure of the liquidus of a theoretical diamond-forming system in a physicochemical experiment with P–T parameters typical of the depths of 670–800 km. The compositions of the 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 ёwas controlled by the peritectic phase relations at the solidus, which was revealed by experimental studies of polythermal sections of the system and their phase diagrams. Of key importance is the effect of the “stishovite paradox,” in which the peritectic reaction between the ultrabasic bridgmanite phase and melt coincides 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 it determines the major defining characteristics in its liquidus structure. Elements of the peritectic liquidus provide the physicochemical basis for the evolution of growth melts that yield diamonds and their paragenetic minerals. On the basis of the experimental results, we constructed a fractional diagram of the syngenesis of diamonds and inclusions. It clearly illustrates the solution–melt mechanism by which diamond genesis occurs, as well as the sequence of growth trapping of primary inclusions by diamonds under the lower-mantle conditions. The physicochemical factors of the genesis of diamonds and their primary inclusions agree: they are 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.