The Role of Garnetization of Olivine in the Olivine–Diopside–Jadeite System in the Ultramafic–Mafic Evolution of Upper-Mantle Magmatism (Experiment at 6 GPa)

The peritectic mechanisms controlling the ultramafic–mafic evolution of magmatism during fractional crystallization and the genesis of peridotite–pyroxenite–eclogite rock series of the garnet–peridotite facies of the upper mantle were substantiated experimentally. The melting phase relations of differentiated mantle material in the olivine–clinopyroxene/omphacite–corundum–coesite multicomponent system were studied by the polythermal sections method; their boundary phases reproduced the compositions of peridotitic and eclogitic minerals. The peritectic reaction between orthopyroxene and melt with the formation of clinopyroxene proceeds at the liquidus of the olivine–orthopyroxene–clinopyroxene–garnet system as the mechanism for “clinopyroxenization of orthopyroxene,” which yields the regressive olivine + clinopyroxene + garnet + melt monovariant cotectic reaction. The further evolution of magmatism was studied experimentally at 6 GPa in the ultramafic–mafic olivine–diopside–jadeite–garnet system with a variable composition of the diopside–jadeite solid solutions (clinopyroxene ↔ omphacite). The peritectic reaction between olivine and melt with the formation of garnet was detected on the liquidus of the triple system olivine–diopside–jadeite as the of olivine garnetization mechanism yielding the omphacite + garnet + melt cotectic reaction with the formation of bimineral eclogite. The structure of the liquidus of the olivine–diopside–jadeite–garnet system was defined, as well as its critical role as the “physicochemical bridge” between the ultramafic olivine-bearing peridotite–pyroxenite composition and mafic silica-saturated eclogitic composition of matter within the garnet–peridotite facies. The experimental physicochemical results illustrate the genetic links between ultramafic and mafic rocks and the mechanisms of continuous fractional magmatic evolution and petrogenesis from olivine-bearing peridotite–pyroxenite rocks to silica-saturated eclogite–grospydite rocks. This explains the complete petrochemical trends for the rock-forming components in clinopyroxenes and garnets from differentiated rocks of the garnet–peridotite facies.