Peritectic Reaction of Olivine in the Olivine–Jadeite–Diopside–Garnet–(C–O–H) System at 6 GPa as the Key Mechanism of the Magmatic Evolution in the Upper Mantle

— The melting relations in the olivine–jadeite–diopside–garnet–(С–О–H fluid) system, including metasomatic СО 2 carbonatization of silicates and Н 2 О dissolution in silicate–carbonate melts, were studied in a physicochemical experiment at 6 GPa (under the conditions of the upper mantle). The influence of the С–O–H fluid (5.0 wt %, with equimolecular contents of Н 2 О and СО 2 ) on the composition and temperature of the quasi-invariant peritectic reaction between olivine and jadeite-bearing melt with the formation of garnet was demonstrated. This reaction is the key element of the liquidus structure of silicate–fluid magmatic and silicate–carbonate–carbon–fluid diamond-forming systems. This is because this reaction controls the final episodes of the fractional ultramafic–mafic evolution of magmatic and diamond-forming melts in the upper mantle. The mode of fractional crystallization contributes to an increase in the concentration of jadeite and hydrous fluid components in the residual silicate–fluid and silicate–carbonate–fluid melts, which controls the sequential change in the total composition of these systems. The importance of the jadeite component in the bulk composition of the multicomponent magmatic system changes with its fractional accumulation from a minor component in ultramafic peridotite to a major (system-forming) component in mafic eclogite. According to the experimental data, the peritectic reaction of olivine retains its primary importance, since the C–O–H fluid components do not introduce radical qualitative changes in the melting relations of the ultramafic–mafic system olivine–jadeite–diopside–garnet–(C–O–H-fluid) at 6 GPa. At the same time, their participation in the composition of the system leads to quantitative topological changes in the parameters of its liquidus structure: a decrease in the temperatures of the solidus and liquidus boundaries by 120 and ~60–80°C, respectively, as well as a shift in the composition of the peritectic reaction of olivine with an increase in the concentration of olivine-bearing component by ~10 wt %. The significance of the peritectic reaction of olivine is discussed in the context of the general scenario of the ultramafic–mafic evolution of upper-mantle magmatism and the genesis of diamond, taking into account the role of the peritectic reaction of orthopyroxene at its early ultramafic stages.