Peritectic Reaction of Olivine in the Carbonate–Silicate–(C–O–H) Diamond-Forming System at 6 GPa

The influence of the components of supercritical C–O–H fluid (at a content of 7.5 wt %) on the melting relations in the multicomponent diamond-forming system olivine–jadeite–diopside–(Mg–Fe–Ca–Na carbonates)–(C–O–H) was studied experimentally at 6 GPa and 700–1200°C under the conditions of the upper mantle. The peritectic reaction of olivine and jadeite-bearing melt with the formation of garnet was suggested as a key mechanism for the ultramafic–mafic evolution of diamond-forming melts. The CO₂ component of the C–O–H fluid reacts with silicate components to form carbonates. The H₂O component, together with carbonate compounds of the diamond-forming system, reduces the temperature of its liquidus and solidus boundaries significantly. After the completion of crystallization of a completely miscible silicate–carbonate–(C–O–H fluid) melt, a phase of supercritical aqueous fluid and hydrous carbonate nesquehonite MgCO₃·3H₂O identified by Raman spectroscopy appear in the subsolidus of the diamond-forming system.