Formation of Richterite in the Enstatite–Diopside System in the Presence of K₂CO₃–Na₂CO₃–CO₂–H₂O Fluid: Implications for the Processes of Mantle Metasomatism

The paper presents data on the formation of K–Na richterite in the enstatite + diopside association with K₂CO₃–Na₂CO₃–CO₂–H₂O fluid at 3 GPa and 1000°C as a model for the formation of this mineral in peridotites of the upper mantle. Richterite formation depends on the (H₂O + CO₂)/(K₂CO₃ + Na₂CO₃) and K₂CO₃/Na₂CO₃ ratios in the starting material. A high concentration of alkaline components in the fluid leads to the decomposition of clinopyroxene, the formation of olivine, and a change in the component composition of the pyroxene and amphibole. Fluids with a high potassium concentration are favorable for the formation of K-richterite similar in composition to that formed in metasomatized peridotites of the upper mantle. In some cases, such a fluid leads to the decomposition of amphibole and stabilization of alkaline melt. An increase in the activity of the sodium component results in richterite similar in composition to richterite from lamproites. The clarified relations can be used to assess the activities of fluid components and conditions for the formation of K-richterite. To update the data bank of the Raman spectra of minerals, the largest and most homogeneous amphibole crystals of different compositions were studied.