Formation of richterite in the enstatite–diopside system in the presence of the K2CO3–Na2CO3–CO2–H2O fluid in application to the processes of mantle metasomatism

The paper presents results of studying the formation reaction of K–Na-richterite in the enstatite + diopside association with the participation of the K2CO3–Na2CO3–CO2–H2O fluid at 3 GPa and 1000°C, simulating 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, as well as a change in the component composition of pyroxene and amphibole. Fluids with a high concentration of the potassium component are responsible 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 the alkaline melt. With an increase in the activity of the sodium component, the fluid contains richterite, which is similar in composition to richterite from lamproites. The obtained patterns can be used to assess the activities of fluid components and the conditions for the formation of K-richterite. To replenish the data bank of Raman spectra of minerals, the largest and most homogeneous amphibole crystals of different compositions were studied.