In this work, we studied the phase relationships in the NaAlSi2O6–CaMgSi2O6–CO2 system in the range of 3–6.5 GPa and 900–1500 ℃ using a multianvil press. The duration of most experiments varied from 5 to 9 days. Equilibrium was examined from both sides using oxide-carbonate mixtures and diopside-jadeite glasses with Ag2C2O4 as starting materials. We found that the subsolidus assemblage is represented by clinopyroxene, coesite, dolomite, and CO2 fluid. The solidus of the system passes through 1015 ℃/3 GPa and 1095 ℃/6 GPa, and has a slope of 38 MPa/℃. The near-solidus carbonate melt contains 1–4 mol% SiO2 and 1–4 mol% Na2O. We found that clinopyroxene can be in equilibrium with CO2 fluid under P–T conditions corresponding to the diamond stability field to at least 6–6.5 GPa and 1050–1100 ℃. Yet, the composition of such clinopyroxene corresponds to almost pure jadeite, Jd# 95–97. Thus, the infiltration of CO2 fluid into eclogite under conditions of the continental lithospheric mantle should be accompanied by partial carbonation of clinopyroxene with the formation of a low-Mg carbonate melt with moderate concentrations of sodium. As a result of the reaction, the compositions of clinopyroxenes of eclogites of Groups A and B should shift towards a high-jadeite composition corresponding to eclogites of Group C. [ABSTRACT FROM AUTHOR]