It was suggested that Na–Mg carbonates might play a substantial role in mantle metasomatic processes through lowering melting temperatures of mantle peridotites. Taking into account that natrite, Na2CO3, eitelite, Na2Mg(CO3)2, and magnesite, MgCO3, have been recently reported from xenoliths of shallow mantle (110–115 km) origin, we performed experiments on phase relations in the system Na2CO3–MgCO3 at 3 GPa and 800–1250°C. We found that the subsolidus assemblages comprise the stability fields of Na-carbonate + eitelite and eitelite + magnesite with the transition boundary at 50 mol% Na2CO3. The Na-carbonate–eitelite eutectic was established at 900°C and 69 mol% Na2CO3. Eitelite melts incongruently to magnesite and a liquid containing about 55 mol% Na2CO3 at 925 ± 25 °C. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 86–88 mol% Na2CO3. Melting point of Na2CO3 was established at 1175 ± 25 °C. The Na2CO3 content in the liquid coexisting with magnesite decreases to 31 mol% as temperature increases to 1250°C. According to our data, the Na- and Mg-rich carbonate melt, which is more alkaline than eitelite, can be stable at the P–T conditions of the shallow lithospheric mantle with thermal gradient of 45 mW/m2 corresponding to temperature of 900 °C at 3 GPa.