Phase relations in the CaCO3–CaSiO3 system have been studied at 6 GPa and 1300–1750°C. At 1300°C, the subsolidus assemblage comprises of aragonite and walstromite. The eutectic was established at 1400°C and 57 mol% CaSiO3. Just below its melting temperature at 1660°C, the CaCO3 solid quenches to calcite, while, at ≤1600°C, it quenches to aragonite. At 1700°C, a walstromite + larnite + liquid assemblage appears, indicating that the system deviates from a simple binary. Incongruent melting of walstromite to produce larnite and a liquid occurs at 1750°C. The diamonds containing inclusions of walstromite and CaCO3 could be formed as a result of the partial reduction of the CaCO3-CaSiO3 melt derived by melting of walstromite-bearing CaCO3 limestones on the top of basaltic crust subducted to a depth of ≥180 km and warming to ≥1400°C.