The join CaCO3-CaSiO3 at 6 GPa with implication to Ca-rich lithologies trapped by kimberlitic diamonds

Phase relations in the CaCO3-CaSiO3 system have been studied at 6 GPa and 1300-1750 degrees C. At 1300 degrees C, the subsolidus assemblage comprises of aragonite and walstromite. The eutectic was established at 1400 degrees C and 57 mol% CaSiO3. Just below its melting temperature at 1660 degrees C, the CaCO3 solid quenches to calcite, while, at <= 1600 degrees C, it quenches to aragonite. At 1700 degrees 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 degrees 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 degrees C.