Melting relations of the hydrous primitive mantle in the CMAS-H₂O system at high pressures and temperatures, and implications for generation of komatiites

The hydrous phase relations of primitive mantle compositions in the CaO-MgO-Al₂O₃-SiO₂-H₂O system with 1, 2, and 5 wt.% H₂O have been investigated at 4, 6.5, and 8 GPa. The stability field of orthopyroxene in the residue expands with increasing H₂O content at 4-8 GPa. Dissolution temperature of garnet decreases more rapidly than that of the other phases at 4 and 6.5 GPa. The dissolution curves of garnet and pyroxenes cross at 6 GPa and 1700°C in the 2 wt.% H₂O-bearing system. Garnet is stable until high degree of melting at 8 GPa. Compositions of aluminum depleted komatiites are consistent with the liquid formed by more than 30% melting of the primitive mantle with H₂O up to 5 wt.% at 8 GPa. Aluminum undepleted komatiite can be formed by more than 50 wt.% melting of dry and wet primitive peridotite (with water up to 2 wt.% H₂O) at around 4-6 GPa. Variation in chemistry of komatiites might be explained by partial melting of primitive mantle with various water contents.