The solids for peridotite-H2O-CO2 is a divariant surface traversed by univariant lines that locate the intersections of subsolidus divariant surfaces for carbonation or hydration reactions occurring in the presence of H2O-CO2 mixtures. Vapor phase compositions are normally buffered to these lines; the buffering capacity of carbonates is much greater than that of amphibole and phlogopite. Near the buffered curve for the solidus of partly carbonated peridotite, extending to higher pressures and lower temperatures from an invariant point near 26 kb-1200 ¿C, there is a temperature maximum on the peridotite-vapor solidus. On the CO2 side of the maximum, above 26 kb, CO2/H2O is greater in liquid than in vapor, and liquids are SiO2-poor; on the H2O side of this maximum in vapor, and liquids are SiO2-poor; on the H2O side of this maximum (including all pressures below 26 kb, H2O/CO2 is greater in liquid than in vapor, and liquids are SiO2-poor; on the H2O side of this maximum (including all pressures below 26 kb), H2/CO2 is greater in liquid than in vapor, and liquids change from forsterite-normative to quartz-normative with increasing H2O/CO2 in vapor. Even traces of H2O and CO2, in minerals or vapor, lower mantle solidus temperatures through hundreds of degrees compared with the volatile-free solidus.