The effect of H_2O and CO_2 on planetary mantles

The solidus for peridotite-H_2O-CO_2 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 H_2O-CO_2 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 CO_2 side of the maximum, above 26 kb, CO_2/H_2O is greater in liquid than in vapor, and liquids are SiO_2-poor; on the H_2O side of this maximum (including all pressures below 26 kb), H_2O/CO_2 is greater in liquid than in vapor, and liquids change from forsterite-normative to quartz-normative with increasing H_2O/CO_2 in vapor. Even traces of H_2O and CO_2, in minerals or vapor, lower mantle solidus temperatures through hundreds of degrees compared with the volatile-free solidus.