The Effect of H20 and Co2 on Planetary Mantles

The solidus for peridotite-HzO-COz is a divariant surface traversed by univafiant lines that locate the intersections of subsolidus divariant surfaces for carbonation or hydration reactions occurring in the presence of HzO-COz 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 peridotitc, extending to higher pressures and lower temperatures from an invariant point near 26 kb-1200øC, there is a temperature maximum on the peridotitc-vapor solidus. On the CO2 side of the maximum, above 26 kb, COz/HzO is greater in liquid than in vapor, and liquids are SlOe-poor; on the HzO side of this maximum (including all pressures below 26 kb), HzO/COz is greater in liquid than in vapor, and liquids change from forsterite-normative to quartz-normative with increasing HzO/COz in vapor. Even traces of HzO and CO2, in minerals or vapor, lower mantle solidus temperatures through hundreds of degrees compared with the volatile-free solidus.