The key to the origin of carbonatite and kimberlite lies in the system CaO-MgO-SiO2-CO2. Increase in pressure causes a carbonation reaction in the peridotite assemblage as follows: forsterite + clinopyroxene + CO2 ⇌ orthopyroxene + carbonate (Ca:Mg::70:30). This reaction passes through 15 kb–960°C with slope 45 b/°C and terminates at an invariant point near 25 kb-1200°C, where melting begins. This intersection of the carbonation reaction with the solidus introduces primary carbonate minerals alongside peridotite minerals on the liquidus surface. At 20 kb the melting temperature of the peridotite assemblage Fo + Opx + Cpx is lowered 75°C by solution of about 5 wt percent CO2. The liquid corresponds to undersilicated basic magma. Stabilization of carbonate increases CO2 solubility in the liquid, and above 25 kb the liquidus reaction involving Fo + Opx + Cpx + CO2 sweeps down through 400°C via a pressure maximum at 32 kb to meet the invariant point at 25 kb. The peridotite solidus curve at higher pressures involves fusion of silicates and carbonates, producing a carbonatitic liquid with more than 45 wt percent CO2. Progressive fusion produces a kimberlitic liquid. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surface. Fractional crystallization of CO2-bearing under-silicated basic magmas at most pressures yields residual kimberlite and carbonatite. Kimberlite and carbonatite magmas rising from the asthenosphere evolve CO2 as they reach a reaction boundary at a depth of about 100 to 80 km. This contributes to their explosive eruption. Free CO2 cannot coexist with subsolidus mantle peridotite with normal temperature distributions. CO2 appears to be as effective as H2O in causing incipient melting in the asthenosphere.