Phase diagrams for the assemblage forsterite+enstatite+CO2 in MgO-SiO2-CO2 show that a subsolidus carbonation reaction intersects the solidus near 44 kb–1530°C and stabilizes the carbonate molecule in the liquid, causing CO2 solubility to increase from about 5 to 10 wt percent to about 40 wt percent. A similar increase occurs in the system CaO-MgO-SiO2-CO2 near 25 kb–1200°C for the liquid coexisting with forsterite+orthopyroxene+ clinopyroxene. The CO2 solubility in diopside and enstatite liquids remains relatively low, because these are not involved in carbonation reactions at this pressure. This accounts for contrasted CO2 solubilities in silicate melts reported in recent experimental studies. Magmas generated in a CO2-bearing mantle below a depth of 80 km may contain up to 40 wt percent dissolved CO2. CO2-rich, SiO2-undersaturated magmas can coexist with mantle peridotite through a wide temperature range. These could represent either primary carbonatite and kimberlite magmas or the carbonated alkali ultrabasic magmas cited by many petrologists as parents for the derivation of continental volcanic and plutonic associations of highly alkalic rocks.