Minor amounts of alkalies (Na and K) can reduce drastically the solidus temperatures of carbonated silicate mantle, by as much as 400–500 °C. Low-degree melting of carbonated peridotite and eclogite at pressures of 3–10 GPa produces Na- and K-bearing carbonatite melt. Mass-balance calculations of samples obtained below apparent solidi show clear deficits of alkalies, suggesting the presence of minor alkali-rich liquid or solid carbonate phases. Here we determine the true solidi in Na- and K-bearing carbonate systems and report the stability of alkaline carbonate phases. Melting of subducting alkaline carbonates would likely occur at transition zone depths to produce mobile carbonatite melt diapirs that migrate upward, modifying and oxidizing the upper mantle and initiating volcanism at the surface.