Carbonate and Carbonate-Rich Liquids in the Earth's Interior, and Critical Fluids in Diamond Inclusions

The near-solidus liquid composition for peridotite-CO_2-H_2O deeper than ~65 km is dolomitic carbonatite; a coexisting vapor phase, if present, is hydrous. Carbonates, CO_2 and H_2O have been found in fluid inclusions in fibrous diamonds, with compositions corresponding to these two components, and to mixtures between them. Interpretations involving mixing or partial melting do not satisfy phase relationships. A possible explanation involves a critical endpoint on the mantle solidus, which permits diamonds growing within a relatively small mantle volume to trap fluids with compositions ranging from carbonatite liquid to dense hydrous vapor, defined by the depth-T position.