Mantle Fluid Compositions Buffered by Carbonates in Peridotite-CO_2-H_2O

The system CaO-MgO-SiO_2 is a useful approximation for the mantle, including the minerals olivine (Fo), orthopyroxene (Opx), and clinopyroxene (Cpx). In the presence of with increasing pressure, peridotite undergoes a series of carbonation reactions. Each of the univariant carbonation reactions terminates at an invariant point where carbonate and silicates melt together (1250°-1450°C with pressures increasing from about 25 kb to 55 kb), producing a carbonate-rich liquid. In the system CaO-MgO-SiO_2-CO_2-H_2O, the carbonation reactions and melting reactions occur at lower temperatures with increasing H_2O content. In the presence of CO_2-H_2O fluids, the mantle peridotite with carbonate acts as a buffer, controlling the CO_2/H_2O ratio as a function of pressure and temperature. Carbon dioxide in the mantle is distributed between calcic dolomite and CO_2-H_2O fluid along normal geotherms, with carbonate, CO_2 and H_2O dissolving in liquid at the top of the asthenosphere. In the absence of phlogopite, the carbonated peridotite buffers the fluid composition towards low CO_2/H_2O ratios, approximately estimated to be 0.4 at 30 kb, and 0.2 at 40 kb. If the ratio of CO_2/H_2O in mantle peridotite exceeds these values, some carbonate is produced, and the buffered pore fluid composition is maintained. Incipient melting of carbonated peridotite produces a carbonatitic liquid, and this is probably true even in the presence of pore fluid enriched in H_2O/CO_2. The interstitial liquid at the top of the seismic low-velocity zone may therefore be carbonatitic, becoming more silicic (but SiO_2-undersaturated-kimberlitic) with increasing depth. Upward migration of this liquid could conceivably produce carbonate-rich peridotites in the lower lithosphere. The compositions of mantle magmas are strongly influenced by the distribution of carbonate, amphibole and phlogopite in the peridotite, as indicated by estimates of their maximum stability ranges on the solidus for peridotite-H_2O-CO_2