EXPERIMENTAL STUDIES ON THE INFLUENCE OF CO2 AND H2O IN THE UPPER MANTLE

Abstract

The system CaO-MgO-SiO2 includes the minerals olivine (Fo), orthopyroxene (Opx), and clinopyroxene (Cpx). In the presence of excess CO2 with increasing pressure peridotite undergoes a series of carbonation reactions. At 1100°C, model mantle assemblage Fo + Opx + Cpx + CO2 is progressively transformed into Fo + Opx + Cd (calcic dolomite) + CO2, and then Opx + Cd + Cm (magnesite solid solution) + CO2 near 20 kbar; and into Cm + Cd + Qz + CO2 just above 30 kbar. For realistic mantle conditions, with only trace amounts of CO2, the starting assemblage at 1100°C is changed to Fo + Opx + Cpx + Cd near 20 kbar, and this changes to Fo + Opx + Cpx + Cm near 45 kbar. Each of the univariant carbonation reaction boundaries between these assemblages terminates at an invariant point at which carbonate and silicates melt together with CO2 (1250°-1450°C with pressures increasing from about 25 kbar to 55 kbar), producing a carbonate-rich liquid. In the system CaO-MgO-SiO2-CO2-H2O, the carbonation reactions and melting reactions occur at lower temperatures as H2O content increases. In the presence of CO2-H2O fluids, the carbonate-bearing peridotite acts as a buffer, controlling the CO2/H2O ratio as a function of pressure and temperature. This in turn controls the composition of the liquid at temperatures just above the solidus as a function of depth. Along normal geotherms in the mantle, CO2 is distributed between calcic dolomite and CO2-H2O fluid, with carbonate, CO2 and H2O dissolving in liquid at the top of the seismic low-velocity zone. Amphibole and phlogopite may become stable in peridotite containing H2O, Al2O3, and K2O. The compositions of near-solidus mantle magmas at various depths are strongly influenced by the presence or absence of carbonate, amphibole, and phlogopite in the peridotite. There is now enough experimental data from various laboratories to permit an estimate of the maximum areas of stability of carbonate, amphibole and phlogopite on the peridotite-CO2-H2O solidus. For high CO2/H2O ratios the interstitial liquid at the top of the low-velocity zone is carbonatitic, becoming more silicic (but SiO2-undersaturated) with increasing depth. Upward migration of this liquid could conceivably lead to the local generation of carbonate-rich peridotites within the lower lithosphere.