Experimental Studies on the Influence of CO_2 and H_2O in the Upper Mantle

Abstract

The system CaO-MgO-SiO_2 includes the minerals olivine (Fo), orthopyroxene (Opx), and clinopyroxene (Cpx). In the presence of excess CO_2, with increasing pressure peridotite undergoes a series of carbonation reactions. At 1100°C, model mantle assemblage Fo + Opx + Cpx + CO_2 is progressively transformed into Fo + Opx + Cd (calcic dolomite) + CO_2, and then Opx + Cd + Cm (magnesite solid solution) + CO_2 near 20 kbar; and into Cm + Cd + Qz + CO_2 just above 30 kbar. For realistic mantle conditions, with only trace amounts of CO_2, 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 CO_2 (1250°-1450°C with pressures increasing from about 25 kbar to 55 kbar), 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 as H_2O content increases. In the presence of CO_2-H_2O fluids, the carbonate-bearing peridotite acts as a buffer, controlling the CO_2/H_2O 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, CO_2 is distributed between calcic dolomite and CO_2-H_2O fluid, with carbonate, CO_2 and H_2O dissolving in liquid at the top of the seismic low-velocity zone. Amphibole and phlogopite may become stable in peridotite containing H_2O, Al_2O_3, and K_2O. 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-CO_2-H _2O solidus. For high CO_2/H_2O ratios the interstitial liquid at the top of the low-velocity zone is carbonatitic, becoming more silicic (but SiO_2-undersaturated) with increasing depth. Upward migration of this liquid could conceivably lead to the local generation of carbonate-rich peridotites within the lower lithosphere.

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