Carbonation and melting reactions in the system CaO-MgO-SiO_2-CO_2 at mantle pressures with geophysical and petrological applications

Abstract

Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO_2-CO_2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO_2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO_2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO_2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO_2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO_2-CO_2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO_2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO_2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO_2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO_2 bubbles in peridotite nodules do not represent free CO_2 in mantle peridotite along normal geotherms. (4) CO_2 is as effective as H_2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO_2 at depths shallower than 80 km produces basic magmas, becoming more SiO_2-undersaturated with depth. (6) The solubility of CO_2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO_2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO_2-bearing SiO_2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.

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