The stability of the assemblage forsterite-enstatite-magnesite in the presence of CO2–H2O vapor, limited by the reaction MgSiO3 + MgCO3 = Mg2SiO4 + CO2, has been determined at 26 kbar pressure. Activity coefficients for CO2 calculated from experimental data (1.40±0.15 at XCO2 = 0.5 and 1.62±0.13 at XCO2 = 0.3) are sufficiently close to values predicted by modified Redlich-Kwong (MRK) functions (Holloway, 1977) to warrant calculation of the divariant decarbonation surface to 60 kbar.

The calculations are possible because of a reliable set of 1-bar free energies for the above reaction in the absence of H2O. Experimental brackets on the reaction from Newton and Sharp (1975) at 19–41 kbar, from Johannes (1969) at 2 kbar, and from new runs at 26 and 30 kbar were reduced to 1-bar free energies using MRK CO2 fugacities. All experimental brackets are consistent with the equation ΔGT,1 baro = 21,337 – 41.05T\°K) ± 400 cal.

From the new calculations it appears that carbonates (magnesite and dolomite) are stable phases in peridotite assemblages in the mantle, even in the presence of very H2O-rich vapor.

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