Free energies of decarbonation reactions at mantle pressures; I, Stability of the assemblage forsterite-enstatite-magnesite in the system MgO-SiO 2 -CO 2 -H 2 O to 60 kbar

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 Δ$GT,1 baro$ = 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.