Carbonation, hydration, and melting relations in the system MgO-H_2O-CO_2 at pressures up to 100 kbar

Abstract

A model has been derived for the subsolidus and melting relationships in the system MgO-H_2O-CO_2 for pressures up to 100 kbar. Using both low- and high-pressure experimental data, the ΔH° and ΔS° for the subsolidus reactions Mg(OH)_2 → MgO + H_2O, MgCO_3 → MgO + CO_2, and MgCO_3 + H_2O → Mg(OH)_2 + CO_2 are estimated to be respectively: ΔH° = 18400 cal/mol and ΔS° = 33.16 cal/ ° mol, ΔH° = 20140 cal/mol and ΔS° = 34.87 cal/° mol and ΔH° = 1740 cal/mol and ΔS° = 1.71 cal/° mol. In the presence of MgCO_3, vapor is buffered to high values of H_2O/CO_2 except for a narrow pressure interval just above the reaction MgCO_3 → MgO + CO_2. As pressure is increased the vapor coexisting with an MgCO_3-bearing assemblage becomes richer in H_2O. MgCO_3 first melts at an invariant point located at 23 kbar and 1550°C. Mg(OH)_2 melts at an invariant point estimated to be near 58 kbar and 1310°C. Mg(OH)_2 + MgCO_3 + vapor melt together at a eutectic at pressures greater than or equal to that of an invariant point located near 46 kbar and 1210°C. The composition of the eutectic liquid on the join Mg(OH)_2-MgCO_3 is estimated to be 73 mole percent Mg(OH)_2 plus 27 mole percent MgCO_3. The system MgO-H_2O-CO_2 provides the thermodynamic basis for estimating the distribution of H_2O and CO_2 between coexisting liquids and vapors in the presence of hydrates and carbonates, and is an essential first step leading to the interpretation of similar phase relationships in the model mantle system CaO-MgO-Al_2O_3-SiO_2-H_2O-CO_2.

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