THE PARTITIONING OF FE AND MG BETWEEN OLIVINE AND CARBONATE AND THE STABILITY OF CARBONATE UNDER MANTLE CONDITIONS

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K(D)(X(Fe)carb X(Mg)ol/X(Fe)ol X(Mg)carb) as a function of Fe/Mg ratio indicates that Fe - Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W(FeMg)ol of 3.7 +/- 0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO3 a W(FeMg)carb of 3.05 +/- 1.50 kJ/mol. The effect of Ca - Mg - Fe disordering is to raise K(D) substantially enabling us to calculate W(CaMg)carb - W(CaFe)carb of 5.3 +/- 2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate ( dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO - MgO - SiO2(CMS) - CO2 system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO2. We also find a stability field of magnesian calcite in lherzolite at 15-25 kbar and 750-1000-degrees-C.