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, dolo mite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, bu t Fe and Mg partition almost equally between disordered calcic carbona te and olivine. Measurement of K(D)(X(Fe)carb X(Mg)ol/X(Fe)ol X(Mg)car b) as a function of Fe/Mg ratio indicates that Fe - Mg carbonates devi ate 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 o btain 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 act ivity-composition relationships and partitioning data have been used t o calculate the effect of Fe/Mg ratio on mantle decarbonation and exch ange reactions. We find that carbonate ( dolomite and magnesian calcit e) is stable to slightly lower pressures (by 1 kbar) in mantle lherzol itic assemblages than in the CaO - MgO - SiO2(CMS) - CO2 system. The h igh pressure breakdown of dolomite + orthopyroxene to magnesite + clin opyroxene is displaced to higher pressures (by 2 kbar) in natural comp ositions relative to CMS. CO2. We also find a stability field of magne sian calcite in lherzolite at 15-25 kbar and 750-1000-degrees-C.