The stability of carbonate under upper-mantle conditions as a function of temperature and oxygen fugacity

The position of the siderite-oxide equilibrium FeCO (sub 3) + 1/6 O (sub 2) = 1/3 Fe (sub 3) O (sub 4) + CO (sub 2) has been experimentally calibrated at 20 kbar and 1000 degrees C at graphite saturation, for mantle compositions. This equilibrium constrains the upper fO (sub 2) limit for carbonate stability given that Fe contents are much less than Mg in both carbonate and spinel. The experimental results are in good agreement with calculations based on existing thermodynamic data. Thus, by using appropriate upper-mantle compositions and activity models for carbonate and spinel, the stability field of carbonate at any P can be constrained as a function of T and fO (sub 2) . In P-T-fO (sub 2) space the upper T limit of carbonate stability is controlled by the intersection of carbonate and carbonate-oxide oxygen buffer planes. With decreasing magnetite activity the carbonate stability field is restricted to lower T, resulting in a shift in the carbonate stability field; at upper-mantle P, carbonate melts can only be produced without the carbonate breaking down when the fO (sub 2) is above FMQ -0.5 log units.