The stability of carbonate under upper-mantle conditions as a function of temperature and oxygen fugacity
Dalton, John A.; Wood, Bernard J.
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 883 - 892
published: Aug 1, 1995
manuscript accepted: Dec 16, 1994
manuscript received: Jul 21, 1994
ArtNo. ESP147050704015, Price: 29.00 €
Abstract The stability of carbonate in the upper-mantle is a function of oxygen fugacity. Carbon-carbonate buffers such as EMOG (enstatite-magnesite-olivine-graphite) provide a lower ƒO2 limit for carbonate stability. We have experimentally calibrated, for mantle compositions, the position of the carbonate-oxide equilibrium: at 20 kbar and 1000°C at graphite saturation. This equilibrium constrains the upper ƒO2 limit for carbonate stability given that Fe contents are much less than Mg in both carbonate and spinel. We find that the experimental results are in good agreement with calculations based on existing thermodynamic data. Using appropriate upper-mantle compositions and activity models for carbonate and spinel, we are thus able, at any pressure, to constrain the stability field of carbonate as a function of temperature and oxygen fugacity. The calculated stability field is generally compatible with current estimates of upper-mantle ƒO2 (FMQ ±1.5 log units). In P-T-ƒO2 space the upper temperature limit of carbonate stability is controlled by intersection of carboncarbonate and carbonate-oxide oxygen buffer planes. With decreasing magnetite activity the carbonate stability field is restricted to lower temperatures resulting in a shift in the carbonate stability field. At uppermantle pressure carbonate melts can only be produced without the carbonate breaking down when the ƒO2 is above FMQ-0.5 log units.