THE STABILITY OF CARBONATE UNDER UPPER-MANTLE CONDITIONS AS A FUNCTION OF TEMPERATURE AND OXYGEN FUGACITY

Authors
Citation
Ja. Dalton et Bj. Wood, THE STABILITY OF CARBONATE UNDER UPPER-MANTLE CONDITIONS AS A FUNCTION OF TEMPERATURE AND OXYGEN FUGACITY, European journal of mineralogy, 7(4), 1995, pp. 883-891
Citations number
37
Categorie Soggetti
Mineralogy
ISSN journal
09351221
Volume
7
Issue
4
Year of publication
1995
Pages
883 - 891
Database
ISI
SICI code
0935-1221(1995)7:4<883:TSOCUU>2.0.ZU;2-J
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 fO(2) limit for carbonate stability. We have experimentally calibrated, for mantle compositions, the posit ion of the carbonate-oxide equilibrium: GRAPHICS at 20 kbar and 1000 degrees C at graphite saturation. This equilibrium constrains the upp er fO(2) limit for carbonate stability given that Fe contents are much less than Mg in both carbonate and spinel. We find that the experimen tal results are in good agreement with calculations based on existing thermodynamic data. Using appropriate upper-mantle compositions and ac tivity models for carbonate and spinel, we are thus able, at any press ure, to constrain the stability field of carbonate as a function of te mperature and oxygen fugacity. The calculated stability field is gener ally compatible with current estimates of upper-mantle fO(2) (FMQ +/- 1.5 log units). In P-T-fO(2) space the upper temperature limit of carb onate stability is controlled by intersection of carbon-carbonate and carbonate-oxide oxygen buffer planes. With decreasing magnetite activi ty the carbonate stability field is restricted to lower temperatures r esulting in a shift in the carbonate stability field. At upper-mantle pressure carbonate melts can only be produced without the carbonate br eaking down when the fO(2) is above FMQ-0.5 log units.