Oxidation state of the mantle
Abstract
Phase equilibrium relations are established in a system MgFeSiHO, with and without C, at high pressures and temperatures. High pressure-temperature equations of state for the fluids including non-ideal mixing are used in the calculations. The computed equilibrium data show that an olivine of appropriate mantle composition (10 to 14 mole% fayalite) is stable over a wide range of temperature and oxygen fugacities in the carbon-free system. If C is introduced, such that the equilibrium assemblage may contain graphite or diamond, the fluid phase in the peridotite + water system consists mostly of H 2O and little CO 2 and CH 4. However, the fluid composition is strongly affected by the Fe content of the system. If Fe is increased (within 10% by moles of the total Fe in the mantle) from undersaturation ( i.e. no free Fe present in equilibrium) to that of saturation ( i.e. Fe stable as a phase) the CH 4 content of the fluid changes from a low of 1% to a high of 89%. The calculated results show a fluid with as much as 75% methane could be in equilibrium with olivine (13 mole% fayalite) without metallic Fe as a coexisting phase. The fO 2 of the primitive mantle with such a fluid composition would be several log units below that of the quartz-fayalite-magnetite buffer.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- January 1989
- DOI:
- 10.1016/0016-7037(89)90275-5
- Bibcode:
- 1989GeCoA..53...89S
- Keywords:
-
- Chemical Composition;
- Earth Mantle;
- Geochemistry;
- Oxidation;
- Fayalite;
- Magnetite;
- Olivine;
- Quartz;
- Saturation;
- Thermochemistry