Carbon-fluid equilibria and the oxidation state of the upper mantle

It has been proposed that the oxidation state of the Earth's upper mantle is buffered by C-O fluids in equilibrium with elemental carbon^1,2. A large body of data on the oxygen fugacities (fo₂) recorded by mantle rocks and their derivative melts now allows us to test this proposal. By comparing the measured fo₂ values with those calculated for carbon-CO₂-CO-carbonate equilibria along appropriate mantle geotherms, we find the data to be wholly consistent with this hypothesis. Moreover, the calculated variation of fo₂ with temperature and pressure accounts for much of the observed correlation between the oxidation state of mantle samples and their tectonic provenance^3,4. The apparent buffering of mantle fo₂ requires modest quantities of mantle carbon and fluid, which do not exceed independently estimated values. The proposal is not compromised if parts of the mantle are fluid-undersatu rated, or if the C-O fluid phase is diluted by other volatile.