A model for C–O–H fluid in the Earth’s mantle

A model is presented for predicting the composition (H2O, CO2, CH4, H2, CO, O2 and C2H6) in the C–O–H fluid system under high temperatures and pressures found in the Earth’s mantle. The model is based on a molecular dynamic equation of state, statistical mechanics calculations and non-stoichiometric global free-energy minimization. Although the model is not fitted to experimental data on C–O–H speciation, it does accurately reproduce these datasets and should extrapolate at least to the depths of ∼80–220 km. The model results suggest that (1) in the upper cratonic mantle, H2O is the dominant fluid species in the C–O–H fluid system; (2) the abundance of CO2 increases with decreasing depth, the trend of CH4 is just the opposite; (3) the boundary between lithosphere and asthenosphere generally divides fluid systems into H2O–CH4+ minor species and H2O–CO2+ minor species, respectively; (4) it is entirely possible to generate methane and ethane and possibly other hydrocarbons under mantle conditions, confirming previously experimental results.