Mantle oxidation state can be quantified as the oxygen fugacity () of the system within the framework of thermodynamics. A higher oxygen fugacity implies a more oxygen-rich and more oxidized environment. At each given pressure-temperature conditions, for any compound or element M that bears the potential to be oxidized by oxygen, we can write
For example, if M is Fe, the redox equilibrium reaction can be Fe+1/2O=FeO; if M is FeO, the redox equilibrium reaction can be 2FeO+1/2O=FeO.
Gibbs energy change associated with this reaction is therefore
Along each isotherm, the partial derivation of ΔG with respect to P is ΔV,
Combining the 2 equations above,
Therefore, 
 (note that ln(e as the base) changed to log(10 as the base) in this formula. 
For a closed system, there might exist more than one of these equilibrium oxidation reactions, but since all these reactions share a same , examining one of them would allow extraction of oxidation state of the system.