Between depths of 30 and 60 km, oxygen fugacity is mainly controlled by Olivine-Orthopyroxene-Spinel oxidation reaction.
Under deeper upper mantle conditions, Olivine-Orthopyroxene-Garnet oxygen barometer is the redox reaction that is used to calibrate oxygen fugacity. 
In this reaction, 4 mole of ferrous ions were oxidized to ferric ions and the other 2 mole of ferrous ions remain unchanged.