Density measurements of eight silicate liquids containing substantial amounts of Fe₂O₃ have been made over a range of 250 °C. These have been combined with published density measurements on multicomponent silicate liquids to yield (by multiple regression) partial molar volumes of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, FeO, MgO, CaO, Na₂O, and K₂O. The data on Fe₂O₃-liquids are neither precise nor abundant enough to show a compositional dependence of V¯Fe2O3liq. In a liquid of constant composition and temperature, the pressure dependence of the oxygen fugacity is given by RT∫1PdlnfO2=∫1P(2V¯Fe2O3liq−4V¯FeOliq)dP which, if ΔV is independent of pressure, necessitates an increase in f_O2 with increasing pressure of about 1 log₁₀ unit for 10 kbars.

Combining an equation relating oxygen fugacity to composition, T, and Fe₂O₃ at 1 bar (Sack et al., 1980) with the results for partial molar volumes, the oxygen fugacity of any magma can be calculated as a function of P and T. If basic magmas have their Fe₂O₃/FeO set in the source regions, and ascend isochemically, then the calculated oxygen fugacities in the mantle increase as pressure increases and silica activity decreases. A P-T grid has been constructed to show the calculated oxygen fugacities in a source region which has equilibrated with some common lava types, based on their FeO and Fe₂O₃ contents.