The following figure illustrates crystallization paths for liquids of two differing water contents. Both have a P total of 4 kb while liquid A has a P H2O of 3 kb and liquid B is anhydrous. Liquid A , which is water-rich, will exsolve a vapor phase at 3 kb P total , cool and solidify before reaching the surface. Liquid B , in contrast, will erupt at the surface. Has a reverse effect on polymerization at low temperature.
Figure summarizes the effects of various volatiles on the solidus.
Thus the mineral assemblage of any rock is, to a certain extent, a function of oxygen fugacity. A greater oxygen fugacity means fewer iron silicates hence fewer mafic minerals. The oxygen combines with iron to form oxides, thus making the rock more silica saturated than it would otherwise be. We can quite literally change the CIPW of a rock from normative ne to normative q by increasing the oxygen fugacity and hence increasing the ferric iron. 64 cm 3 or a 32X expansion.