Abstract A capsule assembly for use at high pressures and temperatures in the piston-cylinder apparatus has been developed which prevents iron loss to capsules and fixes fluid fugacities. The oxygen fugacity of the assembly is fixed by the iron-wustite buffer, and fugacities of other fluid species present are fixed by the buffer and the presence of graphite. The conditions of buffering and no iron loss to capsules allow a very high degree of precision from run to run. This allows determination of subtle melting reactions. The solubility of the fluid phase in albite, basanite and tholeiite liquids was determined using infrared, near-infrared and Raman spectral analysis along with quadrupole mass spectrometry, thermogravimetric analysis and total C-H analysis. Only H 2O and CO show significant solubility compared to their activities. CH 4, CO 2 and H 2 are rejected by the melt. Liquids produced by different degrees of melting of a low-K basanite, which fractionates hornblende, show an alkali trend similar to fractionation trends observed in nature.