P-T Diagrams

The pressure-temperature diagrams of rock compositions display the stability fields for the different phase assemblages at various pressures and temperatures. These diagrams afford information about the possible natural phase relations for rocks and magmas, and indicate the possible combinations of phenocrysts that might constitute a fractionate, and might as well show the P-T regions for the generation of a particular composition by partial melting. The disadvantage of the P-T diagrams for natural compositions is that the particular phase relations estimated, is only valid for the composition investigated. If an incongruent reaction occurs for a given composition, it is not granted that the reaction also occurs for a similar but slightly different composition. The influence of a small difference in composition on the phase relations is especially evident from systems with congruent compounds and thermal divides. The synthetic systems with pure end member compositions will clarify the fundamental phase relations, and indicate the compositional ranges for which a particular phase relation is valid. On the other hand, the synthetic systems do not define accurate P-T values for the natural magmas. Thus, the two types of systems supplement each other, the fundamental phase relations are evident from synthetic systems, while accurate estimates of pressure and temperatures may be obtained from experiments on natural compositions at different P-T values.