What is Pseudosection, isopleth, effective bulk composition?

A pseudosection is a phase diagram, usually in the pressure–temperature plane, computed for one fixed bulk composition: it is divided into fields, each labelled by the assemblage that minimises the Gibbs energy there. An isopleth (pseudosection) is a line of a pseudosection along which a compositional variable of one phase, or its amount, is constant. The effective bulk composition is the part of the rock’s composition that still takes part in the equilibrium, once the cores of zoned crystals, porphyroblasts sealed off from the matrix and other isolated material are removed.