The direct application of transport models to the long standing issues in petrology has often been a frustrating exercise. Rarely can one confidently identify detailed physical processes that drive petrologic diversity. Part of the difficulty originates in the construction of physical and chemical models; they often appear as fundamentally different enterprises. Chemical petrology has traditionally sought to characterize equilibrium states and to advance the formalisms of physical chemistry, the independent variables being taken from the usual thermodynamic ensemble: T, P, etc. Physical petrology differs in that the independent variables are space and time, and the effort is directed at identifying the structures and fluxes that follow from nonequilibrium states. It seems timely then to consider elements that are common to both chemical and physical descriptions of petrologic processes. A unification of this kind is guaranteed to succeed eventually as Nature does not seem to suffer the distinction.

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