Although imperfect, distinction between fluid-driven reactions and gradient reactions is nevertheless convenient when describing the development of concepts pertaining to metamorphic fluid flow that occurred over the last quadrennium. Categorization of fluid-rock effects in this report is not intended to enforce or advocate a form of semantics. Instead, the author finds that it serves to underscore differences and, indeed, similarities between the various processes considered.
Fluid-driven reactions are those for which the sole (or principal) driving force is continued ingress of fluid that enters the flow system out of equilibrium with the reacting rock. In contrast, gradient reactions arise where flowing fluids enter in, and maintain, equilibrium with host rocks. Reaction progress in a fluid-driven reaction occurs as the rock strives to achieve equilibrium with the flowing fluid. Cessation of flow causes desistance of reaction. In the case of gradient reactions, reaction progress occurs in response to the prolonged availability of fluid combined with flow along existent gradients in factors of state other than fluid composition such as temperature and pressure. Reaction ends if either the flux of fluid or gradients in state variables fade.
Fluid-driven reactions result in abrupt changes in physical and chemical properties affected by reaction that propagate in the direction of flow. Gradient reactions allow for gradual changes in the physico-chemical parameter of interest. However, exploration of these concepts has shown that discontinuities similar to those associated with fluid-driven reactions can result from gradient reactions in the event that the capacity of the rock to react with fluid is exhausted (e.g., loss of a reactant mineral). Similarly, fluid-driven reactions can give rise to broad gradients in chemistry and mineralogy in some circumstances. Of course, fluid entering in a state of disequilibrium and flowing along gradients in state variables will produce a hybrid force with components of both fluid-driven and gradient reactions.