MGPV DISTINGUISHED GEOLOGIC CAREER AWARD LECTURE: WHEN THE HEAT IS TURNED UP, LOOK OUT FOR THE HOT WATER

The essence of metamorphism from the standpoint of petrology is mineral reaction. The cause of mineral reaction traditionally is explained by changes in temperature (T) and pressure (P). Fluid composition is an analogous variable and equally important. More fundamentally, reactions are caused by energy transport – heat flow and infiltration of chemically reactive volatiles in the cases of changing T and fluid composition, respectively (P-V work is normally unimportant). The spatial distributions of minerals and mineral reactions that define types of metamorphic terrains are explained by limitations in heat and mass transport during metamorphism. Quantitative understanding of these distributions requires applications of transport theory that couples time, space, heat flow, and fluid flow with chemical reaction. My career has focused on the effects of fluid flow. Infiltration-driven reaction is proven when mineral reactants and products are in equilibrium with a fluid whose composition differs from the composition of volatiles released by the reaction. Infiltration-driven reactions are recognized in all common rock types in regional and contact metamorphic terrains. Equilibrium fluid compositions indicate that infiltrating fluids are typically H₂O-rich. Fundamental properties of fossil fluid flow systems have been deduced from patterns of mineral reaction mapped in the field and interpreted using transport theory, including: (a) delineation of fluid flow channels at a range of spatial scales and documentation of the strong structural control on the location of channels; (b) constraints on the direction of flow, often defined by considering both mineral and stable isotope reactions; (c) the source of fluid; and (d) estimation of the amount of fluid as a time-integrated fluid flux (TIFF), ranging from ~10² to ~10⁵ mol fluid/cm² rock. Recent developments include demonstration that fluid salinity, departure from mineral-fluid equilibrium, and cross-layer diffusion of volatiles at the outcrop scale do not make large estimated TIFFs disappear. My students share credit for my accomplishments for involving me in field problems that I would not have explored otherwise, for developing theory and numerical simulations of fluid flow beyond my abilities, and for pointing out earlier ideas that needed revision.