CHEMICALLY REACTIVE FLUID-FLOW DURING METAMORPHISM

Stable isotopic, mineralogical, and chemical alteration in metamorphic terranes is evidence for reactive fluid flow during metamorphism. In many cases, the amount and spatial distribution of the alteration can be quantitatively interpreted using transport theory in terms of funda mental properties of metamorphic flow systems such as time-integrated flux, flow direction, and Peclet number. Many estimates of time-integr ated flux in the upper and middle crust are surprisingly large, 10(5)- 10(6) cm(3) fluid/cm(2) rock; estimates for the lower crust are much s maller. Rather than pervasive and uniform, reactive fluid flow in all metamorphic environments is channelized on scales of <1-10(4) m. Chann elization results from heterogeneous permeability structures controlle d by features such as lithologic layering, contacts, folds, fractures, and faults. Consequently flow may be in the direction of either decre asing or increasing temperature or isothermal. Site-specific thermal-h ydrologic models of metamorphic terranes that explicitly consider chem ical reactions and dynamic permeability structures will help resolve o utstanding questions with regard to the driving forces and duration of flow, metamorphic permeability distributions, and how deformation con trols fluid flow.