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 fundamental properties of metamorphic flow systems such as time-integrated flux, flow direction, and Peclet number. Many estimates of time-integrated flux in the upper and middle crust are surprisingly large, 10⁵–10⁶ cm³ fluid/cm² rock; estimates for the lower crust are much smaller. Rather than pervasive and uniform, reactive fluid flow in all metamorphic environments is channelized on scales of <1–10⁴ m. Channelization results from heterogeneous permeability structures controlled by features such as lithologic layering, contacts, folds, fractures, and faults. Consequently flow may be in the direction of either decreasing or increasing temperature or isothermal. Site-specific thermal-hydrologic models of metamorphic terranes that explicitly consider chemical reactions and dynamic permeability structures will help resolve outstanding questions with regard to the driving forces and duration of flow, metamorphic permeability distributions, and how deformation controls fluid flow.