CALCULATION OF FLUID FLUXES IN EARTH'S CRUST

Abstract

The movement of fluids in the crust and upper mantle not only lead to important mineral reactions but also play an essential role in the geochemical cycling of elements and in controlling global change. Numerous papers have focused on calculation of fluid fluxes driving metamorphic reactions in the earth’s crust. The extent of reaction in nature has been “inverted” to predict the total amount of fluid that was required to drive that much reaction. These models, although based on thermodynamic equilibrium, have extended the earlier concept of water-rock ratio. Any quantitative treatment of the fluid fluxes and the relationship between these fluxes and other variables such as temperature and mineral abundances requires a kinetic model. A simple model is presented that incorporates the essential dynamics of metamorphic processes including both heat flow by conduction and convection as well as fluid flow in and out of a representative volume. Overall mineral reactions can then take place within this rock volume in response to internal and external factors. The production and subsequent expulsion of excess fluids (H2O and CO2) as a result of these reactions leads to increased fluid fluxes, which the model can also handle. Using this kinetic model, the assumption of thermodynamic equilibrium can be tested and forward calculations can compare the numbers “inverted” for total integrated fluid fluxes based on equilibrium with the “actual” integrated fluid fluxes. Other effects such as changes in the temperature field or the presence of dispersion/diffusion can also be readily quantified with this kinetic model. The nontrivial consequences of heterogeneity in natural systems make the kinetic approach much more essential but at the same time much more “invertible” than earlier approaches. Ultimately, the effects of the rates of overall mineral reactions and their interplay with the other kinetic processes taking place in these open systems have to be evaluated to guide us in developing much more powerful and correct ways of extracting fluid velocities from petrologic field data.