Numerical simulation of kinetically-controlled calc-siucate reactions and fluid flow with transffint permeability around crystallizing plutons

Abstract

Numerical simulations were performed to examine the relationships between variable contact-aureole permeability, the kinetics of calc-silicate reactions, and the fluxes of mixed CO2-H2O fluids around a crystallizing granite laccolith. The role of magmatic water was explicitly considered. The Notch Peak contact-metamorphic aureole in Utah was used to define the stratigraphy of the model domain and rock compositions. Reactions that were considered include the major isograd-defining reactions that occurred in the Notch Peak aureole. The half-space model domain had the laccolith, 2 km thick at the middle apex. Only 1-phase fluid flow was considered. Results show that the evolution of the fluid flow-field is highly dependent on the pressure (P) boundary condition at the top of the model domain. When P at the top boundary is allowed to increase, P in most of the top half of the domain eventually exceeds die lithostatic pressure plus the assumed tensile strength of rocks of 15 MPa. This boundary condition simulates an unvented flow-system. A more realistic boundary condition, one that simulates a system diat is able to vent to the surface, is when Pat the top boundary is held at a hydrostatic pressure. In dus case, the flow-field is determined largely by pressure gradients between die overpressured magmatic fluid exsolving from the pluton and the lower pressures at the domain boundaries. Fracturing is predicted to occur early after pluton emplacement as the pore fluid is heated and metamorphic reactions produce CO2. Although fracturing and reaction-enhanced porosity and permeability influence die local flow-field, die domain-scale flow-field is controlled by long-distance pressure gradients. The domain-scale flow-field and temperature distribution impose die largest control on the distribution of major mineral assemblages in die metamorphic aureole. Transient changes in permeability due to fracturing and volume changes in the solid matrix that accompany reactions have a smaller control on die distribution of minerals. The simulations predict significant overstepping and coeval progress of metamorphic reactions. Reaction rates range from 5x 10-10 to 10-14 kmol/m2/sec, depending on die actual P-T-XC0 2f conditions and die abundance of die rate-controlling mineral. Throughout die metamorphic aureole, XC02f approaches 1 as CO2 evolved by reactions displaces H2O. High pore pressures prevent magmatic H2O from infiltrating die aureole until pressures drop when reactions are approaching completion. Consequently, only in die inner aureole, which is eventually infiltrated by magmatic H2O, can minerals such as wollastonite and vesuvianite be produced. An integrated H2O flux of 3 ×104 kmol/m2 is required to produce die width of die model wollastonite zone by 20 ky. Because of pressure gradients, CO2 diat is produced in die inner aureole flows outward into colder rocks even before diese rocks are heated. The T-XC02f padis in outer aureole rocks cut across die H2O-CO 2 solvus, which predicts diat in nature H2O and CO 2 should unmix and behave as two separate fluid phases. The average yearly CO2 flux at die top of die model domain, 2.3 ×10 2 mol/m2/y, is comparable to fluxes of metamorphic carbonic fluids in active geodiermal fields.