REACTIVE FLUID FLOW IN CONTACT AUREOLES – INSIGHTS FROM NUMERICAL SIMULATIONS OF MINERAL REACTIONS

Numerical simulations of mineral reaction progress in contact aureoles provide information on the temporal evolution of hydrodynamic systems around cooling plutons. Flow of reactive H₂O-CO₂ fluids and progress of metamorphic reactions in an aureole with a horizontal layered permeability structure and transient, reaction-induced permeability increases were simulated using a modified version of the SUTRA code that also allows simultaneous calculation of mineral reaction progress. The intrusion was assumed to be a laccolith, 8 km wide at the base and 4 km high in the middle. Magmatic, metamorphic, and sedimentary fluid sources were considered. The rate of calc-silicate reaction progress was assumed to be related to ΔG's of the considered reactions.

Results show that after magma intrusion, fluid flow is away from the intrusion as the hydrostatic head increases with temperature in the inner aureole and magmatic fluids flow outward. This flow regime remains for several thousand years but becomes less vigorous with time. Local high fluid pressures occur at sites of decarbonation reactions which move away from the intrusion with time. Fluid composition in the aureole initially becomes heterogeneous as CO₂ is evolved into the higher-permeability beds, but eventually the heterogeneity eventually diminishes with time. For a permeability contrast of 100, fluid composition becomes qualitatively homogeneous in most of the aureole by 10 ky, and sooner for a lesser permeability contrast. Progress of mineral reactions is rapid within the first few thousands of years after magma intrusion, but the essential mineral characteristics of the aureole are established by ~10 ky. Further evolution of the hydrodynamic system toward Bénard convective cells after several tens of ky has little effect on the distribution of the model minerals assemblages, which mimic very well assemblages seen in calc-silicate aureoles around laccoliths.