NUMERICAL MODELING OF MIXED H2O-CO2 FLUID FLOW AND ITS INFLUENCE ON CALC-SILICATE REACTIONS IN CONTACT AUREOLES

Our recent two-dimensional (2D) hydrodynamic studies of metamorphic fluid flow in contact aureoles with complex permeability structures have shown that fluid flow is strongly time- and space-dependent, consequently causing heterogeneous redistribution of oxygen isotopes. The results are significantly different from 1D and 2D models of flow in aureoles with homogeneous permeability structures. However, in previous hydrodynamic models, including ours, it was assumed that aureole fluids are pure water, whereas geological data indicate that fluids in calc-silicate contact aureoles contain significant CO₂. The physical and transport properties of a mixed H₂O-CO₂ fluid at elevated P and T are very different from properties of pure water. Thus, the presence of CO₂ may significantly impact the flow field and the distribution of mineral assemblages in contact aureoles. Based on the geology of the Notch Peak aureole, Utah, we investigated the effects of CO₂ on macro-scale fluid flow field and metamorphic reactions using a 2D finite element method. With the assumption of one-phase, mixed, supercritical CO₂-H₂O fluid, the fluid, heat and CO2 transport equations coupled with kinetically controlled calc-silicate reactions were solved with an operator time-splitting technique. Fluid sources simulated in the model include aqueous magmatic water, CO₂-rich metamorphic fluid produced by calc-silicate reactions, and formation fluids. Preliminary results show that the flow pattern of CO₂-rich fluids is more complex than that of pure water. Infiltration of aqueous magmatic water into a homogeneous aureole containing CO₂-rich pore fluid causes vigorous, upward flow of hot fluid along the intrusion-wallrock contact and vigorous downward flow in the outer aureole along zones of sharp CO₂ gradients. However, when reaction-produced CO₂-rich fluid is also considered, fluid flow in the inner aureole remains upward, but there is no vigorous downward flow in the outer aureole as no large CO₂ gradients exist there. Strong transient and heterogeneous flow develops when heterogeneous permeability of wall rocks is considered in the model. The influence of multi-component fluid flow on the distribution of metamorphic mineral assemblages in the Notch Peak contact aureole will be discussed.