Reactive flow of mixed CO2–H2O fluid and progress of calc-silicate reactions in contact metamorphic aureoles: insights from two-dimensional numerical modelling

Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO₂ and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO₂–H₂O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO₂ strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO₂–H₂O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO₂‐poor fluid in the outer aureole. Metamorphic CO₂‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐X_CO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO₂ concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO₂‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO₂‐rich fluid dominates in the inner aureole during early heating, whereas CO₂‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO₂‐rich fluid, and high‐grade rocks reflect the presence of CO₂‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.