Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back

Mineral assemblages in contact-metamorphic aureoles are the products of the interplay between heat transfer and fluid flow induced by intrusion of magma. In wall rocks containing carbonate and silicate minerals, metamorphic reactions produce CO₂, which then becomes part of the hydrodynamic system. Although observed assemblages are the ultimate products of T-XCO₂^fluid-t paths in aureole rocks, the complexities of the paths, and hence the evolution of a hydrodynamic system, are difficult to decipher from them. Numerical simulations were conducted to model the mineralogical evolution in a hydrodynamic system and to evaluate the extent and effects of reaction retardation. Simulations reveal that fluid composition in the inner aureole evolves rapidly toward high XCO₂^fluid as the rocks heat up before an appreciable amount of water is exsolved out of the pluton. After local fluid pressure drops when early reactions are coming to completion, infiltration of magmatic water becomes significant and can drive production of typical inner aureole minerals such as wollastonite. Fluid compositions in the outer aureole reflect largely (1) the initial CO₂ production, as fluids are driven down the pressure gradients from the inner aureole, and (2) the subsequent infiltration of magmatic H₂O. Simulations also suggest temperature overstepping of the onset of reactions and retarded consumption of reactant minerals, which leads to coeval metastable reactions. However, the final simulated mineral assemblage in the inner aureole reflects equilibration with H₂O-rich fluids that is usually seen in the field, although evidence for kinetic retardation may be preserved in some rocks, especially in the outer aureole.