MODELING METAMORPHISM: CURRENT RESULTS AND FUTURE DIRECTIONS

Multidimensional modeling of metamorphic reactions under kinetic control reveals P-T-X pathways that differ from those predicted by equilibrium-based models for both prograde and retrograde reactions. We model the Ca-Mg-Si-O-H system at 3 kbar using a dynamic 2D model for flow and reaction with rates based upon experimental kinetics. The finite-difference/spectral-transform model includes metastable reactions, thermal effects, compaction, buoyancy, and reactive changes of the supercritical CO₂ - H₂O fluids. The deviation from equilibrium can range from exceedingly small (for heating rates associated with regional metamorphism) to tens of degrees C or more (for contact metamorphism). The thermal overstep during prograde heating is a complex function of the nominal heating rate, the reaction rate, and the flow of volatiles driven by decarbonation reactions for siliceous dolomites. Taking these factors into account helps to resolve the apparent discrepancy between rates based on field and laboratory kinetic data.

Future challenges for metamorphic modeling require improved conceptual models, computational techniques, and experimental data. In future work we will move beyond the use of overall reactions, as individual dissolution and precipitation reactions are required to understand vein formation and transport of non-volatile elements. Nucleation kinetics, textural development, and multiphase fluids must be included. Comparisons of model results to field observations of isotopic and petrologic variations near flow conduits and fractures are prime topics for future research.