QUANTIFYING THE METASTABLE CHEMICAL ENERGY AVAILABLE TO METAMORPHIC ASSEMBLAGES

Common thermodynamic models of metamorphic systems assume that chemical equilibration processes (crystal nucleation, growth and resorption) occur under reversible (near–equilibrium) conditions. In reversible thermodynamic models, mineral assemblages and compositions are not path or process dependent; mineral compositions and modes can thus be calculated for a given bulk composition and set of P-T conditions. Although the reversible thermodynamic assumption greatly simplifies petrologic modeling, these models cannot accommodate features such as mineral zoning, incomplete reactions, and variation in porphyroblast and matrix mineral textures.

This study determines the metastable chemical excess energy (MCEM, the difference between the equilibrium energy and the actual energy). In order to calculate MCEM, large numbers (grids of 1000-2000 points) of quantitative electron microprobe analyses are collected. These analyses are phase classified and the Gibbs free energy for each mineral composition is determined at P and T using a thermodynamic model that incorporates non-ideal mixing, heat capacity, thermal expansion and compressibility. The Gibbs free energy values for each mineral analysis are added together and normalized to create a Gibbs free energy for the actual assemblage and composition for any P-T condition. A bulk composition is determined by integrating and normalizing the analyses; the same thermodynamic model is used to determine the equilibrium Gibbs free energy for this bulk composition at any P-T condition. The difference between these energy values is the MCEM.

Preliminary calculated values of minimum MCEM are 1-50 kJ/m3. MCEM is similar to the value for surficial and deformational excess energies reported by Stunitz (1998). MCEM represents a significant amount of excess energy in metamorphic assemblages. MCEM may provide driving energy for rapid nucleation and growth when liberated by rapid kinetics (in many cases induced by deformational processes) and a major component of the non-reversible energy available for textural evolution.