THERIA_G: A Numerical Model to Simulate Prograde Garnet Growth

We present a tool for unraveling the geothermal history of metamorphic rocks from the investigation of garnet chemical zoning. THERIA_G (Gaidies et al., 2008) is based on equilibrium thermodynamics in complex chemical systems and multicomponent diffusion in spherical geometries. It considers the influences on garnet composition of chemical fractionation during garnet growth, intragranular diffusion, and the growth history of the garnet population. Assuming a similar radial growth rate for all garnet porphyroblasts at a given point in time any predefined garnet crystal size frequency distribution (CSD) can be produced. This offers the opportunity to compare garnet chemistries and garnet CSD's that are simulated for predefined conditions with field observations. We propose that the model may allow testing models for the kinetics of garnet nucleation and growth.

The efficiency of intragranular diffusion depends on component mobility and on the gradients of the respective chemical potentials, where the latter are related to compositional gradients in garnet growth zoning. Simulations with THERIA_G demonstrate that the sensitivity of garnet chemical composition to modification by diffusion is different for garnet crystals that nucleated at different stages of the nucleation history. Whereas early-nucleated garnet porphyroblasts tend to have relatively flat chemical zoning patterns in their cores, garnet crystals that nucleated late may have sharp compositional gradients in their growth zoning which are prone to modification through intracrystalline diffusion. Large compositional gradients in garnet also occur at those growth increments that coincide with changes in the thermodynamically stable mineral assemblage. Since such changes may occur repeatedly throughout the entire garnet growth history, modification of the originally growth zoning by intragranular diffusion needs to be considered all over a garnet porphyroblast.

Gaidies, F., de Capitani, C. & Abart, R., 2008. THERIA_G: a software program to numerically model prograde garnet growth. Contributions to Mineralogy and Petrology, 155, 657-671.