THERIA_G: a software program to numerically model prograde garnet growth

Abstract

We present the software program THERIA_G, which allows for numerical simulation of garnet growth in a given volume of rock along any pressure-temperaturetime (P-T-t) path. THERIA_G assumes thermodynamic equilibrium between the garnet rim and the rock matrix during growth and accounts for component fractionation associated with garnet formation as well as for intracrystalline diffusion within garnet. In addition, THERIA_G keeps track of changes in the equilibrium phase relations, which occur during garnet growth along the specified P-Tt trajectory. This is accomplished by the combination of two major modules: a Gibbs free energy minimization routine is used to calculate equilibrium phase relations including the volume and composition of successive garnet growth increments as P and T and the effective bulk rock composition change. With the second module intragranular multi-component diffusion is modelled for spherical garnet geometry. THERIA_G allows to simulate the formation of an entire garnet population, the nucleation and growth history of which is specified via the garnet crystal size frequency distribution. Garnet growth simulations with THERIA_G produce compositional profiles for the garnet porphyroblasts of each size class of a population and full information on equilibrium phase assemblages for any point along the specified P-T-t trajectory. The results of garnet growth simulation can be used to infer the P-T-t path of metamorphism from the chemical zoning of garnet porphyroblasts. With a hypothetical example of garnet growth in a pelitic rock we demonstrate that it is essential for the interpretation of the chemical zoning of garnet to account for the combined effects of the thermodynamic conditions of garnet growth, the nucleation history and intracrystalline diffusion.

Coupling forward modelling of garnet growth with monazite geochronology: an application to the Rappold Complex (Austroalpine crystalline basement)

“…To simulate the growth of a garnet population in a given volume of rock the software program theria_g (Gaidies et al. , 2008a) was used.…”

Thermodynamic equilibrium at heterogeneous pressure

“…This is complementary to already existing techniques to find the pressure (and temperature) variations of rock in time. For example, mineral zoning is commonly used to derive the P-T path; but also phase assemblages are useful by studying inclusions and interpreting phase assemblages and textures in terms of P-T evolution in time (Spear and Selverstone 1983;Gaidies et al 2008;Caddick et al 2010;Lanari et al 2014;Vrijmoed and Hacker 2014).…”

Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt

“…This approach can be used to evaluate the rates of garnet growth within a sample, taking into account diffusional exchange within growth-zoned garnet formed on the prograde path. Unfortunately, this program cannot model open-system behavior involving garnet or compositional variation on grain rims that is a result of garnet resorption (Gaidies et al, 2008). Owing to these limitations, and the fact that the samples examined in this study reach maximum temperatures of 600-650 °C, where garnet diffusional homogenization starts to speed up, we have primarily tried to match the X grs profi le, as intracrystalline diffusion of Ca is typically the slowest of the divalent cations within garnet (Carlson, 2006;Caddick et al, 2010).…”

“…In order to further defi ne the P-T evolution of samples BKC-23 and BKC-10, modeling was conducted using Theria_G software (Gaidies et al, 2008). This approach can be used to evaluate the rates of garnet growth within a sample, taking into account diffusional exchange within growth-zoned garnet formed on the prograde path.…”