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.
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