Crystallization kinetics during regional metamorphism of porphyroblastic rocks

Numerical simulations of diffusion‐controlled nucleation and growth of garnet porphyroblasts in regionally metamorphosed rocks constrain interfacial energy and rates of nucleation and Al intergranular diffusion. The 13 rocks analysed in this study were collected from seven localities exhibiting a diverse range of crystallization conditions. Kinetic parameters governing nucleation and intergranular diffusion were adjusted iteratively to achieve fits between simulated and natural porphyroblastic textures. Model fits were assessed primarily from textural characteristics precisely measured by high‐resolution X‐ray computed tomography. Interfacial energy for heterogeneous nucleation ranges from 0.007 to 0.255 J m^?2 for the sample suite, assuming shape factors in the range 0.01–1.0. Nucleation rates change through space and time due to growth and impingement of Al depletion zones surrounding porphyroblasts. In some models, the overall rock‐wide nucleation rate rises steeply, achieves a steady state, and then falls rapidly as reactants are consumed; in others, the steady state is not achieved, but instead the rate simply peaks before falling. Maximum rock‐wide nucleation rates range from 10^?14.7 to 10^?10.7 nuclei cm^?3 s^?1, and maximum local rates range from 10^?13.7 to 10^?9.7 nuclei?cm^?3 s^?1 depending on Al supersaturation. Diffusive fluxes of Al are well constrained by the simulated textures, but rates of intergranular diffusion are subject to uncertainties in Al solubility and interconnected porosity. Best estimates of Al diffusivities at 600 °C span 10^?12.3 to 10^?10.5 m² s^?1 for the sample suite, a narrow range considering natural variability and the uncertainties in solubility and porosity. Eliminating some models suspected of higher uncertainty for these quantities yields diffusivities at 600 °C near 10^?11.0 m² s^?1, with dispersion of less than half an order of magnitude. These simulations, which are among the first attempted for regionally metamorphosed rocks, emphasize that: (i) nucleation rates vary markedly in time and space during crystallization; (ii) nucleation extends well beyond equilibrium conditions; (iii) Al diffusivity likely varies over only a narrow range across common metamorphic circumstances; and (iv) better determinations of both Al solubility and interconnected porosity are needed to constrain rates of Al intergranular diffusion more precisely.