Simulating major element diffusion in garnet using realistic 3D geometries

Chemical diffusion of major elements in garnet is a common phenomenon in amphibolite to granulite facies metamorphic rocks. The study of this process has led to important constraints on the rate and timescale of metamorphism, for instance using geospeedometry and forward thermodynamic modelling. However, to date, most models have assumed spherical coordinates and simple geometries when modelling diffusion in garnet. In this study, we present a framework for running 3D multicomponent diffusion models from real grain geometries obtained by micro-computed tomography. We introduce an open-source code, DiffusionGarnet.jl, written for high performance in the Julia programming language. We demonstrate the high efficiency of the numerical solver, a stabilised explicit method, and its scalability using GPU acceleration. This approach is applied to two garnet grains with different characteristics, a euhedral well-shaped grain and a deformed sub-euhedral grain with a high connectivity to the matrix from core to rim. Starting from a similar initial composition and at constant conditions of 700 °C and 0.8 GPa for 10 Myr, the models show results with very different characteristics. The euhedral grain shows results similar to those predicted with a spherical assumption, largely preserving its original zoning. In contrast, the sub-euhedral grain shows significant re-equilibration, nearly erasing completely its initial zoning. This behaviour is caused by the high connectivity with the matrix. In addition to providing a robust solver for 3D diffusion modelling, these results demonstrate the role of grain geometry and matrix connectivity on intra-grain diffusion and highlight the power of 3D approaches to properly study the complexity of natural grains.