Modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics

Radial microcracks around inclusions in garnet are important indicators of ultrahigh-pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains. We investigate the growth of microcracks from inclusion corners using a two-dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining (lithostatic) pressure or significant decompression to initiate. Results suggest that inclusions (∼100 μm diameter) develop microcracks during modest decompression. The magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size. The potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion. We integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism.