Abstract

Fluids play a key role in the transfer of both heat and mass within the crust and in the attainment of chemical equilibrium in metamorphic rocks, controlling both reaction rates and deformation mechanisms. However, our understanding of fluid movement through the deep crust is limited because rocks readily recrystallize, and fluids are invariably lost from grain boundaries. Little is known about how textures evolve during metamorphism since minerals rarely preserve much textural history that can be directly related to grain-boundary processes. Here we report on a study of the surfaces of apatite within gneisses. A variety of textural features is preserved on these surfaces, including imprinted texture of fossil grain boundaries. This texture is linked to deformation that caused a temporary excursion to fine grain size in the surrounding silicates. The geometry of paleoporosity is also recorded by microhollows on the apatite surface and gives a unique insight into both fluid behavior within the crust and the textural evolution of rocks.

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