Sheltered inside crystals, mineral inclusions preserve crucial information on the geological history of our planet. These inclusions allow estimation of the pressure and temperature trajectories of metamorphic rocks, but also can tell about rock mineralogy, provenance, and geodynamic and petrogenetic processes. This information is recovered under the axiom that mineral inclusions have remained immutable objects that do not change shape after their entrapment. Conversely, we show that post-entrapment shape modifications do occur in high-T rocks from granulite-facies conditions. Here we analyze the shape and orientation of quartz inclusions within garnet, a common metamorphic mineral. Our data reveal a progressive change in shape from irregularly-shaped forms in rocks formed at T<550 °C, to combined dodecahedron and icositetrahedron geometries imposed by the host garnet in granulites equilibrated at T>750 °C. Lack of fluid at the quartz-garnet phase boundary indicates that inclusion shape change occurs by thermally-induced grain boundary diffusion, driven by the minimization of the surface energy of the host-inclusion system. Since inclusions acquire a negative crystal shape after entrapment, our discovery redefines the conceptual framework for the application of elastic thermobarometry, and motivates a reevaluation of the criteria for mineral inclusion syngenesis in metamorphic rocks.
