Garnets as open systems during regional metamorphism

If a garnet fractures during metamorphism, its interior will be in chemical communication with matrix phases (solids and fluids) and may participate in late prograde or retrograde metamorphic reactions. The core of a growth-zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally, in the vicinity of fractures, or extensively, resulting in dissolution and reprecipitation of the garnet interior and/or production of new phases. The results of petrologic techniques such as thermobarometry, pressure-temperature path calculations, and geochronometry may be misinterpreted if sites of garnet and mineral-inclusion compositional modification are not identified. Brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions. The conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure, retrograde pressure-temperature path, and thermoelastic properties of the inclusion and garnet. Inclusion-induced fracturing is favored by nearly isothermal decompression, a common metamorphic pressure-temperature path. Other fractures may form as a result of propagation of an inclusion-host boundary or by mechanisms external to the garnet.