GARNETS AS OPEN SYSTEMS DURING REGIONAL METAMORPHISM

If a garnet fractures during metamorphism, its interior will be in che mical communication with matrix phases (solids and fluids) and may par ticipate in late prograde or retrograde metamorphic reactions, The cor e of a growth-zoned garnet is unlikely to be in equilibrium with matri x phases and may react locally, in the vicinity of fractures, or exten sively, resulting in dissolution and reprecipitation of the garnet int erior and/or production of new phases, The results of petrologic techn iques such as thermobarometry, pressure-temperature path calculations, and geochronometry may be misinterpreted if sites of garnet and miner al-inclusion compositional modification are not identified, Brittle fr acturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions. The conditions at which differenti al expansion of an inclusion relative to its host garnet are sufficien t 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 pr essure-temperature path. Other fractures may form as a result of propa gation of an inclusion-host boundary or by mechanisms external to the garnet.