A Theoretical Study on the Formation of Growth Zoning in Garnet Consuming Chlorite

Chemical zoning of garnet is often used to deduce P–Tpaths of rocks by inverse calculation. To validate the derivedP–T paths, it is desired to establish a method to predictthe chemical compositions of garnet theoretically. This studyproposes a new forward calculation of the formation of Mg–Fe–Mngarnet from chlorite, which solves the non-linear simultaneousequations using nested iterative calculations. Growth of garnetconsuming chlorite and quartz was modelled in a MnO–FeO–MgO–Al₂O₃–SiO₂–H₂Osystem, using the most recent thermodynamic data for the minerals.The prograde P–T history of the Sambagawa metamorphicbelt, SW Japan, was modelled. To reproduce growth zoning, crystallizedgarnet was removed step by step from the system; perfect diffusionwas assumed for chlorite. The proposed model derived the evolutionof molar amounts and chemical compositions of Mg–Fe–Mnchlorite and garnet. It successfully reproduced the shape ofthe observed chemical profile of garnet, although the temperaturecondition was higher than general observations. The Mn contentof the garnet core was generally high, and Mg/Fe ratio alwaysstarted rising rapidly after Mn was depleted. Thermodynamicproperties of minerals, initial chlorite composition, P–Tpath, H₂O partial pressure, and Ca content in garnet were variedto test the behaviour of the system. The properties of Mn phasesinfluenced only the chemical composition of the garnet core.The temperature range in which garnet grew depended on the H₂Opartial pressure or the Ca content in garnet. KEY WORDS: chemical equilibrium; chemical zoning; garnet; forward modelling; Sambagawa metamorphic belt