A model to explain the Mn enrichment in the rim of zoned garnet

抄録

Zonal structure of garnet grown in metamorphism maintaining surface equilibrium was examined on the model system Mn₃Al₂Si₃O₁₂-Fe₃Al₂Si₃O₁₂-H₂O. The model assumes that the diffusion of Mn and Fe in both garnet and chlorite is limited, or it is practically negligible in garnet. It is shown that if the rate of temperature change during metamorphism is large in comparison with diffusion rate, a zonal structure, in which the MnO content of garnet decreases from the center of the crystal towards rim and increases again at the rim, giving rise to the minimum MnO content at an intermediate zone, can be formed without accompanying resorption. Two natural examples which support the conclusion on the model system are discussed.