Abstract
A study of the compositional zoning in garnets that have undergone subsequent metamorphism to staurolite or sillimanite grade confirms the suggestions of some earlier workers that diffusion has taken place in high-grade garnets to eliminate the zoning that developed originally during growth. There is an abrupt change in zoning patterns of garnets within the staurolite-sillimanite transition zone over an approximate temperature range of 50°, with a mean temperature around 640° C. The change in diffusion coefficients with temperature required to eliminate zoning by increased diffusion at the higher grades gives values for Q, the activation energy for diffusion, in the same range as reported values obtained experimentally for comparable diffusion in other silicates. The change in garnet zoning may be used to define an isograd which is not very sensitive to garnet composition or duration of metamorphism. High-grade garnets that have been homogenized by diffusion often show compositional variations at the rims that are attributed to retrograde effects, and it seems probable that at very high temperatures, in excess of 700° C, garnets may be able to adjust composition throughout, during cooling at geological rates.
