An experimental measurement of cation diffusion in almandine garnet

Abstract

COMPOSITIONAL zoning in minerals is a disequilibrium phenomenon which is in principle potentially useful for interpreting the thermal history of the host rock. Almandine-rich garnets from low and medium grade regionally metamorphosed rocks are commonly zoned^1–8. The manganese content typically varies from core to rim, being high in the centre and low at the rim, producing a bell-shaped profile, whereas Fe is antipathetic to Mn. With the exception of the diffusion models of Anderson and Buckley⁹, such zoning has generally been explained in terms of fractionation-depletion processes^2–4 where diffusion in the growing mineral is assumed to be negligible. However, in garnets from high grade rocks^8,10,11, zoning is usually weak, absent or reversed, and recent authors^8,12,13 have favoured the homogenisation of pre-existing or developing zoning by volume diffusion. To evaluate the importance of cation diffusion to garnet zoning, Fe–Mn interdiffusion coefficients have been measured over a restricted temperature range.