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 zoned1–8. 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 processes2–4 where diffusion in the growing mineral is assumed to be negligible. However, in garnets from high grade rocks8,10,11, zoning is usually weak, absent or reversed, and recent authors8,12,13 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.