The diffusion coefficient of Sm in almandine garnet was determined (figure 24) as a function of temperature at 1bar and at an fO2-value corresponding to that of a wüstite-Fe buffer, and of a few other rare-earth elements in pyrope garnets. Both garnets were shown to have metastably survived diffusion annealing at conditions beyond their stability fields. The experimental diffusion profiles were analyzed by using secondary ion mass spectrometry, and Rutherford back-scattering spectroscopy, for 2 samples. Transmission electron microscopic study of an almandine crystal, that was diffusion-annealed, did not reveal any near-surface fast diffusion path. Using reasonable approximations and a theoretical analysis of vacancy diffusion, the experimental data were used to develop an expression for rare-earth diffusivity in garnet as a function of the temperature, pressure, fO2-value, ionic radius and matrix composition. Modelling of the rare-earth evolution in the melt and in residual garnet suggested that, for dry melting conditions, the rare-earth pattern in the melt should usually conform closely to that expected for equilibrium melting. For much lower solidus temperatures than would prevail in the presence of a H2O-CO2 fluid, the concentration of light rare-earths in the melt could be significantly lower than that under equilibrium melting condition.

Rare Earth Diffusion Kinetics in Garnet - Experimental Studies and Applications. M.Tirone, J.Ganguly, R.Dohmen, F.Langenhorst, R.Hervig, H.W.Becker: Geochimica et Cosmochimica Acta, 2005, 69[9], 2385-98