Oxygen diffusion in grossular and some geological implications

Computer simulation techniques were used to investigate defect formation and the diffusion of O and OH groups in grossular garnet. Diffusion of both species along an octahedral edge was simulated, and activation energies for the jump were calculated. The activation energy for O diffusion in the extrinsic regime is predicted to be 106 kJ/mol, whereas for the high-temperature intrinsic regime, an activation energy of 383 kJ/mol is calculated. Extrinsic diffusion of the OH group along the same pathway also has an activation energy of 106 kJ/mol. However, when structurally bound H₂O in the form of hydrogarnet defects is introduced into the system, the OH diffusion activation energy is reduced to 67 kJ/mol. The high activation energy combined with experimentally available diffusion coefficients for O in garnet suggest that O mobility is very low at metamorphic temperatures. -from Authors