A Thin Film Approach for Producing Mineral Diffusion Couples

Few diffusion coefficient values have been measured for silicate minerals at pertinent geologic conditions because of experimental restrictions. Until recently, analysis of diffusion couples was conducted principally with electron microprobes which have rather poor spatial resolution (micrometer scale). Ion microprobe analyses, however, eliminate many of the previous experimental restrictions; in depth profile mode they have excellent spatial resolution (tens of angstroms) and diffusion couples can be analyzed normal to the interface. Diffusion couples analyzed by ion microprobe must be well-defined and uniform; previous methods using solution precipitates to form the diffusion couples were heterogeneous and had limited success. A new approach, the thermal evaporation of 25MgO under high vacuum onto a crystalline substrate (oxide, silicate), produces a 1000 Å thick 25MgO _x (x < 1) thin film. This method yields an excellent diffusion couple for low-temperature diffusion experiments. Diffusion anneal experiments using this approach for garnet provide a Mg self-diffusion coefficient of D = 0.60 ± 0.09 × 10−21 m2/s at 1000°C (logfO₂ = −11.3, P = 1 atm, X_Almandine = 0.24).