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Analytical transmission electron microscopy (ATEM) can be used to measure quantitative compositional profiles in minerals on a sub-µm scale. This technique allows investigation of low-temperature diffusion processes and short duration diffusion events, which have not been possible to study previously because of experimental and analytical difficulties.

In this study we used ATEM to measure compositional profiles produced during Fe-Mg interdiffusion experiments in olivine under controlled fO₂ conditions. The accuracy of the measurements can be tested by comparing compositional profiles measured by both ATEM and EMPA (for profile lengths where both methods are applicable) as well as by retrieving diffusion coefficients from both TEM and EMPA data. For example, we determined the Fe-Mg diffusion coefficient from profiles measured on a sample which was annealed for only 24 minutes (as opposed to a typical anneal time of ~250 hours) and found it to be identical, within error, to those obtained from longer anneals. This implies that point defects equilibrate extremely rapidly in the interfacial region and thus that it should be possible to conduct shorter annealing experiments at high pressures, which have not been done previously. For diffusion anneals below 900°C, conventional single crystal diffusion couples are found to be unsuitable because of weak bonding of the interface, which makes sample preparation difficult. Therefore, we developed a new technique employing a thin film of fayalite attached by a vapor deposition process onto a substrate of San Carlos olivine. Analysis of the thin film shows a Fe/Si ratio of 2:1. From electron diffraction as well as HRTEM it can be seen that the film is partly crystalline, with randomly oriented domains up to 250 nm in size showing lattice fringes (Fig. 3.4-3). This film makes an ideal diffusion couple because the substrate can be cooled during the PVD process, precluding the induction of an initial diffusion profile prior to the diffusion anneal. Diffusion anneals have been carried out using these thin film diffusion couples and the resulting interface regions were successfully prepared for TEM analysis. This demonstrates the potential of using such thin-film diffusion couples in combination with ATEM for obtaining low temperature diffusion data. A preliminary determination of the diffusion coefficient from a couple annealed at 800°C is consistent with extrapolation of high temperature data, which suggests there is no change in diffusion mechanism for olivine between 800-1300°C.
 

Fig. 3.4-3: HRTEM photograph showing interface between a fayalite film and San Carlos olivine substrate. The film consists of randomly oriented, nanometer-scale crystalline domains.