Microstructural observations of recovered samples support this reaction mechanism. SEM images of samples with limited transformation (Fig. 4a, sample Fa52a with 6% of transformation) indicate that ringwoodite nucleated at the olivine grain or sub-grain boundaries. This is consistent with the proposed stress-induced mechanism of reaction, since the maximum densities of defects should be concentrated at grain boundaries (e.g. Poirier, 1981 and Mosenfelder et al., 2001). This nucleation stage is followed by the growth of ringwoodite grains enriched in iron compared to the original olivine (light grey areas on SEM images Fig. 4b). The Etazolate of the sample microstructures as a function of the extent of transformation is consistent with nucleation at grain boundary and interface-controlled growth as reported in early works on the olivine–ringwoodite transformation (Brearley et al., 1992, Mosenfelder et al., 2001, Kubo et al., 2004 and Smyth et al., 2012). While the initial grain size of olivine after annealing is about 7±2 μm7±2 μm, the ringwoodite grain size is limited to ca. 2 μm after transformation (Figs. 4b and 4c). This evidences an important grain size reduction of ca. 70% associated to the olivine–ringwoodite transformation. Finally, SEM images of run Fa66c (Fig. 4c), transformed up to 80%, still displayed chemically heterogeneous olivine grains. The initial olivine crystals thus fell to fully re-equilibrate with the ringwoodite composition, indicating that cations interdiffusion is the slowest process of the reaction.
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