Mechanism of the olivine–ringwoodite transformation in the presence of aqueous fluid
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The mechanism of the high pressure transformation of olivine in the presence of aqueous fluid was investigated by high pressure experiments conducted nominally at the wadsleyite + ringwoodite stability field at 14.5 GPa and 700 and 800°C. The microstructures of recovered samples were observed using an analytical transmission electron microscope (ATEM) for which foils were prepared using a focused ion beam technique. Glass films approximately 1 μm in width always occupied the interface between olivine and hydrous ringwoodite. ATEM measurements showed that the chemical compositions of the glass films had approximately the same Mg/Fe ratio as that of olivine, but a higher Si content. Micro-structural and -chemical observations suggest that these glass films formed as quenched glass from the aqueous fluid dissolving olivine and that hydrous ringwoodite was crystallized from the fluid. This indicates that the transformation of olivine to hydrous ringwoodite was prompted by the dissolution–reprecipitation process. The dissolution–reprecipitation process is considered an important mineral replacement mechanism in the Earth’s crust by which one mineral is replaced by a more stable phase or phases. However, this process has not previously been reported for deep mantle conditions.
KeywordsDissolution–reprecipitation Olivine Ringwoodite Transformation mechanism
We thank S. Karato and T. Kubo for helpful reviews and constructive comments to improve the manuscript. We also thank H. Ishisako for making the thin sections, Y. Shibata for helping with the EPMA measurement, the staff of the Techniques Center at Hiroshima University for making the experimental parts, and K. Kanagawa for supplying the peridotite sample. We are grateful to A. Putnis and Y. Takahashi for helpful comments and suggestions. This study was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science to J.A.
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