Growth of ringwoodite reaction rims from MgSiO3 perovskite and periclase at 22.5 GPa and 1,800 °C
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The growth rate of ringwoodite reaction rims between MgSiO3 perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10−15 and 5.6 × 10−15 m2/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO2 component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO3 perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.
KeywordsReaction rim Growth kinetics Diffusion Ringwoodite MgSiO3 perovskite Post-spinel transformation
We thank E. Ito, A. Yoneda, T. Kubo, and G. Maruyama for valuable discussions regarding the manuscript, S. Yamashita and N. Chertkova for their technical assistance with the Raman spectrometer, and K. Komatsu for his technical assistance with the FTIR spectrometer. We also thank R. Milke and an anonymous reviewer for their constructive reviews that have helped us improve the manuscript significantly. This work was supported by Misasa International Student Intern Program 2010 at the Institute for Study of the Earth’s Interior, Okayama University. A. Shimojuku is a research fellow of the Japan Society for the Promotion of Science.
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