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Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite


The Earth’s mantle transition zone could potentially store a large amount of water, as the minerals wadsleyite and ringwoodite incorporate a significant amount of water in their crystal structure1,2. The water content in the transition zone can be estimated from the electrical conductivities of hydrous wadsleyite and ringwoodite, although such estimates depend on accurate knowledge of the two conduction mechanisms in these minerals (small polaron and proton conductions), which early studies have failed to distinguish between3,4. Here we report the electrical conductivity of these two minerals obtained by high-pressure multi-anvil experiments. We found that the small polaron conductions of these minerals are substantially lower than previously estimated. The contributions of proton conduction are small at temperatures corresponding to the mantle transition zone and the conductivity of wadsleyite is considerably lower than that of ringwoodite for both mechanisms. The dry model mantle shows considerable conductivity jumps associated with the olivine–wadsleyite, wadsleyite–ringwoodite and post-spinel transitions. Such a dry model explains well the currently available conductivity–depth profiles5 obtained from geoelectromagnetic studies. We therefore conclude that there is no need to introduce a significant amount of water in the mantle transition to satisfy electrical conductivity constraints.

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Figure 1: Electrical conductivity of wadsleyite and ringwoodite as a function of reciprocal temperature.
Figure 2: Electrical conductivity profiles beneath the Pacific, and the estimated water content in the mantle transition zone.

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We thank E. Ito, D. Yamazaki for critical discussion, S. Yamashita and N. Bolfan-Casanova for interpretation of Fourier-transform infrared spectra, H. Utada for beneficial discussion of conductivity structure and C. Oka for technical assistance. This research was supported by a Grant-in-Aid for Scientific Research to T.K. and T.Y. from the Japan Society for the Promotion of Science and the COE-21 program to the Institute for Study of the Earth’s Interior, Okayama University.

Author Contributions T.K. and T.Y. organized the project and completed the manuscript. The conductivity measurements of wadsleyite and ringwoodite were made by G.M. and T.Y., respectively. The Fourier-transform infrared analysis was made by T.M.

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Correspondence to Takashi Yoshino.

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This file contains Supplementary Methods, Supplementary Discussion, Supplementary Figures S1-S5 with Legends and additional references. (PDF 1445 kb)

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Yoshino, T., Manthilake, G., Matsuzaki, T. et al. Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite. Nature 451, 326–329 (2008).

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