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Stability of hydrous silicate at high pressures and water transport to the deep lower mantle

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Abstract

The presence of water strongly influences the structure, composition and dynamics of the Earth’s deep mantle1,2,3,4. Hydrous magnesium-rich silicates play an important role in transporting water into the deep mantle when oceanic plates subduct as slabs. The highest-pressure form of such hydrous silicates, phase D, was reported to dissociate into an assemblage of nominally anhydrous phases plus water at pressures of about 44 GPa, equivalent to 1,250 km depth5. In this way, free water would be released in the middle region of the lower mantle, which suggests that no water can reach the deeper regions of the lower mantle. Here we use in situ X-ray measurements in conjunction with a multi-anvil apparatus using sintered diamond anvils to show that hydrous phases are stable under lower mantle conditions up to 50 GPa. We found that phase D transforms to an assemblage with another hydrous silicate (MgSiH2O4), named phase H, at pressures above about 48 GPa. Our results suggest that phase H is the dominant hydrous silicate in descending slabs, and may be found at depths far deeper than 1,250 km. We conclude that a significant amount of water is retained in the descending slabs and may be delivered to the deepest part of the lower mantle.

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Figure 1: Back-scattered electron images of the run products.
Figure 2: Phase relations of DHMSs.
Figure 3: Selected X-ray diffraction patterns of in situ observations (M1372).
Figure 4: Stability fields of major hydrous phases in the subducted slabs in a pyrolite +2 wt% H2O system.

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Acknowledgements

We thank T. Shinmei and T. Inoue for their helpful comments, Kunimoto and F. Isobe for assistance with the experiments. In situ X-ray measurements were conducted at SPring-8 (proposal no. 2013A0082). M.N. was supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.

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M.N., T.I., J.T. and Y.N. designed the study and wrote the manuscript. M.N. carried out most of the experiments and analyses of the data with the help of Y.T., K.F. and Y.H. All authors discussed the results and commented on the manuscript.

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Correspondence to M. Nishi.

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The authors declare no competing financial interests.

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Nishi, M., Irifune, T., Tsuchiya, J. et al. Stability of hydrous silicate at high pressures and water transport to the deep lower mantle. Nature Geosci 7, 224–227 (2014). https://doi.org/10.1038/ngeo2074

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