Sun, Yoshino, Sakamoto and Yurimoto (2018) proposed, from hydrogen–deuterium interdiffusion experiments on wadsleyite, that hydrous material in the roof of a slab stagnating in the mantle transition zone can exceed its water storage capacity and release a supercritical fluid, whose high electrical conductivity could explain the conductive anomalies observed in mantle wedges above stagnant slabs. Their analysis was deliberately limited to one-dimensional end-member treatments (instantaneous fluid removal versus zero percolation), a single-phase wadsleyite water-solubility law, and the electrical conductivity of shock-compressed water from a single 1982 experiment. Substantial partial progress exists — Fe- and temperature-dependent water solubility of wadsleyite and ringwoodite, and electrical conductivity of hydrous ultramafic melts at 13 GPa — but the percolative migration rate of the fluid, the fluid-generation threshold of multiphase peridotite, the conductivity of composition- and salinity-variable supercritical fluids at transition-zone pressure and temperature, and a geometry-respecting quantitative reconciliation with magnetotelluric and seismic observations all remain open.

Background

The mantle transition zone (MTZ) is the layer between the 410 km and 660 km seismic discontinuities, where the upper-mantle mineral olivine transforms to its high-pressure polymorphs wadsleyite and ringwoodite. These minerals are nominally anhydrous (they contain no water in their ideal chemical formula) but can incorporate hydrogen as point defects, storing up to about 1–3 wt% H2O, so the MTZ is a candidate reservoir of the deep water cycle. Subducting oceanic plates commonly flatten and stagnate within or above the MTZ beneath regions such as northeastern China and Japan; the roof of such a stagnant slab is cold and can remain hydrous for long times.

Two geophysical threads motivate the source paper. First, magnetotelluric surveys — which infer subsurface electrical conductivity from natural time-varying electromagnetic fields — reveal high-conductivity anomalies in the mantle wedge above stagnant slabs. Whether these anomalies record proton conduction (conduction by hydrogen ions diffusing through a mineral lattice) in hydrous wadsleyite and ringwoodite is a long-standing controversy: Huang, Xu and Karato (2005) attributed transition-zone conductivity to water, whereas Yoshino, Manthilake, Matsuzaki and Katsura (2008) argued for a dry transition zone. Second, at pressures above the second critical endpoint of the silicate–H2O system — the pressure and temperature beyond which liquid water and silicate melt become one continuously miscible phase — any liberated hydrous phase is a supercritical fluid, neither a dilute aqueous fluid nor a normal melt. Mibe et al. (2007) and Wang et al. (2020) bracketed the second critical endpoint of peridotite–H2O at only a few GPa, so at MTZ pressures any free hydrous phase is expected to be supercritical.

Sun, Yoshino, Sakamoto and Yurimoto (2018) measured hydrogen diffusivity in wadsleyite single crystals by hydrogen–deuterium (H–D) interdiffusion couples — an experiment in which a hydrogen-bearing crystal is welded to a deuterium-bearing twin and the isotope exchange profile, read by secondary ion mass spectrometry, yields the self-diffusivity — at 15–18 GPa and 1000–1400 K, extending their earlier ringwoodite study (Sun et al. 2015) and the group's olivine work (Sun et al. 2019). Combining the fast measured hydrogen diffusion with the wadsleyite water-solubility laws then available (determined for Fe-free, single-phase wadsleyite), they modeled the roof of a hydrous stagnant slab at the top of the MTZ. In their scenario, as the slab warms, a hydrous layer in the slab roof crosses water saturation and releases a supercritical fluid. For the fluid's electrical conductivity they had to rely on the single shock-compression measurement of water and ammonia by Mitchell and Nellis (1982), because no systematic conductivity data for supercritical fluids at MTZ conditions existed. With these ingredients the one-dimensional model could reproduce the order of magnitude of mantle-wedge conductivity anomalies, but only in two bounding end members: instantaneous removal of the fluid as it forms, and zero percolation (fluid retained in place). The authors themselves listed what would be needed to make the scenario quantitative: the actual percolative migration rate of the fluid in the slab roof, a water-solubility law for multiphase peridotite (which also contains garnet, with lower water solubility, and ringwoodite), treatment of lateral heterogeneity and three-dimensional slab geometry rather than a one-dimensional column, and systematic measurements of the electrical conductivity of supercritical fluids of realistic composition and salinity at MTZ pressure and temperature. Reviews of transport properties (Zhang and Xia 2021; Zhang, Guo, Yoshino and Xia 2021) consolidated the diffusion and conductivity databases but did not fill these gaps.

Problem Statement

Make the supercritical-fluid generation scenario for the roof of stagnant slabs in the mantle transition zone quantitative. Specifically, determine, with constraints sufficient for comparison against geophysical observations: (1) the rate at which the supercritical fluid generated by water saturation of hydrous wadsleyite-bearing peridotite in the slab roof percolates and migrates, replacing the bounding end members of instantaneous removal and zero percolation used by Sun et al. (2018); (2) the water-storage capacity and fluid-generation threshold of multiphase peridotite (wadsleyite plus garnet and other phases, with realistic iron content and temperature dependence), rather than of single-phase wadsleyite alone; (3) the electrical conductivity of supercritical fluids of realistic composition and salinity at mantle transition-zone pressure and temperature, superseding reliance on the single 1982 shock-compression measurement of pure water and ammonia; and (4) the spatial extent, temporal evolution and geophysical signatures of the resulting fluid layer in a model that includes lateral heterogeneity and realistic subducting-slab geometry rather than a one-dimensional column. These four results — each a value or range with an uncertainty and a domain of validity (pressure, temperature and, where relevant, space and time) — jointly answer the source question: whether, where and in what amount a supercritical fluid in the roof of a stagnant slab can account for the electrical conductivity anomalies observed in the mantle wedge above stagnant slabs, and whether they are together consistent — or inconsistent — with the magnetotelluric (high-conductivity) and seismological (seismic-velocity) observations of the mantle wedge above stagnant slabs, such as those beneath northeastern China and Japan.

References

  1. Sun, W., Yoshino, T., Sakamoto, N., Yurimoto, H. (2018). Supercritical fluid in the mantle transition zone deduced from H–D interdiffusion of wadsleyite. Earth and Planetary Science Letters 484, 309–317. doi:10.1016/j.epsl.2017.12.032, https://doi.org/10.1016/j.epsl.2017.12.032
  2. Sun, W., Yoshino, T., Sakamoto, N., Yurimoto, H. (2021). Hydrogen diffusion mechanism in the mantle deduced from H-D interdiffusion in wadsleyite. Earth and Planetary Science Letters 561, 116815. doi:10.1016/j.epsl.2021.116815, https://doi.org/10.1016/j.epsl.2021.116815
  3. Sun, W., Yoshino, T., Kuroda, M., Sakamoto, N., Yurimoto, H. (2019). H-D Interdiffusion in Single-Crystal Olivine: Implications for Electrical Conductivity in the Upper Mantle. Journal of Geophysical Research: Solid Earth 124, 5696–5707. doi:10.1029/2019JB017576, https://doi.org/10.1029/2019JB017576
  4. Sun, W., Yoshino, T., Sakamoto, N., Yurimoto, H. (2015). Hydrogen self-diffusivity in single crystal ringwoodite: Implications for water content and distribution in the mantle transition zone. Geophysical Research Letters 42, 6582–6589. doi:10.1002/2015GL064486, https://doi.org/10.1002/2015GL064486
  5. Fei, H., Katsura, T. (2021). Water Solubility in Fe-Bearing Wadsleyite at Mantle Transition Zone Temperatures. Geophysical Research Letters 48, e2021GL092836. doi:10.1029/2021GL092836, https://doi.org/10.1029/2021GL092836
  6. Fei, H., Katsura, T. (2020). High water solubility of ringwoodite at mantle transition zone temperature. Earth and Planetary Science Letters 531, 115987. doi:10.1016/j.epsl.2019.115987, https://doi.org/10.1016/j.epsl.2019.115987
  7. Druzhbin, D., Fei, H., Katsura, T. (2021). Independent hydrogen incorporation in wadsleyite from oxygen fugacity and non-dissociation of H2O in the reducing mantle transition zone. Earth and Planetary Science Letters 557, 116755. doi:10.1016/j.epsl.2021.116755, https://doi.org/10.1016/j.epsl.2021.116755
  8. Zhang, B., Zhao, C., Yoshino, T. (2021). Fe–Mg interdiffusion in wadsleyite and implications for water content of the transition zone. Earth and Planetary Science Letters 554, 116672. doi:10.1016/j.epsl.2020.116672, https://doi.org/10.1016/j.epsl.2020.116672
  9. Zhang, B., Xia, Q. (2021). Influence of water on the physical properties of olivine, wadsleyite, and ringwoodite. European Journal of Mineralogy 33, 39–75. doi:10.5194/EJM-33-39-2021, https://doi.org/10.5194/EJM-33-39-2021
  10. Zhang, B.-H., Guo, X., Yoshino, T., Xia, Q.-K. (2021). Electrical conductivity of melts: implications for conductivity anomalies in the Earth's mantle. National Science Review 8, nwab064. doi:10.1093/nsr/nwab064, https://doi.org/10.1093/nsr/nwab064
  11. Yoshino, T. (2022). Electrical conductivity of hydrous silicate melt atop of the 410 km seismic discontinuity. Goldschmidt2022 Abstracts. doi:10.46427/gold2022.9434, https://doi.org/10.46427/gold2022.9434
  12. Yoshino, T., Xie, L. (2026). Electrical Conductivity of Hydrous Ultramafic Melts With Implications for Origin of Low Velocity Layer Atop of the 410 km Seismic Discontinuity. Geophysical Research Letters 53, e2026GL122472. doi:10.1029/2026gl122472, https://doi.org/10.1029/2026gl122472
  13. Ding, J., Guo, X., Zhang, L., Li, Y., Mao, Z., Ni, H. (2025). Electrical Conductivity of H2O-Rich Silicate Melt: Implications for Subduction Zone Magnetotelluric Anomalies. Journal of Geophysical Research: Solid Earth 130, e2024JB030224. doi:10.1029/2024jb030224, https://doi.org/10.1029/2024jb030224
  14. Ding, J., Ni, H., Guo, X. (2023). Electrical conductivity of potassium-bearing hydrous silicate melts and supercritical fluids. Goldschmidt2023 Abstracts. doi:10.7185/gold2023.17378, https://doi.org/10.7185/gold2023.17378
  15. Zhao, B., Zhu, J., Chen, Q., Yoshino, T. (2026). Electrical Conductivity of Carbonatite Melts to 20 GPa: Constraints on Partial Melting Atop the 410-km Discontinuity and in the Lower Mantle Transition Zone. Journal of Geophysical Research: Solid Earth 131, e2025JB033390. doi:10.1029/2025JB033390, https://doi.org/10.1029/2025JB033390
  16. Wang, J., Takahashi, E., Xiong, X., Chen, L., Li, L., Suzuki, T., Walter, M. J. (2020). The Water-Saturated Solidus and Second Critical Endpoint of Peridotite: Implications for Magma Genesis Within the Mantle Wedge. Journal of Geophysical Research: Solid Earth 125, e2020JB019452. doi:10.1029/2020jb019452, https://doi.org/10.1029/2020jb019452
  17. Mibe, K., Kanzaki, M., Kawamoto, T., Matsukage, K. N., Fei, Y., Ono, S. (2007). Second critical endpoint in the peridotite-H2O system. Journal of Geophysical Research: Solid Earth 112, B10206. doi:10.1029/2005jb004125, https://doi.org/10.1029/2005jb004125
  18. Mitchell, A. C., Nellis, W. J. (1982). Equation of state and electrical conductivity of water and ammonia shocked to the 100 GPa (1 Mbar) pressure range. The Journal of Chemical Physics 76, 6273–6281. doi:10.1063/1.443030, https://doi.org/10.1063/1.443030
  19. Huang, X., Xu, Y., Karato, S. (2005). Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite. Nature 434, 746–749. doi:10.1038/nature03426, https://doi.org/10.1038/nature03426
  20. Yoshino, T., Manthilake, G., Matsuzaki, T., Katsura, T. (2008). Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite. Nature 451, 326–329. doi:10.1038/nature06427, https://doi.org/10.1038/nature06427
  21. Gerya, T. (2024). Is mantle transition zone a water reservoir? Yes, but only transient. EGU General Assembly 2024, EGUsphere conference abstract. doi:10.5194/egusphere-egu24-6602, https://doi.org/10.5194/egusphere-egu24-6602
  22. Gerya, T., Moccetti Bardi, N., Karato, S., Murakami, M. (2026). Transient water storage in the mantle transition zone governed by subduction and water-induced buoyancy. EGU General Assembly 2026, EGUsphere conference abstract. doi:10.5194/egusphere-egu26-3055, https://doi.org/10.5194/egusphere-egu26-3055
  23. Muir, J. M. R., Zheng, X., Zhang, F. (2025). Gradually Hydrating Upper Mantle and a Moderately Wet Mantle Transition Zone Determined From Wadsleyite-Olivine Phase Relations. Journal of Geophysical Research: Solid Earth 130, e2025JB031482. doi:10.1029/2025JB031482, https://doi.org/10.1029/2025JB031482
  24. Yang, X. (2025). Discriminating ionic mobility between diffusivity and electrical conductivity experiments on Earth's silicate materials. American Mineralogist 110, 1209–1225. doi:10.2138/am-2024-9523, https://doi.org/10.2138/am-2024-9523