Fig. 1: Water circulation in the mantle.
The deep water cycle down to the mantle transition zone is characterized by the release of fluid water9 (blue arrows). For water to be transported deeper, into the lowermost mantle, it relies on high-pressure hydrous phases such as FeOOH (stable in the subducted basaltic crust and sediments depicted by the thick black line). Hou and colleagues1 show that at the core–mantle boundary, where the temperature is greater than the superionic transition temperature (Ttr = 1,700–2,000 K), the hydrogen ions in the FeOOH structure are moving freely through the crystal lattice and electrical conductivity is high, offering an exciting geophysical possibility for exploration of Earth’s deepest water cycle. Crystal structure insets adapted with permission from ref. 1, Springer Nature Ltd.
