page_title: Deep water cycle
section_title: Storage capacity
hierarchical_section_title: Deep water cycle / Storage capacity
caption_attribution_description: English: Phase transformations of olivine moving through the upper mantle, transition zone, and lower mantle. In the core, water might be stored as hydrogen bound to iron.
context_page_description: The deep water cycle, or geologic water cycle, involves exchange of water with the mantle, with water carried down by subducting oceanic plates and returning through volcanic activity, distinct from the water cycle process that occurs above and on the surface of Earth. Some of the water makes it all the way to the lower mantle and may even reach the outer core. Mineral physics experiments show that hydrous minerals can carry water deep into the mantle in colder slabs and even "nominally anhydrous minerals" can store several oceans' worth of water.
context_section_description: An upper bound on the amount of water in the mantle can be obtained by considering the amount of water that can be carried by its minerals (their storage capacity). This depends on temperature and pressure. There is a steep temperature gradient in the lithosphere where heat travels by conduction, but in the mantle the rock is stirred by convection and the temperature increases more slowly (see figure). Descending slabs have colder than average temperatures.
  The mantle can be divided into the upper mantle (above 410 km depth), transition zone (between 410 km and 660 km), and the lower mantle (below 660 km). Much of the mantle consists of olivine and its high-pressure polymorphs. At the top of the transition zone, it undergoes a phase transition to wadsleyite, and at about 520 km depth, wadsleyite transforms into ringwoodite, which has the spinel structure. At the top of the lower mantle, ringwoodite decomposes into bridgmanite and ferropericlase.
  The most common mineral in the upper mantle is olivine. For a depth of 410 km, an early estimate of 0.13 percentage of water by weight (wt%) was revised upwards to 0.4 wt% and then to 1 wt%. However, the carrying capacity decreases dramatically towards the top of the mantle. Another common mineral, pyroxene, also has an estimated capacity of 1 wt% near 410 km.
  In the transition zone, water is carried by wadsleyite and ringwoodite; in the relatively cold conditions of a descending slab, they can carry up to 3 wt%, while in the warmer temperatures of the surrounding mantle their storage capacity is about 0.5 wt%. The transition zone is also composed of at least 40% majorite, a high pressure phase of garnet; this only has capacity of 0.1 wt% or less.
  The storage capacity of the lower mantle is a subject of controversy, with estimates ranging from the equivalent of 3 times to less than 3% of the ocean. Experiments have been limited to pressures found in the top 100 km of the mantle and are challenging to perform. Results may be biased upwards by hydrous mineral inclusions and downwards by a failure to maintain fluid saturation.
  At high pressures, water can interact with pure iron to get FeH and FeO. Models of the outer core predict that it could hold as much as 100 oceans of water in this form, and this reaction may have dried out the lower mantle in the early history of Earth.
caption: Phase transformations of olivine moving through the upper mantle, transition zone, and lower mantle. In the core, water might be stored as hydrogen bound to iron.
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