The water content of Earth’s mantle is a key measurement of Earth’s water budget. Global recycling of water on Earth drives important forms of volcanism such as island arcs, controls the flow of upper-mantle fluid and plays a role in the evolution of mantle plumes.
While subducted oceanic crust dehydrates at shallow depth, faster and colder slabs can carry water much deeper. Average water abundance in the mantle changes substantially across the boundary between the upper mantle and the transition zone, as well as across the boundary between the transition zone and the lower mantle.The transition zone contains more than ten times more water than the upper mantle. The abundance of water in the lower mantle is unknown but is believed to hold less than the transition zone. The average abundance of chemically bound water in these different regions of Earth, as well as possible occurrences of smaller layers of water rich rock are central to our understanding of Earth’s water budget over extended geologic time.
The actual water content of different mantle regions depends not only on thermodynamic limits of water solubility but also on the process in real Earth that carries water to beyond 410 km depth.
Scientists have shown that the mineral ice-VII occurs as inclusions in natural diamond and serves as an indicator for water rich regions. Ice-VII, which is the residue of aqueous fluid present during diamond growth, crystallizes upon ascent of the host diamonds but remains at high pressures. Ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and around the 660 km boundary.