Hidden Ocean Locked Up Deep in Earth’s Mantle

Deep within the Earth’s rocky mantle lies oceans’ worth of water locked up in a type of mineral called ringwoodite, new research shows.

The results of the study will help scientists understand how plate tectonics moves water between the surface of the planet and interior reservoirs.

The Earth’s mantle is the hot, rocky layer between the planet’s core and crust. Scientists have long suspected that the mantle’s so-called transition zone, which sits between the upper and lower mantle layers 255 to 410 miles (410 to 660 kilometers) below Earth’s surface, could contain water trapped in rare minerals. However, direct evidence for this water has been lacking, until now.

A water-rich mineral

Ringwoodite is a rare type of mineral that forms from olivine under very high pressures and temperatures, such as those present in the mantle’s transition zone. Laboratory studies have shown that the mineral can contain water, which isn’t present as liquid, ice or vapor; instead, it is trapped in the ringwoodite’s molecular structure as hydroxide ions (bonded oxygen and hydrogen atoms).

In March, another research group discovered an unusual diamond from the mantle that encased hydrous ringwoodite. Though the find suggested the transition zone could contain a lot of water, it was the first and only ringwoodite specimen from the mantle scientists have ever analyzed (all other samples were produced in the lab or found in meteorites), and may not be representative of other mantle ringwoodite.

The new findings will help scientists better understand Earth’s water cycle.

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