Top > Topics > Discovery by Ehime University and others that overturns conventional theory Stable iron hydroxide under ultra-high pressure in the deep mantle

2017/7/10

Discovery by Ehime University and others that overturns conventional theory Stable iron hydroxide under ultra-high pressure in the deep mantle

University Journal Online Editorial Department

　A research group led by Assistant Professor Masayuki Nishi of Ehime University (in addition to Tokyo Institute of Technology, Tokyo University, etc.) has found that iron hydroxide with a new crystalline structure consisting of iron, water, and oxygen is stable in an ultra-high temperature and high pressure environment deep in the earth's mantle. The existence was revealed for the first time in the world.The research results were published in the international scientific journal "Nature" (online version).

　The mass of water that can be stored inside the earth is estimated to be several times that of seawater on the surface of the earth.Water reacts with rocks on the surface of the earth to form hydrous minerals, and is transported to the mantle (30 to 2,900 kilometers deep) deep in the earth by subduction of the plate.However, iron hydroxide (a type of hydrous mineral) responsible for water transport is dehydrated and decomposed at 1,900 atmospheres, which corresponds to a depth of 80 kilometers, and it was said that water could not be transported to any deeper depth.

　This time, the research group performed theoretical calculations based on quantum mechanics using a supercomputer such as the K computer, and predicted that iron hydroxide would not decompose at around 80 atm but would change to a pyrite (pyrite) type crystal structure.Using an ultra-high pressure generator at the large-scale radiation facility SPring-8, iron hydroxide was subjected to high pressure equivalent to the deep part of the earth's mantle, demonstrating the theoretically predicted crystal structure change and the presence of hydrogen in the structure.

　This result is a discovery that overturns the conventional theory that iron hydroxide is dehydrated and decomposed in the deep part of the earth's mantle, and it is expected that the role and circulation of water in the deep part of the earth will be elucidated.The iron hydroxide with the new structure demonstrated this time is likely to exist stably even under high pressure at the boundary between the mantle and the core, and the general circulation of water in the deep part of the earth and the mantle ascending flow (plume) at the mantle core boundary. It is thought that it will greatly affect the material composition and movement of the deep part of the earth, such as the occurrence of water and the dissolution of water in the molten iron, which is the main material of the Earth's core.

Paper information:[Nature] The pyrite-type high-pressure form of FeOOH

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