Water found at 660 km below the surface. Full completion of the water cycle?

The full water cycle is now thought to be completed by a fourth unknown form of water held within the Earth’s mantle, at 410 – 660 km depth. Scientists have been seeking evidence to support this theory for last stage of the water cycle for decades.

The discovery was accomplished by laboratory experiments where a simulation of the conditions, by geodynamical modelling in the mantle, such as the high pressures created by the weight of the 400 km rock and mantle temperatures (Schmandt et al., 2014). Furthermore, a dense network of 2,000 seismometers and three different micro-beam analytical techniques were applied across the United States.

The water is trapped within the molecular structure of minerals in the mantle. As the water molecules split to hydroxyl radical (OH), which can bind with minerals crystal structure at temperatures above 1,095℃ and at high pressures produced by the weight of 400 km rock, such as the mineral ringwoodite, where the water is stored. This mineral was discovered within a diamond sample from Juina area of Mato Grosso, Brazil, where artisan miners extracted the host diamond from shallow river gravels. It is believed that the diamond had been transported to the surface of the Earth by volcanic activity in a rock named kimberlite (Pearson, 2008). It was suggested that minerals in the transition zone of the mantle (410-660 km) may act as a water reservoir resulting the solubility properties of water causing the partial melting of certain elements (Huang et al., 2005).

This discovery may imply that water from the Earth’s surface can be transported to great depths by plate tectonics, eventually leading to partial melting of the rocks found deep in the mantle. The water content of Earth’s mantle transition zone (between the upper and lower mantle) is incredibly challenging to quantify. The content of water in the mantle may potentially influence many properties, including rheology, electrical conductivity, melting point and preservation of chemical heterogeneity. The presence of this fourth form of water may be the reason for the unique properties and features of the tectonic movement and volcanic activity making the Earth such dynamic planet (Pearson, 2014).

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Denitsa Tarnovska

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