Underground ocean? Scientists discover water in the depths of the earth

Scientists have discovered evidence of water hundreds of kilometers deep.

An international research team led by a professor at Goethe University is analyzing diamond inclusions.

The boundary layer between the Earth’s upper and lower mantle is known as the transition zone (TZ). It lies between 410 and 660 kilometers (between 255 and 410 miles) below the surface. The mineral green olive oil, known as peridot, which makes up about 70% of the Earth’s upper mantle, changes its crystalline structure at extreme pressures of up to 23,000 bar at TZ. At a depth of about 410 kilometers (255 miles), at the upper edge of the transition zone, it turns into more dense wadslite, and at a depth of 520 kilometers (323 miles), it turns into more dense ringwoodite.

“These mineral shifts significantly impede the movement of rocks in the mantle,” explains Professor Frank Brinker of the Institute of Earth Sciences at Goethe University in Frankfurt. For example, mantle plumes – rising plumes of hot rock from the deep mantle – sometimes stop right below the transition zone. The movement of the mass in the opposite direction also stops. Brinker says, “The conductive plates often have difficulty penetrating the entire transition zone. So there is a whole cemetery of these plates in this sub-European region.”

Diamond Goethe University

Diamonds from Botswana have revealed to scientists that large amounts of water are stored in the rock at a depth of more than 600 km. Credit: Tingting Gu, Gemological Institute of America, New York, NY, USA

However, it was not yet known what the long-term effects of the “sucking” of materials in the transition zone will have on its geochemical composition and whether greater amounts of water are present there. Brinker explains: “The subducted plates also carry deep-sea sediments on their backs to the Earth’s interior. These sediments can contain significant amounts of water and carbon dioxide. But until now it was not clear how much enters the transition zone in the form of more stable minerals and carbonates, Thus it was also not clear whether large amounts of water were actually stored there.”

Undoubtedly, the current conditions will favor this. The thick minerals wadsleyite and ringwoodite can hold large amounts of water (unlike olivine at lower depths), so much so that the transition zone can hypothetically absorb six times as much water in our oceans. “We learned that the boundary layer has a tremendous capacity to store water,” Brinker says. “However, we didn’t know if she actually did.”

The answer has now been provided by an international study. The research team analyzed a diamond from Botswana, Africa. It originated at a depth of 660 km, right at the surface between the transition zone and the lower mantle, where the dominant mineral is ringwoodite. Diamonds from this location are very rare, even among the extremely rare diamonds of ultra-deep origin, which represent only 1% of the total diamonds. Studies have found that the stone has a high water content due to the presence of many ringwoodite inclusions. The study team was also able to determine the chemical composition of the stone.

They were almost exactly the same as those found in every part of the mantle rock found in basalt anywhere in the world. This showed that the diamond definitely came from an ordinary piece of Earth’s mantle. “In this study, we showed that the transition zone is not a dry sponge, but rather contains large amounts of water,” Brinker says, adding, “This also brings us one step closer to Jules Verne’s idea of ​​an ocean within Earth.” The difference is that there is no ocean. There are, but there are watery rocks which, according to Brinker, will feel neither wet nor drip.

Watery ringwoodite was first discovered in diamonds from the transitional zone as early as 2014. Brinker was also involved in that study. However, it was not possible to determine the exact chemical composition of the stone because it was so small. Therefore, it remained unclear how the first study of the mantle was in general, as the water content of this diamond could also be caused by a strange chemical environment. By contrast, the inclusions in 1.5 cm (0.6 in) diamonds from Botswana, which the research team investigated in this study, were large enough to allow the exact chemical composition to be determined, and this provided definitive confirmation of preliminary findings from 2014.

The high water content in the transition zone has far-reaching consequences for the dynamic situation within the Earth. What this leads to can be seen, for example, in the plumes of the hot mantle coming from below, which get stuck in the transition zone. There, they heat the water-rich transition zone, which in turn leads to the formation of new, smaller mantle plumes that absorb water stored in the transition zone.

If small, water-rich mantle plumes now migrate upward and penetrate the boundary into the upper mantle, the following occurs: the water in the mantle plumes is released, lowering the melting point of the emerging material. So it dissolves instantly and not just before it reaches the surface as it usually does. As a result, the massifs in this part of the Earth’s mantle are generally no longer solid, which gives mass movements more dynamism. The transition zone, which acts as a barrier to the dynamics there, suddenly became an engine of global material circulation.

Reference: “Aquamarine Fragments of Earth’s Mantle 660 km Diamond Sampling Interruption” By Tingting Gu, Martha J. Pamato, David Novella, Matteo Alfaro, John Fornelli, Frank E. Brinker, Wei Wang and Fabrizio Nistola, Sep 26, Nature Geoscience is a monthly peer-reviewed scientific journal published by the Nature Publishing Group covering all aspects of the Earth sciences, including theoretical research, modeling, and fieldwork. Other related work has also been published in fields including atmospheric sciences, geology, geophysics, climatology, oceanography, palaeontology, and space science. It was established in January 2008.

“data-gt-translate-attributes=”[{” attribute=””>Nature Geoscience.
DOI: 10.1038/s41561-022-01024-y

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