Water is found in the transition zone between the Earth's upper and lower mantle. A rare diamond formed 660 meters below the Earth's surface was analyzed by the German-Italian-American research team. For a long time the theory was that ocean water accompanies subducting slabs and enters the transition zone. The Earth's interior is included in the water cycle.
The boundary layer that separates the upper and lower mantles is called the transition zone. It is between 410 and 660 kilometers deep. The olive-green mineral olivine, which constitutes 70% of the Earth's upper mantle, is altered by the immense pressure of up to 23,000 bar in the TZ. At the upper boundary of the transition zone, at a depth of about 410 kilometers, it becomes denser and denser ringwoodite.
The movement of rock in the mantle is hampered by mineral transformations. The hot rock from the deep mantle sometimes stops below the transition zone. Mass movement in the opposite direction comes to a halt. The transition zone is difficult to break through withducting plates. There is a lot of such plates in this area.
Until now, no one knew what the long-term effects of sucking material into the transition zone would be. The subducting slabs carry the deep-sea mud into the Earth. Large amounts of water and CO 2 can be found in these sediments. Until now, it was unclear how much enters the transition zone in the form of hydrous minerals and carbonates, and whether large quantities of water actually are stored there.
The conditions are favorable for that. The transition zone can absorb six times the amount of water in our oceans because of the dense minerals. The boundary layer has a lot of capacity to store water. We weren't sure if it actually did so.
The answer has been supplied by the international study in which the Frankfurtgeoscientist was a part. A diamond from Africa was analyzed. It was formed at the interface between the transition zone and the lower mantle, where ringwoodite is the main mineral. Diamonds from this region are very rare and only 1% of them are super-deep origin. The analyses showed that the stone has many ringwoodite inclusions. The stone's chemical composition was determined by the research group. It was the same as every basalt fragment found in the world. The diamond came from a piece of the Earth's mantle, according to this proof. The study shows that the transition zone is not a dry sponge and that it holds a lot of water. There is no ocean down there, but hydrous rock which would not feel wet or dry.
The first diamond with hydrous ringwoodite was found in the transition zone. He was involved in that study as well. The stone was too small to determine its chemical composition. It was not clear how representative the first study was of the mantle as the water content of that diamond could have come from an exotic chemical environment. The 1.5 centimeter diamond from Botswana, which the research team investigated in the present study, was large enough to allow the precise chemical composition to be determined, and this provided final confirmation of the preliminary results.
The high water content of the transition zone has far-reaching consequences. The hot mantle plumes coming from below can be seen in the transition zone. The heat up of the water-rich transition zone leads to the formation of smaller mantle plumes that absorb the water in the transition zone. The melting point of the emerging material is lowered if the smaller water-rich mantle plumes migrate further upwards. It will melt immediately and not just before reaching the surface. The rock mass in this part of the Earth's mantle is no longer as strong as it used to be. The transition zone acts as a barrier to the dynamics there and now becomes a driver of the global material circulation.
More information: Tingting Gu et al, Hydrous peridotitic fragments of Earth's mantle 660 km discontinuity sampled by a diamond, Nature Geoscience (2022). DOI: 10.1038/s41561-022-01024-y Journal information: Nature Geoscience
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