An international research team led by a professor from Goethe University is analyzing diamond inclusions.
The boundary layer between Earth’s upper and lower mantle is known as the transition zone (TZ). It is located between 410 and 660 kilometers (between 255 and 410 miles) below the surface. The olive-green mineral olivine, commonly known as peridot, which makes up about 70% of the Earth’s upper mantle, changes its crystal structure at the extreme pressure of up to 23,000 bar in the TZ. At a depth of about 410 kilometers (255 miles), at the upper edge of the transition zone, it changes to denser wadsleyite, and at a depth of 520 kilometers (323 miles) it changes to even denser ringwoodite .
“These mineral transformations strongly impede rock movements in the mantle,” explains Professor Frank Brenker from the Institute of Geosciences at Goethe University Frankfurt. For example, mantle plumes – rising columns of hot rock from the deep mantle – sometimes stop directly below the transition zone. Mass movement in the opposite direction also stops. Brenker says, “Subduction plates often struggle to cross the entire transition zone. So there is a whole graveyard of such plates in this area under Europe.
However, until now it was not known what the long-term effects of the “sucking up” of materials into the transition zone were on its geochemical composition and whether greater amounts of water were present there. Brenker explains: “Subduction slabs also transport deep-sea sediments into the Earth’s interior. These sediments can contain large amounts of water and CO2. But until now it was not known exactly how much was entering the transition zone in the form of more stable hydrated minerals and carbonates – and so it was also unclear whether large amounts of water were actually stored there. .
Current circumstances would no doubt favor it. The thick minerals wadsleyite and ringwoodite can hold significant amounts of water (unlike olivine at shallower depths), so much so that the transition zone could hypothetically absorb six times the amount of water from our oceans. “So we knew that the boundary layer has a huge water storage capacity,” says Brenker. “However, we didn’t know if that was actually the case.”
The answer has just been provided by an international study. The research team analyzed a diamond from Botswana, Africa. It originated at a depth of 660 kilometers, directly at the interface between the transition zone and the lower mantle, where the dominant mineral is ringwoodite. Diamonds from this place are very rare, even among the extremely rare diamonds of very deep origin, which represent only 1% of all diamonds. The studies revealed that the stone had a high water content due to the presence of numerous ringwoodite inclusions. The study team was also able to establish the chemical composition of the stone.
It was almost exactly the same as that of virtually all mantle rock fragments found in basalts all over the world. This showed that the diamond definitely came from a normal piece of Earth’s mantle. “In this study, we demonstrated that the transition zone is not a dry sponge, but contains considerable amounts of water,” says Brenker, adding, “This also brings us closer to Jules Verne’s idea of an ocean inside the Earth.” The difference is that there’s no ocean there, just hydrated rock that Brenker says doesn’t feel wet or dripping.
Hydrated ringwoodite was first detected in a transition zone diamond as early as 2014. Brenker also participated in this study. However, it was not possible to determine the precise chemical composition of the stone because it was too small. It was therefore unclear how representative the first study was of the mantle in general, as the water content of this diamond could also result from an exotic chemical environment. In contrast, the inclusions in the 1.5 centimeter (0.6 inch) diamond from Botswana, which the research team investigated in the current study, were large enough to determine the precise chemical composition, which provided the final confirmation of the preliminary results. from 2014.
The high water content of the transition zone has important consequences for the dynamic situation inside the Earth. What this leads to can be seen, for example, in the plumes of the warm mantle coming from below, which get stuck in the transition zone. There, they heat up the water-rich transition zone, which leads to the formation of new, smaller mantle plumes that absorb the water stored in the transition zone.
If these small, water-rich mantle plumes now migrate higher and cross the boundary of the upper mantle, the following occurs: the water in the mantle plumes is released, which lowers the melting point of the emerging material. It therefore melts immediately and not just before reaching the surface, as usually happens. As a result, the rock masses of this part of the Earth’s mantle are not as resistant overall, which gives more dynamism to mass movements. The transition zone, which otherwise acts as a barrier to dynamics, suddenly becomes an engine of global material circulation.
Reference: “Hydrated peridotitic fragments of the Earth’s mantle 660 km of discontinuity sampled by a diamond” by Tingting Gu, Martha G. Pamato, Davide Novella, Matteo Alvaro, John Fournelle, Frank E. Brenker, Wuyi Wang and Fabrizio Nestola, September 26
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