An international research team led by a Goethe University professor analyzes diamond inclusions.
The boundary layer between the upper and lower mantles of the Earth is known as the transition zone (TZ). It is located between 410 and 660 kilometers (between 255 and 410 miles) under the surface. The olive-green mineral olivine, commonly known as peridot, which makes up around 70% of the Earth’s upper mantle, changes its crystalline structure at the extreme pressure of up to 23,000 bar in the TZ. At a depth of around 410 kilometers (255 miles), at the upper edge of the transition zone, it changes into denser wadsleyite, and at a depth of 520 kilometers (323 miles), it transforms into even denser ringwoodite.
“These mineral transformations greatly hinder the movements of rock in the mantle,” explains Professor Frank Brenker from the Institute for Geosciences at Goethe University in Frankfurt. For example, mantle plumes – rising columns of hot rock from the deep mantle – sometimes stop directly below the transition zone. The movement of mass in the opposite direction also comes to standstill. Brenker says, “Subducting plates often have difficulty in breaking through the entire transition zone. So there is a whole graveyard of such plates in this zone underneath Europe.”
However, until now it was not known what the long-term effects of “sucking” material into the transition zone were on its geochemical composition and whether larger quantities of water existed there. Brenker explains: “The subducting slabs also carry deep-sea sediments piggyback into the Earth’s interior. These sediments can hold large quantities of water and CO2. But until now it was unclear just how much enters the transition zone in the form of more stable, hydrous minerals and carbonates – and it was therefore also unclear whether large quantities of water really are stored there.”
The current circumstances would undoubtedly favor this. The thick minerals wadsleyite and ringwoodite can hold significant amounts of water (unlike olivine at lower depths), so much so that the transition zone could hypothetically absorb six times the quantity of water in our oceans. “So we knew that the boundary layer has an enormous capacity for storing water,” Brenker says. “However, we didn’t know whether it actually did so.”
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 kilometers, directly at the interface 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 super-deep origin, which account for just 1% of all diamonds. The studies found that the stone had a high water content due to the presence of many 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 every fragment of mantle rock found in basalts anywhere in the world. This showed that the diamond definitely came from a normal piece of the Earth’s mantle. “In this study, we have demonstrated that the transition zone is not a dry sponge, but holds considerable quantities of water,” Brenker says, adding: “This also brings us one step closer to Jules Verne’s idea of an ocean inside the Earth.” The difference is that there is no ocean down there, but hydrous rock which, according to Brenker, would neither feel wet nor drip water.
Hydrous ringwoodite was first detected in a diamond from the transition zone as early as 2014. Brenker was involved in that study, too. However, it was not possible to determine the precise chemical composition of the stone because it was too small. It, therefore, remained unclear how representative the first study was of the mantle in general, as the water content of that diamond could also have resulted from an exotic chemical environment. By contrast, the inclusions in the 1.5-centimeter (0.6 inches) diamond from Botswana, which the research team investigated in the present study, were large enough to allow the precise chemical composition to be determined, and this supplied final confirmation of the preliminary results from 2014.
The transition zone’s high water content has far-reaching consequences for the dynamic situation inside the Earth. What this leads to can be seen, for example, in the hot mantle plumes coming from below, which get stuck in the transition zone. There, they heat up the water-rich transition zone, which in turn leads to the formation of new smaller mantle plumes that absorb the water stored in the transition zone.
If these smaller water-rich mantle plumes now migrate further upwards and break through the boundary to the upper mantle, the following happens: The water contained in the mantle plumes is released, which lowers the melting point of the emerging material. It, therefore, melts immediately and not just before it reaches the surface, as usually happens. As a result, the rock masses in this part of the Earth’s mantle are no longer as tough overall, which gives the mass movements more dynamism. The transition zone, which otherwise acts as a barrier to the dynamics there, suddenly becomes a driver of global material circulation.
Reference: “Hydrous peridotitic fragments of Earth’s mantle 660 km 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, 26 September 2022, Nature Geoscience.