The transition zone between the Earth’s upper and lower mantle contains a significant amount of water, according to an international study. The research team analyzed the rare diamond formed 660 meters below the Earth’s surface using techniques including Raman spectroscopy and FTIR spectrometry. The study confirmed something that had long been only a theory, namely that ocean water accompanies subducting plates and thus enters the transition zone. This means that the water cycle of our planet includes the interior of the Earth.
“These mineral transformations greatly hinder the movement of rock in the mantle,” explains prof. Frank Brenker of the Institute for Geosciences at Goethe University Frankfurt. For example, mantle plumes—rising columns of hot rock from the deep mantle—sometimes stop just below the transition zone. The movement of mass in the opposite direction also stops. Brenker says, “Subducting plates often have trouble breaking through the entire transition zone. So there’s a whole graveyard of such plates in this zone beneath Europe.”
Until now, however, it was not known what the long-term effects of “sucking” material into the transition zone are on its geochemical composition and whether there is a greater amount of water there. Brenker explains: “Subducting plates also transport deep-sea sediments back into the Earth’s interior. These sediments can hold large amounts of water and CO2. However, until now it was not clear how much of it enters the transition zone in the form of more stable, hydrated minerals and carbonates – and therefore there was no clear whether a large amount of water is actually stored there.’
Prevailing conditions would certainly help this. The dense minerals wadsleyite and ringwoodite can (unlike olivine at shallower depths) store large amounts of water – so much, in fact, that the transition zone would theoretically be able to absorb six times the amount of water in our oceans. “So we knew that the boundary layer has a huge capacity to store water,” says Brenker. “But we didn’t know if that was really the case.”
The answer has now been provided by international studies in which the Frankfurt geoscientist participated. The research team analyzed a diamond from Botswana, Africa. It was formed at a depth of 660 kilometers, right at the interface of the transition zone and the lower mantle, where ringwoodite predominates. Diamonds from this area are very rare even among rare super-deep origin diamonds, which make up only one percent of diamonds.
Analyzes revealed that the stone contains numerous ringwoodite inclusions – which exhibit a high water content. Furthermore, the research group managed to determine the chemical composition of the stone. It was almost exactly the same as 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 shown that the transition zone is not a dry sponge, but contains a significant amount 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 wet rock, which Brenker says is neither wet nor dripping with water.
Hydrous ringwoodite was first detected in a diamond from the transition zone back in 2014. Brenker was also involved in this study. However, it was not possible to determine the exact chemical composition of the stone because it was too small. It therefore remained unclear how representative the first mantle study was in general, as the water content of this diamond could also have resulted from an exotic chemical environment. In contrast, the inclusions in the 1.5 cm diamond from Botswana that the research team examined in this study were large enough to allow the exact chemical composition to be determined, providing final confirmation of the preliminary results from 2014.
The high water content of the transition zone has far-reaching consequences for the dynamical situation inside the Earth. What this leads to can be seen, for example, in the hot mantles coming from below that 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 the water stored in the transition zone. If these smaller water-rich mantle plumes now migrate further up and breach the upper mantle boundary, the following happens: The water contained in the mantle plumes is released, lowering the melting point of the emerging material.
So it melts immediately and not just before it reaches the surface as it usually does. As a result, the rock masses in this part of the earth’s mantle are no longer as rigid overall, which gives mass movements more dynamism. The transition zone, which otherwise acts as a barrier to the local dynamics, suddenly becomes the driver of the global material circulation.
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