Jakarta –
An international research team led by a professor from Goethe University, analyzed the diamond inclusions. This research found signs of the existence of oceans underground, deep inside the Earth.
Diamond inclusions are the boundary layer between Earth’s upper and lower mantles, also known as the transition zone (TZ). This zone is located between 410 and 660 kilometers below the surface.
Olivine or the olive green mineral commonly known as peridot, makes up about 70% of Earth’s upper mantle. This mineral changes its crystal structure at extreme pressures of up to 23 thousand bar at the TZ.
At a depth of about 410 kilometers, at the upper edge of the transition zone, it changes into the denser wadsleyite, and at a depth of 520 kilometers, it changes into the denser ringwoodite.
“This mineral transformation greatly inhibits the movement of rock in the mantle,” explained Professor Frank Brenker from the Institute for Geosciences at Goethe University in Frankfurt, Germany, quoted from SciTechDaily.
For example, mantle plumes (columns of hot rock rising from the deep mantle) sometimes stop just below the transition zone. The mass movement in the opposite direction also stopped.
“The subducting plate often has difficulty penetrating across the transition zone. So there are such plate burials in this European lower zone,” Brenker said.
However, until now it was not known what the long-term effect of ‘sucking’ material into the transition zone would be on its geochemical composition, and whether larger amounts of water were present there.
“The subduction layer also carries deep-sea sediments piggybacking into Earth’s interior. These sediments can hold large amounts of water and CO2. But until now it was unclear how much made it into the transition zone in the form of the more stable hydro and carbonate minerals, and therefore it’s also not clear whether large amounts of water are actually stored there.”
Circumstances as described in this study, seem to support the existence of water inside the Earth. The thick minerals wadsleyite and ringwoodite can store such significant amounts of water that the transition zone could hypothetically absorb six times the amount of water in our oceans.
“So we know that the boundary layer has a very large capacity to store water. However, we don’t know if that actually is the case,” Brenker said.
The answer is now revealed by an international study. The research team analyzed diamonds from Botswana, Africa. The diamond comes from a depth of 660 kilometers, directly at the interface between the transition zone and the lower mantle, where the predominant mineral is ringwoodite.
Diamonds from this location are extremely rare, even among the extremely rare diamonds of super depth, which account for only 1% of all diamonds. Studies found that the rock has a high water content due to the presence of many ringwoodite inclusions. The research team was also able to determine the chemical composition of the rock.
It contains almost exactly the same as almost every mantle rock fragment found in basalt anywhere in the world. This suggests that the diamond must have come from a normal cut in the Earth’s mantle.
“In this study, we have shown that the transition zone is not a dry sponge, but holds a sizeable amount of water,” said Brenker.
“This study also brings us one step closer to Jules Verne’s ideas about the oceans inside the Earth. The difference is, down there there are no oceans, but hydrous rocks,” he continued.
Hydrated ringwoodite was first detected in diamonds from the transition zone in early 2014. Brenker was also involved in the study. However, it is impossible to determine the exact chemical composition of the stone because it is too small.
It is therefore unclear how representative the first studies of the Earth’s mantle are in general, because the diamond’s water content could also have resulted from an exotic chemical environment.
In contrast, the inclusions in the 1.5 cm diamond from Botswana, which the research team investigated in this study, were large enough to allow for a precise chemical composition, and this provided final confirmation of the study’s initial results in 2014.
The high water content in the transition zone has far-reaching consequences for the dynamic situation within the Earth. What causes this can be seen, for example, in hot mantle fluff coming from below, which gets 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 these smaller, water-rich mantle plumes now migrate further up and break through the boundary into the upper mantle, the water contained in the mantle plumes is released, which lowers the melting point of the material that emerges.
As a result, the rock masses in this part of the Earth’s mantle as a whole make mass motion more dynamic. The transition zone, which previously served as a barrier to dynamics there, suddenly became a driving force for global material circulation.
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