When we refer to the interior of the Earth, we realize that we know very little about what is happening there. But new technologies have made it possible to get to know some areas better describing its composition and dynamics.
Despite this, some doubts remain, especially on the presence of water under the earth’s crust. Some stories say there are. In Jules Verne’s fictional book Journey to the Center of the Earth of 1864, for example, there was already talk of internal oceans, but it was only in 2014 that the first scientific study showed evidence of the presence of water at a depth of 500 km.
The study by experts from the United States, Italy and Germany, published in an article in the journal Nature Geoscience, provides new evidence of the existence of significant amounts of water between the Earth’s upper and lower mantle – at approximately 410 and 660 km depth.
“The study confirms something that has long been just a theory, that is ocean water accompanies subduction losses and thus enters the transition zone.
“This means that our planet’s water cycle includes the interior of the Earth”, explains the Geosciences Institute of Goethe University in Frankfurt, to which three of the study participants belong.
Low pressure in depth
Between the upper mantle and the lower mantle of the Earth lies the so-called transition zone (TZ). The pressure can reach 23 million millibars, resulting in a change in the crystal structure of the olive mineral, which makes up about 70% of the Earth’s upper mantle.
At the upper limit of the TZ, at a depth of 410 kilometers, it becomes a denser mineral, wadsleyite, and at 520 kilometers it becomes another mineral, ringwoodite, which is even denser than the first. “Mineral transformations make it very difficult for rocks to move through the mantle”, explains Professor Frank Brenker of the Institute of Geosciences.
Until now, it was not known what the long-term effects of the suction of materials in the transition zone on their geochemical composition and whether greater amounts of water were present. thinks Professor Brenker that subduction losses also carry deep-sea sediments into the Earth.
“These sediments can contain large amounts of water and CO2. But until now it was not clear how much water enters the transition zone in the form of hydrated minerals and more stable carbonates. and therefore it was not even known whether large quantities of water were actually stored there “.
Analysis of a found diamond
The authors analyze a Botswana diamond formed at a depth of 660 kilometers, between the transition zone and the lower mantle, where ringwoodite is the predominant mineral.
Analysis revealed that the stone contains numerous ringwoodite inclusions, which show a high water content. In addition, it is possible to determine the chemical composition of the stone.
Jules Verne’s idea of an ocean inside the Earth is far from reality. There would be no real ocean in the depths, but rather an area of hydrated rock.
Brenker comments: “In this study, we have shown that the transition zone is not a dry sponge, but contains significant amounts of water. It also brings us closer to Jules Verne’s idea of an ocean inside the Earth. The difference is that there is actually no ocean in the depths, but hydrated rock “, explains Professor Brenker.
Hydrated ringwoodite was first detected in a transition zone diamond in 2014; Brenker also participated in this study. However, it was not possible to determine the exact chemical composition of the stone because it was too small.