When we refer to the interior of the earth, we find that we know very little about what is going on there. But new technologies have made it possible to learn more about specific areas by describing their composition and dynamics.
Nevertheless, some doubts remain especially the presence of water under the earth’s crust. Some stories say they exist. For example, in Jules Verne’s 1864 novel Journey to the Center of the Earth there was talk of inland seas, but it wasn’t until 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 for the existence of significant amounts of water between the upper and lower mantle – at depths of about 410 and 660 km.
“The study confirms something that was only theory for a long time, namely that Ocean water accompanies subduction losses and therefore enters the transition zone.
“The water cycle of our planet thus includes the interior of the earth”, explains the Institute for Geosciences at the Goethe University in Frankfurt, to which three of the study participants belong.
negative pressure at depth
Between the upper mantle and the lower mantle of the earth is the so-called transition zone (TZ). The pressure can reach 23 million millibars, resulting in a change in the crystal structure of the olive-green mineral that 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, even denser than the first. “Mineral alterations make it very difficult for rocks to move through the mantle”, explains Professor Frank Brenker from the Institute for Geosciences.
It was not previously known what long-term effects the suction of materials in the transition zone will have on its geochemical composition and whether increased water volumes are present. Professor Brenker thinks that subduction losses also transport deep-sea sediments into the earth’s interior.
“These sediments can contain large amounts of water and CO2. However, until now it was unclear how much water enters the transition zone in the form of hydrated minerals and more stable carbonates. and therefore it was not known whether large amounts 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 inclusions of ringwoodite, which have a high water content. In addition, the chemical composition of the stone could be determined.
Jules Verne’s idea of an ocean inside the earth is far from reality. At depth there would be no actual ocean but 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 at depth, but hydrated rock”, explains Professor Brenker.
Hydrated ringwoodite was first discovered in a transition zone diamond in 2014; Brenker also took part in this study. However, The exact chemical composition of the stone could not be determined because it was too small.