According to the Geosciences Institute of Goethe University in Frankfurt, the transition zone between the upper and lower mantles of the Earth contains significant amounts of water.
A systematic review
In a study just presented in Nature (here it is), the researchers analyzed a rare diamond formed 660 kilometers below the Earth's surface, using techniques such as Raman spectroscopy and FTIR spectrometry.
The findings confirm a long-standing theory that, accompanying subducting tectonic plates, ocean water enters the transition zone underground. This means, in summary, that the water cycle of our planet also includes the interior of the Earth.
The transition zone
"Transition zone" is the name given to the boundary layer that separates the upper mantle of the Earth from the lower one. It is found at a depth of between 410 and 660 kilometers.
The immense pressure of the transition zone, up to 23.000 bar, causes the mineral olivine, which makes up about 70% of the Earth's upper mantle, to alter its crystalline structure. At the upper limit of the transition zone, some 410 kilometers underground, it transforms into (denser) wadsleyite; at 520 kilometers it turns into ringwoodite (even denser).
Wadsleyite and ringwoodite can (unlike olivine found at shallower depths) store large amounts of water, to the point that the transition zone could theoretically absorb six times the amount of water present in our oceans. And now we have confirmation that he is doing it, we still don't know to what extent.
The diamond studied in Frankfurt was formed, as mentioned, at a depth of 660 kilometers: right between the transition zone and the lower mantle, where ringwoodite is the predominant mineral.
And the ringwoodite components within the stone have a high water content. "With this study," he says Frank Brenker, lead author, "we have shown that the transition zone is not a dry sponge, but contains significant amounts of water."
"This brings us one step closer to Jules Verne's idea of an ocean inside the Earth," says the researcher.
With the difference that there would be a "ocean" of rock soaked in water down there, and the Nautilus underground would have some trouble crossing it.