A diamond reveals abundant water dragged into the Earth’s mantle

Sep. 27 () –

The transition zone between the upper and lower mantle of the Earth contains considerable amounts of water carried by subducting plates, according to a study published in NatureGeoscience.

The German-Italian-American research team analyzed a rare diamond formed 660 meters below the surface of the Earth using techniques including Raman spectroscopy and FTIR spectrometry. The study confirmed something that was long just a theory, namely that ocean water accompanies subducting slabs and thus enters the transition zone. This means that our planet’s water cycle includes the interior of the Earth.

The transition zone (TZ) is the name given to the boundary layer that separates the upper and lower mantles of the Earth. I know It is found at a depth of 410 to 660 kilometers. The immense pressure of up to 23,000 bar in the TZ causes the olive-green mineral olivine, which makes up around 70% of the Earth’s upper mantle and is also called peridot, to alter its crystalline structure. At the upper limit of the transition zone, at a depth of about 410 km, it becomes denser wadsleyite; at 520 km it metamorphoses into even denser ringwoodite.

“These mineral transformations greatly hinder the movements of the rocks in the mantle,” he explains. it’s a statement Professor Frank Brenker of the Institute of Geosciences at the Goethe University in Frankfurt. For example, mantle plumes, rising columns of hot rock from the deep mantle, sometimes stop directly below the transition zone. Mass movement in the opposite direction also stops. Brenker says: “Subducting plates often have a hard time traversing the entire transition zone. Therefore, there is an entire graveyard of such plaques in this area below Europa.”

Until now, however, it was not known what the long-term effects of the “suction” of material in the transition zone were on its geochemical composition and whether larger amounts of water existed there. Brenker explains: “Subduction slabs also transport deep-sea sediments into the Earth’s interior. These sediments can contain large amounts of water and CO2. But until now it was unclear how much enters the transition zone in the form of more minerals and stable and hydrated carbonates, and therefore it was also unclear whether large amounts of water are actually stored there.”

Undoubtedly, the prevailing conditions would be conducive to it. The dense minerals wadsleyite and ringwoodite can (unlike olivine at shallower depths) store large amounts of water; so large, in fact, that the transition zone could theoretically absorb six times the amount of water in our oceans. “So we knew that the boundary layer has an enormous capacity to store water,” says Brenker. “However, we didn’t know if he actually did it.”

An international study in which the Frankfurt geoscientist was involved has now provided the answer. The research team analyzed a diamond from Botswana, Africa. It was formed at a depth of 660 km, right at the interface between the transition zone and the lower mantle, where ringwoodite is the predominant mineral. Diamonds from this region are very rare, even among rare diamonds of super deep origin, which make up only 1% of diamonds.

ACCUMULATES IN HYDRATED ROCK

Analysis revealed that the stone contains numerous ringwoodite inclusions, which exhibit a high water content. In addition, the research group was able to determine the chemical composition of the stone. It was almost exactly the same as virtually all mantle rock fragments found in basalts anywhere in the world. This showed that the diamond definitely came from a normal part of the Earth’s mantle.

“In this study we have shown that the transition zone is not a dry sponge, but contains considerable amounts of water,” says Brenker, adding: “This also brings us one step closer to Jules Verne’s idea of ​​an ocean within from the earth”. The difference is that there is no ocean down there, but rather hydrated rock that, according to Brenker, would not feel wet or drip water.