An underground ocean? Scientists discover water deep in the earth

An international research team led by a Goethe University professor analyzed the diamond inclusions.

The boundary layer between the Earth’s upper and lower mantle is known as the transition zone (TZ). It lies between 410 and 660 kilometers (between 255 and 410 miles) below the surface. The olive-green mineral olivine, commonly known as peridot, which makes up about 70% of Earth’s upper mantle, changes its crystal structure at extreme pressures of up to 23,000 bars in the TZ. At a depth of about 410 km (255 mi), at the upper end of the transition zone, it changes to dense wadeslite, and at a depth of 520 km (323 mi) it changes to denser ringwoodite.

“These mineral transformations greatly inhibit the movement of rocks in the mantle,” explains Professor Frank Brenker of the Institute for Geosciences. Goethe University in Frankfurt. For example, mantle plumes – rising columns of heated 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 difficulty breaking through the entire transition zone. So there is a whole graveyard of such plates in this region beneath Europe.”

However, until now it was not known whether the long-term effects of the material “sucked up” in the transition zone were on its geochemical composition and whether abundant water existed there. Brenker explains: “Subducting slabs carry deep-sea sediments piggyback into the Earth’s interior. These sediments can hold large amounts of water and CO2. But until now it was not clear how much enters the transition zone in the form of more stable, hydrous minerals and carbonates – and so it was also unclear whether there was much water stored there.

The current situation will undoubtedly favor it. The coarser minerals wadslate and ringwoodite can hold significant amounts of water (unlike olivine at lower depths), so much so that the transition zone can hypothetically absorb six times the amount of water in our oceans. “So we knew that the boundary layer has a huge capacity to store water,” Brenker said. “However, we didn’t know if it actually did that.”

The answer has now been provided by an international study. The research team analyzed a diamond from Botswana, Africa. It originates at a depth of 660 km, directly at the interface between the transition zone and the lower mantle, where the dominant mineral is ringwoodite. Diamonds from this location are very rare, even among the extremely rare diamonds of ultra-deep origin, which are only 1% of all diamonds. The study showed that the rock had a high water content due to the presence of many ringwoodite inclusions. The study team was also able to establish the chemical composition of the rock.

It was similar to virtually every fragment of mantle rock found in basalt anywhere in the world. This shows that the diamond must have come from a common part of the Earth’s mantle. “In this study, we have shown that the transition zone is not a dry sponge, but holds a considerable amount of water,” said Brenker, adding: “This brings us one step closer to Jules Verne’s idea of ​​an ocean inside the Earth.” The difference is which ocean is here. No, but water rocks, which according to Brenker, won’t feel wet or dripping water.

Hydrous ringwoodite was first identified in a diamond from the transition zone in early 2014. Brenker was also involved in that study. However, as the stone is too small, its exact chemical composition could not be determined. Thus, it remains unclear how representative the first study was of the mantle in general, as the water content of that diamond could also result from an exotic chemical environment. In contrast, the 1.5-centimeter (0.6 inch) diamond inclusion from Botswana, which the research team investigated in the current study, was large enough to allow the exact chemical composition to be determined, and it provided final confirmation of the preliminary results. From 2014.

The high water content of the transition zone has far-reaching consequences for dynamic conditions in Earth’s interior. What this leads to is seen, for example, in hot mantle plumes coming from below, which get stuck in the transition zone. There, they heat the water-rich transition zone, leading to the formation of new small mantle plumes that absorb water stored in the transition zone.

If these small water-rich mantle plumes now migrate upward and break through the boundary into the upper mantle, the following occurs: Water within the mantle plumes is released, lowering the melting point of the emerging material. Therefore, it melts immediately and not before it reaches the surface, as usually happens. As a result, the rock masses in this part of the Earth’s mantle are no longer solid overall, giving mass movement more mobility. The transition zone, which otherwise acts as a barrier to mobility there, suddenly becomes a driver of global material circulation.

References: Tingting Gu, Martha G. Pamato, David Novella, Matteo Alvaro, John Fornell, Frank E. “Sampling of hydrous peridotitic fragments of the Earth’s mantle at 660 km separation” by Brenker, Wuyi Wang and Fabrizio Nestola, September 26 DOI: 10.1038/s41561-022-01024-y