Botswana Diamond Proves Transition Zone Between Earth Mantles Has Water Reserves, Not a Dry Sponge

It was previously believed that the Earth's transition zone was a dry sponge, but a recent study has revealed that there are water reserves there. More information about this region between the mantles can be found by looking at a Botswana diamond.

The transition zone (TZ) is the layer that separates the Earth's lower and upper mantle. It is between 255 to 410 miles below the surface.

Peridot and Ringwoodite

Around 70% of the Earth's upper mantle is made up of the olivine mineral, also known as peridot, which is olive-green in color. Under extreme pressure that can reach 23,000 bar in the TZ, olivine undergoes a crystalline structure change. It transforms into denser wadsleyite at a depth of about 410 kilometers, at the top portion of the transition zone, and into even denser ringwoodite at a depth of 520 kilometers.

Professor Frank Brenker from Frankfurt's Goethe University-Institute for Geosciences said that the movement of rock in the mantle is significantly hampered by these mineral transformations.

As an illustration, mantle plumes, which are rising columns of hot rock out from deep mantle, occasionally come to an abrupt stop just below the transition zone. Mass that is moving in the opposite direction also stops moving. According to Brenker, subducting plates frequently struggle to penetrate the whole transition zone. This means there is a sizable cemetery of subducting plates as a result in this area beneath Europe.

Piggybacking Into the Earth

However, up until this point, it was unclear what long-term effects the "sucking" of particles into the transition zone would have on its geochemical makeup and whether there would be more water present.

According to Brenker, the subducting slabs also piggyback deep-sea sediments into the Earth's interior. Large amounts of Carbon dioxide and water can be stored in these sediments, but it has been difficult to determine whether or not there is significant water storage in that region because it has been unclear exactly how much entered the transition zone in a more stable form like hydrous minerals and carbonates.

There is no doubt that the current situation favors this. The transition zone could theoretically absorb six times as much water as our oceans due to the thick minerals wadsleyite and ringwoodite's ability to hold large amounts of water, unlike peridot at lower depths. Brenker says that although the boundary layer has a huge capacity to hold water, it was unclear whether it actually did so.

Botswana Diamond, Ocean Inside the Earth

A recent international study that examined a Botswana diamond from Africa has revealed the solution. It was created at a depth of 660 kilometers, right at the boundary between the lower mantle and the transition zone, where ringwoodite is the dominant mineral.

Even among the incredibly rare diamonds of remarkably deep-earth origin, which make up just 1% of all diamonds on earth, diamonds originating from this location are extremely uncommon. Due to the abundance of ringwoodite inclusions, the studies determined that the stone carries a high water content. The research team was also able to determine the stone's chemical makeup.

It was nearly identical to every piece of mantle rock found in basalts throughout the world. This demonstrated that the diamond unquestionably originated from a typical component of the Earth's mantle.

According to Brenker, their study, published in Nature, has shown that the transition zone, TZ, is not a dry sponge but rather holds a significant amount of water. Additionally, it advances our understanding of Jules Verne's theory that the Earth contains an ocean. The deviation is that instead of an ocean, Brenker claimed that there is hydrous rock, which neither feels wet nor drips water.

Hydrous Rock Minerals

In a diamond originating from the transition zone, hydrous ringwoodite was discovered for the first time in 2014, according to Scientific American. In that investigation, Brenker also took part. However, due to the stone's small size, it was impossible to pinpoint its precise chemical makeup.

As a result, it was unclear how representative the initial study was of the mantle as a whole because the diamond's water content could have also been a product of an unusual chemical environment. Contrarily, the inclusions in the 1.5 cm Botswana diamond that the research team examined in the current study were sufficiently large for the precise chemical composition to be defined, and this provided definitive proof of the preliminary results from 2014.

The high water content of the transition zone has significant effects on the Earth's dynamic environment. Hot mantle plumes rising from below that become trapped in the transition zone are one example of what this results in. There, the water-rich transition zone is heated, which causes the emergence of new, smaller mantle plumes that then take up the water that has been stored in the transition zone.

The water in the mantle plumes is forced to release, which decreases the melting point of the emerging material if these narrower water-rich mantle plumes migrate further upward and cross the limit to the upper mantle.

As a result, it melts right away rather than just until it reaches the surface, as is typical. The mass movements are more dynamic as a result of the decreased overall toughness of the rock masses in this region of the Earth's mantle. The transition zone, which usually serves as a barrier to the dynamics in the area, unexpectedly becomes a major force in the circulation of materials around the world, Sci-Tech Daily reports.