The transition zone (TZ) is the boundary layer that separates the upper mantle and lower mantle of the Earth. It is located between 410 and 660 kilometers deep. The immense pressure of up to 23,000 bar in the TZ changes the crystalline structure of the olive-green mineral olivine, which accounts for roughly 70% of the upper mantle and is also known as peridot.At the top of the transition zone, at a depth of about 410 kilometers, it changes into the denser wadsleyite. At a depth of 520 kilometers, it changes into the denser ringwoodite.
As Prof. Frank Brenker from the Institute of Geosciences at eart in Frankfurt explains, “These mineral transformations greatly impede the movement of rock in the mantle.” For instance, mantle plumes — rising columns of hot rock from the mantle’s depths — occasionally terminate directly below the transition zone. Also halted is the movement of mass in the opposite direction. Brenker says, “Subducting plates frequently struggle to penetrate the entire transition zone. Therefore, there is an entire graveyard of such plates beneath Europe.
Prior to this discovery, it was unknown what the long-term effects of “sucking” material into the transition zone were on its geochemical composition or whether there were greater quantities of water there. Brenker explains: “The subducting slabs also transport deep-sea sediments into the interior of the planet. These sediments can store substantial quantities of water and carbon dioxide. Until now, however, it was unknown how much water enters the transition zone in the form of more stable hydrous minerals and carbonates, and it was also unknown whether significant quantities of water are actually stored there.
Certainly, the prevailing conditions would be conducive to that. The dense minerals wadsleyite and ringwoodite can (unlike the olivine at shallower depths) store enormous amounts of water; in fact, the transition zone could theoretically absorb six times the amount of water in the world’s oceans. Brenker states, “Therefore, we know that the boundary layer has an enormous capacity for water storage.” However, we did not know if this was the case.
An international study in which the geoscientist from Frankfurt participated has now provided the answer. The research team examined an African diamond from Botswana. It was formed at a depth of 660 kilometers at the interface between the transition zone and the lower mantle, where ringwoodite predominates. Even among the extremely rare super-deep diamonds, which account for only 1% of all diamonds, diamonds from this region are extremely rare.The analyses revealed that the stone contains numerous inclusions of ringwoodite, which have a high proportion of water. In addition, the research team was able to determine the stone’s chemical composition. It was virtually identical to virtually every fragment of mantle rock found in basalts across the globe. This demonstrated that the diamond originated from a typical portion of the Earth’s mantle. “In this study, we demonstrated that the transition zone is not a dry sponge but instead contains significant amounts of water,” Brenker says, adding, “This brings us one step closer to Jules Verne’s concept of an ocean within the Earth.” There is no ocean there, only hydrous rock, which, according to Brenker, would neither feel wet nor drip water.
In 2014, hydrous ringwoodite was first identified in a diamond from the transition zone. Brenker also participated in this study. Due to the stone’s diminutive size, it was not possible to determine its precise chemical composition. As a result, it remained unclear how representative the initial study was of the mantle as a whole, as the water content of that diamond could have been the result of an exotic chemical environment. The inclusions in the 1.5-centimeter diamond from Botswana, which was the subject of the current study, were large enough to enable the precise chemical composition to be determined, confirming the preliminary findings from 2014.
The high water content of the transition zone has far-reaching effects on the dynamic situation within the Earth. This can be seen, for instance, in the hot mantle plumes that rise from below and become trapped in the transition zone. There, they warm the water-rich transition zone, resulting in the formation of new, smaller mantle plumes that absorb the water stored in the zone. The following occurs if these smaller water-rich mantle plumes migrate further upwards and breach the upper mantle boundary: The release of water from the mantle plumes lowers the melting point of the emerging material. As a result, it melts immediately, as opposed to just before it reaches the surface, as is typically the case. As a result, the rock masses in this region of the Earth’s mantle are not as brittle, resulting in more dynamic mass movements. The transition zone, which usually slows down the movement of things in the region, becomes an unexpected driver of the flow of materials around the world.
