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Flaw diamond reveals existence of sixth ocean in Earth’s mantle?

It’s a scientists’ delight to explore the environment where diamonds were formed millennia earlier

By Kiran N. Kumar

A rare 1.5-carat diamond from Earth’s mantle containing a beautiful blue flaw called inclusion suggests that the mantle of our planet contains another ocean, the sixth one waiting to be explored.

The flaw resembling a fish eye in deep blue surrounded by a white haze is a mineral ringwoodite from 660 kilometers down, between the upper and lower mantle.

Read: Rare ‘folded diamonds’ discovered, probably from another planet (September 14, 2022)

A similar diamond found earlier was destroyed in the scientists’ hands while trying to analyze it. The second diamond is the only one of its kind now.

The rare diamond from the Karowe mine in Botswana, discovered by Tingting Gu, a mineral physicist now at Purdue University, is a delight for scientists to explore the environment where diamonds were formed millennia earlier, though they are undesirable for the jewelry industry as they eclipse a diamond’s sparkle.

The discovery of the diamond from lower-mantle minerals – ringwoodite + ferropericlase + low-Ni enstatite (MgSiO3) in a polyphase inclusion, together with other principal lower-mantle minerals and hydrous phases, enabled scientists to place its origin at ~23.5 GPa and ~1,650 °C temperature, corresponding to the depth at the 660 km.

The petrological character of the inclusions or flaws in the rare diamond indicates that ringwoodite breaks down into bridgmanite and ferropericlase in a water-saturated environment at the 660 km and reveals that the peridotitic composition and hydrous conditions extend at least across the transition zone and into the lower mantle, explained the authors of a paper published in Nature.

“It is incredibly rare to even have a super deep diamond, and then to have inclusions is even rarer,” says Suzette Timmerman, a mantle geochemist and postdoctoral fellow at the University of Alberta, who was not involved in the new discovery. Finding a ringwoodite inclusion is even more mind-boggling, she says.

The dense mineral ringwoodite can store large quantities of water, so large that the transition zone would theoretically be able to absorb six times the amount of water in our oceans.

“So we knew that the boundary layer has an enormous capacity for storing water,” says Prof. Frank Brenker from the Institute for Geosciences at Goethe University in Frankfurt.

The global study in which the Frankfurt geoscientist was involved, analyzed the diamond that was formed at a depth of 660 km, at the interface between the transition zone and the lower mantle, where ringwoodite prevails.

Diamonds from this zone of super-deep origin account for only one percent of diamonds and the stone contained numerous ringwoodite inclusions – which exhibit a high water content, said the research finding.

Read: Rare diamond suggests presence of sixth ocean below Earth’s crust (October 4, 2022)

Chemical composition  

After determining its chemical composition, it was found almost exactly the same as that of any other diamond found in basalts anywhere in the world, reiterating the fact that its origin is definitely from the Earth’s mantle.

A similar diamond found in 2014 was believed to have formed within 135 km of depth. The new rare diamond also had two other minerals in the new inclusion, ferropericlase and enstatite, which can  occur together at 660 km and deeper, pinpointing the layer of the mantle where the diamond was formed.

If the 2014 ringwoodite discovery was the first hint, the second discovery has confirmed it now, says Timmerman.

“If you only have one sample, it could just be a local hydrous region,” she says, “whereas now that we have the second sample, we can already tell it’s not just a single occurrence. It’s likely to be widespread.”

Now researchers are keen to figure out where this water comes from.

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