Ultra-Rare Diamond Reveals Secrets of Oceans of Water Deep Inside the Earth, Scientists Say

More water than all of Earth's oceans is thought to reside deep inside the mantle, and a newly-discovered gem just shed more light on the mystery.
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Image: Tingting Gu
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An extremely rare diamond that formed more than 400 miles under Earth’s surface has revealed that the deep layer of our planet known as the mantle may be more saturated with water than previously known, reports a new study. The results provide an unprecedented glimpse of the movement of water through the still-mysterious mantle, which has implications for understanding how Earth, and perhaps other planets, became habitable to life.

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Scientists led by Tingting Gu, a mineral physicist at Purdue University, studied a diamond found at Karowe mine in Botswana that originates from the lower mantle, a region that extends from 400 miles to 1,800 miles under Earth’s surface, where it meets the planet’s core. Gu first came across the diamond while at the Gemological Institute of America, when a colleague pointed out it might contain tiny pockets of material, known as inclusions.

Using non-invasive spectroscopic techniques, Gu realized that the inclusions contain material from the lower mantle, which survived the long journey up through what’s known as the transition zone to the upper mantle and ultimately to the surface of the planet. The minerals in the inclusions indicate that the gem formed “in a water-saturated environment” which means that “hydrous conditions extend at least across the transition zone and into the lower mantle,” which is deeper than previously known, according to a study published on Monday in Nature Geoscience.

“The diamond in this study is a very unique type,” said Gu in a call, noting it belongs to a class called type IaB that is distinguished by idiosyncratic arrangements of nitrogen. “The diamond was probably sitting in the mantle for a long while, and probably from the deep region.”

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This is not the first type IaB diamond that has surfaced from the planetary depths, but its combination of minerals—ferropericlase, enstatite, and ringwoodite—have never been seen before in one of these mantle gems. Ringwoodite is able to absorb water, suggesting that hydrous conditions may extend past the so-called 660-kilometer boundary, or discontinuity, where the transition zone ends and the lower mantle begins.

“The occurrence of ringwoodite together with the hydrous phases indicate a wet environment at this boundary,” the team said in the study. “Although the formation of upper-mantle diamonds is often associated with the presence of fluids, super-deep diamonds with similar retrogressed mineral assemblages rarely have been observed accompanied with hydrous minerals.”

In other words, the new diamond has shown that water continues to persist into the lower mantle, a finding that will help scientists reconstruct the enigmatic dynamics of this shadowy region. The new study also builds on past research that has probed these hidden reservoirs of water, which may be comparable in volume to all of the water found in Earth’s oceans. 

Gems like this diamond, that originate far underground, hold many of the answers to the question of how life came to arise on Earth and whether it might exist elsewhere. Water is the key ingredient for life as we know it, which is why scientists involved in the search for alien life are especially focused on identifying planets where liquid water might exist. 

“I think water is quite an important discovery in this study,” Gu said. “The hydrous environment very deep inside the Earth is actually very critical for the origin of life and why Earth is habitable. We need to understand where that water comes from.” 

Gu noted that it’s currently unclear whether the water located deep in the mantle is an ancient reservoir supplied by asteroid and comet strikes early in the solar system, or if water cycles deep into the mantle from higher layers. Both options are “amazing” in the context of understanding life and habitability, she said, because they would reveal new insights about the history and future of water on Earth. 

Update: This article has been updated to include comments from lead author Tingting Gu.