Don't be disgusted by diamond impurities! Maybe they can reveal the source of water deep in the earth

Rare diamonds suggest water is deeper inside Earth than scientists thought

Minerals in diamonds include ringwoodite, enstatite and ferropericrystalline. (Image credit: Nathan D. Renfro and Tingting Gu)

A rare type of diamond suggests that water may be able to penetrate deeper into the Earth's interior than scientists previously thought.

Earth's surface is already 70 percent covered in water, and more than 200 miles (322 kilometers) below the surface there's water in minerals, including in the mantle, a semi-plastic layer that floats above the crust. Scientists have long believed that the amount of water in minerals decreases dramatically as the upper mantle transitions to the hotter, denser lower mantle.

However, in a new study published in the journal Nature Geoscience on September 26, 2022, researchers found inclusions of suspected tiny fragments of other minerals in a type of diamond. This diamond may exist at the boundary between the upper and lower mantle and can hold more water. The result suggests that the water on Earth may be deeper than scientists thought, which may affect our understanding of deep water circulation and plate tectonics. They can hold more water and appear to exist at the boundary between the upper and lower mantle. The result suggests that the water on Earth may be deeper than scientists thought, which may affect our understanding of deep water circulation and plate tectonics.

The results were unexpected, says lead author Tingting Gu, who worked on the study while a fellow at the Gemological Society of New York City and is now a mineral physicist at Purdue University in Indiana.

Gu and her colleagues examined Type IaB diamonds, which are extremely rare and come from the Karowe mine in Botswana. They form deep underground and remain hidden for a long time. To study the diamonds, they used "non-destructive" forms of analysis, including Raman microspectroscopy, which uses lasers to reveal the physical properties of a material non-invasively, and X-ray diffraction, which observes the internal structure of the diamond without cutting it open.

Gu said that among the inclusions in the diamonds, the researchers found a mineral called ringwoodite, which has the same chemical composition as olivine and is the main material of the upper mantle, but only forms under extreme temperatures and pressures. Scientists had never found it in meteorite samples until 2014. The study authors noted that ringwoodite is usually found in the transition zone between the upper and lower mantle, about 255 to 410 miles (410 to 660 kilometers) below the Earth's surface.

Bridgmanite and ferropericlase are the main minerals in the lower mantle, while the newly discovered ringwoodite has a much higher water content than them.

However, the minerals surrounding ringwoodite are not typical of the transition zone, but rather typical of the lower mantle. Because the minerals surrounding ringwoodite retain the properties of the minerals deep within the Earth, the researchers can trace back the temperatures and pressures to which the minerals were subjected; they estimate the depth of the minerals to be about 410 miles (660 kilometers) below the surface, close to the edge of this transition zone. The analysis further showed that in a hydrated or water-saturated environment, ringwoodite could be in the process of breaking down into minerals more similar to those in the lower mantle, a finding that suggests water could be percolating from the transition zone all the way into the lower mantle.

The inclusions in this 1.5-carat diamond are evidence of minerals that formed in the lower mantle. (Image credit: Tingting Gu)

The research team noted that although previous studies have found some mineral forms from the lower mantle in diamond inclusions, the combination of materials in this inclusion is unique. The researchers said that based on previous findings, they cannot yet determine whether these minerals indicate the presence of hydrous minerals in the lower mantle. Because no one has directly sampled rocks about 7 miles (11 kilometers) below the Earth's surface, the current discovery is one of the few mineral sources from the Earth's mantle.

Gu Tingting said the results could have important implications for understanding the deep water cycle, or the circulation of water between the Earth's surface and the deep interior.

Gu Tingting also pointed out: "If water can be stored deeper, the time scale of water circulation will actually be longer." This means that if water is stored deep underground, it will take longer to be renewed.

The findings could also affect models of plate tectonics. Gu said she hopes scientists will be able to incorporate the study's findings into models of how water in the mantle affects processes such as convection currents in Earth's interior. Such currents heat the Earth's mantle unevenly, causing the hotter parts to rise and move Earth's plates over millions of years.

While inclusions are sometimes viewed as flaws that make a diamond less than desirable, Gu believes they can provide valuable scientific information.

“If there are inclusions in the diamond you buy, don’t be nervous, you never know what it could be,” Gu said.

BY:Rebecca Sohn

FY: Continent

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