Geochemistry, Mineralogy, Volcanology News

Diamond Impurities Reveal Water Deep Within the Mantle

A high-pressure form of ice, trapped within diamonds forged in the lower mantle, suggests that aqueous fluids reside deeper in Earth than we knew.

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New research has discovered water-rich material from hundreds of kilometers beneath Earth’s surface, in the form of a high-pressure variety of water ice suspended within deep-mantle diamonds. Scientists found that the ice is likely a residue of a more chemically complex fluid that lives in the upper and lower layers of the mantle and influences geological processes.

“The ice is important because it tells us something about the presence of water in the great depths of the Earth,” said Oliver Tschauner, professor of geoscience at the University of Nevada, Las Vegas, and lead author on the study. “This has implications for the Earth’s water budget and also for the mobility of chemical elements that dissolve preferentially in aqueous fluids, like heat-generating elements,” including potassium, thorium, uranium, and others.

“The extent of heat-generating chemicals at certain depths in the Earth is an important parameter in geodynamical models,” Tschauner noted.

He and his team examined deep-mantle diamonds from around the world to measure the chemical makeup of their inclusions. To their surprise, they found that some of the inclusions contained ice VII, a mineral that forms at very high pressures. The ice’s crystal structure more precisely revealed the pressures at which the inclusions formed, and from that the researchers learned that the diamond inclusions grew 150–800 kilometers beneath Earth’s surface, spanning the upper mantle and shallow lower mantle.

These diamonds contain the first direct evidence that a water-rich fluid lives in the shallow lower mantle and may significantly contribute to Earth’s overall water content, according to the team.

Frozen in Diamonds

Although jewelers typically consider diamonds with flaws like inclusions to be lower quality, those inclusions can hold significant scientific value. In addition to trapping material within a diamond’s rigid structure, inclusions can record the conditions in which the diamonds formed, giving scientists a window into the environment deep within Earth.

“Diamond is a remarkable vessel for sampling the geochemistry of the deep mantle,” said Steven Jacobsen, “because of its ability to seal off trapped inclusions from the reactive environment during ascent, like a tiny indestructible spaceship.” Jacobsen, a mineral physicist at Northwestern University in Evanston, Ill., was not involved with this research.

Tschauner and his team measured the X-ray diffraction and X-ray fluorescence patterns of dozens of inclusions within 10 diamonds from Botswana, South Africa, China, Sierra Leone, and Zaire. They were trying to determine what lay trapped within the diamonds but were not looking specifically for evidence of water ice.

Oliver Tschauner holds a small piece of one of the diamond samples tested during the experiments
Oliver Tschauner holds a small piece of one of the diamond samples tested during the experiments that used the Advanced Light Source at Lawrence Berkeley National Laboratory, Berkeley, Calif. Credit: Marilyn Chung/Berkeley Lab

They found that four of the diamonds (the samples from China, South Africa, and Botswana) contained inclusions with ice VII, a phase of water that is stable at pressures exceeding 2.4 gigapascals.

The presence of a dominant ice phase surprised the team. “The topic that we focused on originally was carbon dioxide,” Tschauner said. “But during our search for carbon dioxide we found the ice VII inclusions, which we did not expect.”

Deep Fluid Deposits

Here’s the scenario that the researchers pieced together: Deep within Earth are regions with a complex water-rich fluid. The still-forming diamonds enclosed droplets of that fluid and trapped them in pockets a few micrometers wide.

The water component of the fluid, Tschauner explained, would be a liquid at the searing temperatures and enormous pressures—from around 870°C to 2,200°C and from 1 to 30 gigapascals, respectively—found in that region of the mantle.

As the diamonds rose toward the surface, he continued, temperatures within the inclusions dropped, but the pressures held constant. At the still enormous pressure and newly lower temperature, the liquid water crystalized into ice VII. Tiny variations in samples’ x-ray and infrared signatures revealed the pressure within the inclusions and at which the diamonds originally formed. By comparing those pressures to current models of Earth’s mantle, the researchers constructed a rough map of where the diamonds, and also the water-rich fluid, came from.

Of the 13 inclusions that contained ice VII, the team found that most formed at pressures consistent with the deep upper mantle, about 400–550 kilometers beneath the surface. Two of the ice VII inclusions, however, suggested pressures more consistent with the shallow lower mantle, 610–800 kilometers deep.

Although scientists have previously discovered shallower diamonds with water inclusions, these are the first to come from such depth. The researchers published these results in Science on 9 March.

These findings from Earth’s depths have consequences for people and other organisms that rely on water to live. “It seems likely the water molecules in the air we breathe and in the water we drink have been recycled through the mantle at least once over geologic time,” Jacobsen said. “The discovery forces us to rethink how the geochemical lines of communication are open between the Earth’s surface and deepest mantle.”

More Diamonds Needed

Given that he had only 10 diamonds to work with at the time, Tschauner said that the next step for his team will be scrutinizing more deep-mantle diamonds for evidence of ice inclusions.

“We definitely want to more systematically sample different regions on Earth where the diamonds occur,” said Tschauner. “Some of the diamonds that we looked at were not from locations that are known to carry deep diamonds.…It might be that there are many more diamonds from great depths at many more locations than previously thought.”

With more diamonds, the team hopes to get a better idea of how much of the aqueous fluid exists beneath the transition zone. That puzzle piece, Tschauner explained, will help scientists hone in on Earth’s total water budget and mantle composition.

“What other inclusions from great depths can we find?” he asked. “We want to eventually reconstruct the composition of the fluid.”

—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern

 

Citation: Cartier, K. M. S. (2018), Diamond impurities reveal water deep within the mantle, Eos, 99, https://doi.org/10.1029/2018EO095949. Published on 02 April 2018.
© 2018. The authors. CC BY-NC-ND 3.0