Diamonds reveal water in deep mantle

by Rachel Crowell
Tuesday, June 5, 2018

A new study analyzing the structural and chemical properties of diamonds from various locations in Africa and China found inclusions of ice-VII, a form of crystallized water created under very high pressure that has never before been found in nature. Credit: Marilyn Chung/Berkeley Lab.

Inclusions in diamonds often render them undesirable to consumers, but they can provide researchers with striking insights into Earth’s composition. Recently, scientists probing diamond samples for the presence of carbon dioxide stumbled instead upon inclusions of ice-VII — a type of crystallized water that forms at very high pressures, and has never before been found in nature.

The discovery of the icy inclusions, reported in Science, could provide new insights into water in the deep mantle and its relationship to the global water supply. “Water-rich regions in Earth’s deeper mantle are suspected to play a role in the global water budget,” wrote Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas, and his colleagues in the study. Ice-VII “points toward fluid-rich locations in the upper transition zone [at a depth of about 410 kilometers] and around the 660-kilometer boundary.”

“This study reveals something unusual,” says Evan Smith, a diamond geologist at the Gemological Institute of America, who wasn’t involved with the study. “We have seen a variety of different aqueous diamond-forming fluids trapped in lithospheric diamonds, but not in superdeep diamonds” — those that originate deeper than about 200 kilometers.

“Any evidence of water deep inside Earth, in the convecting mantle, is an important finding,” Smith adds. “It is still unclear how much water is stored in the mantle, what its role is, and how it might relate to water at the surface.”

Tschauner’s team used hard X-ray diffraction and other techniques, including infrared analysis, to examine the structure and chemistry of diamonds from Africa and China and found inclusions of ice-VII in 13 of them. The discovery demonstrates that the inclusions aren’t isolated to diamonds formed in only one region, and that deep diamonds may be more common than previously thought, Tschauner said in a statement. Until now, only about 60 diamonds originating from depths greater than about 300 kilometers had been found.

However, Smith notes that “compared to other superdeep diamonds, these ice-VII-containing diamonds do not seem to have the other typical inclusions representing high-pressure minerals like majoritic garnet. So, for now, the ‘superdeep' origin of the ice-VII-containing diamonds is based entirely on pressure calculations from X-ray diffraction data,” he says.

Ice-VII inclusions remain at high pressures within their host diamonds, allowing researchers to use knowledge of ice-VII’s structure to determine the minimum pressures at which the diamonds formed. Chemical analysis, specifically the state of nitrogen inside the diamonds, revealed information about temperatures, which, along with pressure, was used by researchers to determine the depths at which the diamonds formed.

“Of 13 occurrences of ice-VII, eight fall into a narrow pressure interval from 8 to 12 gigapascals,” with one other sample having formed at a pressure of about 6 gigapascals, and two more forming at about 24 to 25 gigapascals, they wrote.

These observations imply the diamonds were formed in “regions in the deep mantle where aqueous fluid was present during [their] growth,” they add. The diamonds with the greatest retained pressures are thought to have formed up to 800 kilometers below Earth’s surface.

Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C., who studies deep diamonds but wasn’t involved with the study, says he finds the research “quite novel, because preservation of such high residual pressure is unexpected.”

However, Shirey says the diamonds in the study “are rather poorly characterized and difficult to understand. One is from a locality not known to carry such superdeep diamonds.” In others, he says, the nitrogen chemistry is “hard to explain.”

“It is not immediately clear how to connect these findings to other diamonds and other studies to date,” Smith says. The study “has people scratching their heads,” but it will prompt others to attempt to replicate and build on these findings.

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