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Diamond Traps Rare Mineral, Brings to the Surface Tantalizing Clues from the Deep

ByAllison EckNOVA NextNOVA Next
This diamond contains a message from the depths of the Earth.

A Brazilian diamond weighing less than one-tenth of a gram, forged hundreds of miles below the surface of the Earth, is acting as an emissary from the depths of our planet.

And it’s bringing us news from the deep—there might be enormous amounts of water hidden down there, far beneath our feet.

Graham Pearson, a mantle geochemist at the University of Alberta, led a team of researchers who analyzed the impurities trapped in this “ultra-deep” diamond. They wanted to see what minerals populate the transition zone between the upper and lower layers of Earth’s mantle, some 250 to 400 miles deep. Typically, as rocks make their way to the Earth’s surface, they lose their original crystalline structure—as pressure on the minerals decreases, they begin to rearrange themselves. This means that the stories they tell of their home environment are lost.

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But when this diamond cooled, it locked the structure of the minerals inside. Diamonds are so strong that they can keep the impurity under immense pressure, like a capped soda bottle holding in its carbonation. The diamond then made its way to the surface, bringing its cargo with it. This diamond is a courier of knowledge—mineral information encapsulated in precious stone.

And it had a lot to say. Inside, Pearson and his colleagues discovered ringwoodite—a high-pressure form of the mineral olivine. Ringwoodite had previously been found only in meteorites or created artificially in the lab. Here’s Richard A. Lovett, writing for Nature :

Unlike better-studied forms of olivine, ringwoodite can hold a substantial amount of water. The sample therefore had the potential to help resolve a long-standing controversy over just how much water the transition zone contains. Using infrared spectroscopy, Pearson’s team found that its tiny fleck of ringwoodite contained about 1% water by weight. “That may not sound like much,” Pearson says, “but when you realize how much ringwoodite there is, the transition zone could hold as much water as all the Earth’s oceans put together.”

Other scientists aren’t jumping to conclusions. They say that a single sample of ringwoodite can’t speak for all of Earth’s interior, and that the water would be in a different molecular form than liquid water. Nonetheless, the finding also opens up questions about where water came from:

If the water has been there since Earth formed, its ratio of deuterium to normal hydrogen could be different from that found in sea water today—and closer to the composition of the Earth’s primordial water. If so, that ratio could provide clues as to whether the water came from asteroids or from comets, says Humberto Campins, an asteroid researcher at the University of Central Florida in Orlando.

In other words, not only could there be oceans’ worth of water down there, it could be trillions of gallons that were around when the Earth was just a watery orb. Pearson is hesitant to crack the mineral to find out—it’s only 40 micrometers across—but geologists may stumble upon another courier from the deep that contains clues about our planet’s aqueous early days.

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Photo Credit: Richard Siemens, University of Alberta

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