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A pea-sized diamond picked up a decade ago by a Brazilian prospector has unlocked evidence that, hundreds of kilometres under our feet, Earth’s mantle holds as much water as all of our planet’s oceans.
The discovery by a University of Alberta team bolsters theories about the existence of a water-saturated zone between the Earth’s rocky layers that would explain volcanic activities and the interaction of tectonic plates.
The diamond’s route from the alluvial plains of Brazil’s Mato Grosso province to the University of Alberta’s labs is a tale blending an exotic setting with reminders of Jules Verne’s classic science fantasy of a sea in the centre of Earth. The research, published this week in the journal Nature, confirms the terrestrial existence of ringwoodite, a high-pressure form of a common silicate.
What was most striking about the ringwoodite discovery, by a team led by University of Alberta professor Graham Pearson, was that 1.5 per cent of it was water, bound chemically to the mineral. Based on projections of how much ringwoodite scientists believe is in the Transition Zone, between 410 and 670 kilometres down, Dr. Pearson estimated that it contains “a very, very large amount of water.”
“The amount of water is possibly up to all the water contained in all the world’s oceans,” he said.
The tiny ringwoodite sample, too small to be visible to the human eye, was trapped in the diamond, which itself was encased in kimberlite, a volcanic rock ejected to the Earth’s surface during the Cretaceous period, 90 million years ago, when dinosaurs roamed the planet.
Erosion exposed the gem amid river gravel in a tributary of the Rio Aripuana, and a garimpeiro, an artisanal miner, sieved it out and sold it to a diamond geologist a decade ago.
Ringwoodite is a high-pressure form of a common silicate mineral, olivine, also known as peridot. Until now, it had been found only in shattered remains of meteorites and the mathematical models of scientists parsing seismic sensor results.
“It’s not stable at the Earth’s surface. It’s only when trapped within a diamond and brought to Earth quickly that you see it. This is the first time it’s been seen,” said Dr. Pearson, who is the University of Alberta’s Canada Excellence Research Chair in Arctic Resources.
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