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Physical Chemistry

Ringwoodite Paints Watery Picture Of Earth’s Mantle

Discovery of mineral predicted to exist several hundred miles below Earth’s surface validates long-held geological theory

by Elizabeth K. Wilson
March 17, 2014 | A version of this story appeared in Volume 92, Issue 11

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Credit: Richard Siemens/U of Alberta
A diamond sample contains a tiny inclusion of hydrous ringwoodite. The 40-µm inclusion is not visible at this scale.
Rough diamond with a ringwoodite inclusion inside.
Credit: Richard Siemens/U of Alberta
A diamond sample contains a tiny inclusion of hydrous ringwoodite. The 40-µm inclusion is not visible at this scale.

Geologists have long had good circumstantial evidence that a great cache of water lies where Earth’s upper and lower mantle meet, about 500 km below the surface. Scientists hypothesized that at these depths a mineral existed known as ringwoodite, which can contain large amounts of water at high pressures. But even though models predict ringwoodite should exist in this region, and samples of ringwoodite have been found in meteorites, a terrestrial example of the mineral has never been found. Now, Graham Pearson of the University of Alberta and colleagues have reported in Nature the discovery of a tiny sample of ringwoodite encased in a diamond that made its way to Earth’s surface from the mantle (2014, DOI: 10.1038/nature13080). The sample contains about 1% water, which, if representative of conditions at the meeting of the upper and lower mantle, would translate into as much water there as is contained in all of Earth’s oceans. “Most people (including me) never expected to see such a sample,” says Hans Keppler of Bayreuth University, in Germany, in a perspective accompanying the paper. “This is an amazing finding.”

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