Peridot-Like Impurity Trapped in ‘Ultra-Deep’ Diamond Signals the Presence of a Vast Water Reserve 300 Miles Below the Earth’s Surface

March 17th, 2014

A microscopic peridot-like impurity trapped under high pressure in a Brazilian “ultra-deep” diamond signals the presence of a vast water reserve about 320 miles below the Earth’s surface, according to a report published in Nature.

Scientists at the University of Alberta reported that the microscopic crystal of ringwoodite, a green mineral with a chemical composition similar to that of the August birthstone peridot, contained 1.5% water. With ringwoodite comprising much of the Earth’s “transition zone,” the scientists theorized that the reservoir could be immense.

“When you realize how much ringwoodite there is, the transition zone could hold as much water as all the Earth’s oceans put together,” said Graham Pearson, a mantle geochemist at the University of Alberta and the lead author of the study.

Ringwoodite had never been found on the Earth’s surface because it is only exists under immense pressure in the Earth's "transition zone," an area sandwiched between the upper mantle and lower mantle. In the case of the Brazilian diamond, the ringwoodite was trapped within its super-strong host, allowing it to maintain its original high-pressure form. At lower pressure, the material is called olivine. Peridot is gem-quality olivine.

Before this discovery, the presence ringwoodite in the Earth’s mantle was theoretical. The material had been seen in meteorites and was synthetically produced in laboratories.

According to Nature.com, most diamonds form at depths of about 100 miles and are propelled to the surface by volcanic eruptions through kimberlite pipes. The diamond containing the ringwoodite is considered an “ultra-deep” diamond because it formed in the Earth’s “transition zone,” more than 300 miles below the surface.

“These high-pressure diamonds give you a window into the deep Earth,” Pearson said.

Pearson’s diamond specimen is no beauty compared to the faceted gems in a jeweler’s showcase. It looks battered from its 300-plus mile journey to the surface. It has a silvery-brown color and is 5mm wide. The trapped ringwoodite grain, by comparison, measures a mere .04mm across.

Ringwoodite was named after the Australian earth scientist Ted Ringwood, who was a pioneer in studying olivine and other mantle materials at high pressures.

Photos: RICHARD SIEMENS/UNIVERSITY OF ALBERTA