Researcher Graham Pearson holds a diamond within which scientists have found the first confirmation of the terrestrial existence of ringwoodite. The discovery is proof that, hundreds of kilometres under our feet, the Earth’s mantle holds as much water as all of our planet’s oceans. Dr. Pearson is Canada Excellence Research Chair in Arctic Resources at the University of Alberta. (Richard Siemens)
How a tiny diamond implies oceans of water hidden deep inside the Earth Add to ...
TU THANH HA
The Globe and Mail
Published Wednesday, Mar. 12 2014, 4:05 PM EDT
A pea-sized diamond picked up a decade ago by a Brazilian prospector has allowed an Alberta-led research team to present proof that, hundreds of kilometres under our feet, the Earth’s mantle holds as much water as all of our planet’s oceans.
The research, published this week in the journal Nature, is the first confirmation of the terrestrial existence of ringwoodite, a high-pressure form of a common silicate mineral.
Until now, ringwoodite had only been found in shattered remains of meteorites or in the mathematical models of geologists parsing seismic sensor results.
The discovery bolsters theories about how the presence of a sponge-like transition zone between layers of the planet’s rocky mantles explains the way volcanoes and tectonic plates interact.
The route that took the diamond from the alluvial plains of Brazil’s Mato Grosso province to the pages of a scientific journal is a tale that blends Indiana Jones-style exoticism and reminders of Jules Verne’s classic science-fantasy of a sea in the centre of Earth.
The ringwoodite was found in a speck of mineral trapped within a diamond that was itself encased in kimberlite, a volcanic rock ejected to the Earth’s surface during the Cretaceous period, 90 millions years ago, when dinosaurs still roamed the Earth.
Erosion exposed the precious stone, which lay amid river gravel of a tributary of the Rio Aripuana, until a garimpeiro, an artisanal miner, sieved it out and retailed it to a Las Vegas diamond geologist a decade ago.
Ringwoodite is a high-pressure form of a common silicate mineral, olivine, also known as peridot.
“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 Graham Pearson, Canada Excellence Research Chair in Arctic Resources at the University of Alberta.
He said that until now, scientists had inferred ringwoodite’s existence in the deep Earth from remote sensing and seismologic data.
“That inference has been known for 50 years without anybody ever been able to put their hands on a piece of ringwoodite to completely prove that it’s correct,” Dr. Pearson said in an interview.
The diamond came from a part of the Earth called the Transition Zone, between 410 to 670 kilometres under the ground.
What was most striking about the ringwoodite Dr. Pearson’s team extracted was that it was about 1.5 per cent water.
“It doesn’t sound like a lot but when you calculate how much ringwoodite there is in the Transition Zone, that equates to 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.”
Water weakens rock and lowers its melting point. The ringwoodite’s subterranean water would play a role in creating magma and destabilizing the roots that anchor continental plates.
French geophysicist Nathalie Bolfan-Casanova, who didn’t take part in the research, said Dr. Pearson had made a very important finding.
Finding the first terrestrial sample of ringwoodite opens a window into how diamonds are created and the existence of important water reservoirs within the Earth’s mantle, said Dr. Bolfan-Casanova, a researcher at the Magmas and Volcanoes Lab of Université Blaise-Pascal Clermont-Ferrand.
“It is already exceptional to have a sample that preserved such a high-pressure phase … and, in addition, with water!” Dr. Bolfan-Casanova said in an e-mail.
The diamond, labelled JUc29, came from a batch purchased by diamond geologist Mark Hutchison, a director at Trigon GeoServices Ltd., a prospecting and research firm based in Las Vegas.
For years Dr. Hutchison has been purchasing diamonds from Juina, a town in the Brazilian interior, about 300 kilometres from the border with Bolivia.
Juina diamonds have a reputation for being of poor quality, which makes them perfect for researchers who are interested in the minerals encased in the gems’ tiny flaws, Dr. Hutchison said in an interview.
He described Juina as a mining community where garimpeiros sell their stones on wooden tables in little shacks. The surrounding area is a lush, flat grassland where meandering rivers expose diamond-rich gravel flood plains.
The garimpeiros scoop up the gravel with diesel pumps, then sieve the material through shaking tables.
Dr. Hutchinson acquired diamond JUc29 a decade ago and identified it as a potential research target. It was sent to research labs in Germany and Italy for X-ray and spectroscopic tests and it took four years before the ringwoodite and its water-content were identified.
“Jules Verne speculated a hundred years ago that there was an ocean floating in the deep Earth,” Dr. Pearson said, referring to the French author’s classic novel Journey to the Center of the Earth.
“Well this is an ocean mass of water but it’s not free water. It’s bound to silicate rock.”
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Analysis of ringwoodite shows it contains a significant amount of water, confirming theories about large volumes of water trapped 410 to 660 kilometres below the Earth's surface
1. Water forced downward
2. Ringwoodite contains water
3. Ringwoodite pushed up by volcanic activity and trapped in diamonds within kimberlite rocks
Source: Professor Graham Pearson, University of Alberta
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