Water, water everywhere: Ringwoodite revelation

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  • Published: Apr 1, 2014
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Water, water everywhere: Ringwoodite revelation

Subterranean hydroxylated blues

Raman and infrared spectroscopy, X-ray diffraction and other techniques have been used to examine a rare mineral sample of hydroxylated ringwoodite trapped in a rough diamond

Raman and infrared spectroscopy, X-ray diffraction and other techniques have been used to examine a rare mineral sample of hydroxylated ringwoodite trapped within a $20 rough diamond for years. Critical water measurements of the sample at the Arctic Resources Geochemistry Laboratory could well reveal the presence of water based on isotopic microanalysis, suggesting that there may be vast oceans between 410 and 660 kilometres below the Earth's surface.

A chunk of carbon could harbour the secret of subterranean oceans deep below the Earth's surface according to research by Graham Pearson of the University of Alberta and colleagues there and internationally. The team has discovered the first-ever terrestrial sample of a potentially hydroxylated mineral called ringwoodite - a high-pressure polymorph of olivine, better known as the precious gem peridot, chemically magnesium iron silicate. Analysis of the mineral shows it to contain significant quantities of water - about 1.5 per cent of its mass - as hydroxyl groups. The polymorph is stable at the pressures that exist at depths associated with the boundary at 410 to 660 kilometres beneath the Earth's surface between the upper and lower mantle.

"This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," explains Pearson, He and his colleagues report details in the journal Nature. "That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together," Pearson adds.

International analysis

Scientists have previously identified ringwoodite of extraterrestrial origin; it was first identified in the Tenham meteorite in 1969. It is named for Australian geoscientist Ted Ringwood (1930–1993), who studied polymorphic phase transitions in the common mantle minerals olivine and pyroxene at extreme pressures. However, until now, no terrestrial sample had been unearthed. The sample analysed by Pearson and colleagues was found in 2008 in the Juina area of Mato Grosso, Brazil, where artisanal miners uncovered a rough diamond from shallow river gravels. The researchers explain that this diamond was brought to the surface by hitching a ride within volcanic rock known as kimberlite, the most deeply derived of all volcanic rocks.

Pearson's team bought the brown, rough diamond, just 3 millimetres across, for $20, it was almost worthless in terms of cutting and polishing to make a gemstone, while searching for minerals of interest. The ringwoodite within the host diamond is invisible to the naked eye, buried beneath the surface, but it was spotted by student John McNeill in preliminary analysis in 2009.

"It's so small, this inclusion, it's extremely difficult to find, never mind work on," Pearson says, "so it was a piece of luck as are many scientific discoveries." The team used Raman and infrared spectroscopy as well as X-ray diffraction over the years since to identify the ringwoodite inclusion. They then performed critical water measurements at Pearson's Arctic Resources Geochemistry Lab, part of the Canadian Centre for Isotopic Microanalysis. Other aspects of the research were carried out by researchers at the Geoscience Institute at Goethe University, Germany, University of Padova, Italy, Durham University, UK, University of Vienna, Austria, US company Trigon GeoServices and Ghent University in Belgium.

Rock melt

The finding represents the culmination of half a century of theoretical and experimental studies by geophysicists, seismologists and others hoping to understand the makeup of the Earth's interior. Knowing that water exists beneath the crust has implications for the study of volcanism and plate tectonics, affecting how rock melts, cools and shifts below the crust. "One of the reasons the Earth is such a dynamic planet is because of the presence of some water in its interior," Pearson explains. "Water changes everything about the way a planet works."

Related Links

Nature, 2014, 507, 221-224: "Hydrous mantle transition zone indicated by ringwoodite included within diamond"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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