Hell is wetter than you thought: Diamond suggests that beneath the Earth's crust is an 'ocean’s worth of water'

Battered-looking diamond shows the Earth has a rather wet interior

A rare diamond that survived a trip from deep within the Earth's interior may have confirmed some theories that beneath the planet’s crust is an ocean’s worth of water.

In a study published on Wednesday in the journal Nature, scientists have found a battered-looking diamond from Brazil actually contains a water-rich inclusion of the olivine mineral ringwoodite, suggesting there is a very large amount of water held in the transition zone of the Earth’s mantle.

Samples from the transition zone are considered to be “exceedingly rare” and are only found in a small number of unusual diamonds, geochemist Hans Keppler said.

Most diamonds form at depths of about 150 to 200 kilometres (90 to 125 miles), but "ultradeep" diamonds come from the transition zone, which is 410 to 660 kilometres below the surface, Graham Pearson, a mantle geochemist at the University of Alberta in Edmonton said.

This particular diamond weighs just one tenth of a gram.

Mr Pearson said their discovery was almost accidental as his team had been looking for another mineral when they paid about $20 (£12) for a three-millimetre-wide brown diamond from the Juina district in Brazil in 2008.

 

His team studied the diamond and discovered a grain 40 micrometres across that turned out to be "ringwoodite" - a high-pressure form of olivine, a mineral that makes up much of the upper mantle of the Earth.

The upper mantle is a layer that lies between the planet’s crust and lower mantle.

Unlike other forms of olivine, ringwoodite can hold a substantial amount of water, meaning the sample could potentially resolve a long-standing debate over just how much water the transition zone contains.

The team found that its tiny speck of ringwoodite contained about 1 per cent  of its weight in water - a significant amount.

“That may not sound like much,” Mr Pearson told Nature News, “but when you realise how much ringwoodite there is, the transition zone could hold as much water as all the Earth’s oceans put together.”

"It's actually the confirmation that there is a very, very large amount of water that's trapped in a really distinct layer in the deep Earth," Mr Pearson added.

However, the results taken from one single crystal may not represent the entire transition zone, according to some scientists.

Speaking to Nature, Norm Sleep, a geophysicist at Stanford University in California compared the situation to panning for gold and discovering a large nugget. “It would be unwise to assume that all the gravel in the stream is gold nuggets,” he countered.

Mr Pearson agreed that further analysis is needed to test his theory, as other studies of the mantle have shown the water content in the transition zone is "spotty", and this sample may have come from one of the wet spots.

But tests could prove difficult because of the very small size of the mantle ringwoodite. “We have to think really carefully on what we do next on this sample because it’s very small: 40 micrometres,” he said. “That means you can only think of doing one or two additional analyses.”

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