14th June 2014

New evidence for "oceans" of water lying deep inside the Earth

Researchers have presented new evidence indicating vast amounts of water in a transition layer below Earth's crust. Although not in liquid form, this discovery may represent the planet's single largest reservoir.

Earlier this year, a sample of the mineral ringwoodite provided strong evidence of water in huge volumes below the Earth at depths of 250 to 430 miles (400 to 700 km). Now researchers have built upon that discovery with additional findings, based on seismic wave patterns and laboratory simulations.

Geophysicist Steve Jacobsen (Northwestern University) and seismologist Brandon Schmandt (University of New Mexico) found deep pockets of magma located about 400 miles below North America – a likely signature of the presence of water at these depths. Their discovery suggests that water from Earth's surface can be driven to such great depths by plate tectonics, leading to partial melting of the rocks found deep in the mantle. The study results, published yesterday in the journal Science, will aid scientists in better understanding how the Earth formed, its current composition and inner workings and how much water is trapped in mantle rock.

"Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," said Jacobsen. "I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades."

The study combined Jacobsen's laboratory experiments – in which mantle rock was simulated under high pressure at 400 miles below Earth's surface – with Schmandt's observations using vast amounts of seismic data from the USArray, a dense network of 2,000 seismometers across the United States. The pair's findings converged to produce evidence that melting occurs about 400 miles deep in the Earth. H2O stored in mantle rocks, such as those containing ringwoodite, is key to this process, the researchers said.

A sample of ringwoodite, able to contain hydroxide ions (oxygen and hydrogen atoms bound together). Credit: Jasperox (CC BY 3.0)

"Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles," said Schmandt. "If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle – and that is consistent with what we found."

The total amount of water is so huge that it could fill the volume of Earth's oceans three times over. It exists as neither liquid, ice nor vapour, however – but in a fourth form. The weight of 250 miles of solid rock creates such high pressure, along with temperatures of more than 1100°C (2000°F), that a water molecule splits to form a hydroxyl radical (OH), which can be trapped inside a mineral's crystal structure.

"The ringwoodite is like a sponge, soaking up water," Jacobsen says. "There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water. This mineral can contain a lot of water under conditions of the deep mantle."

The melting the researchers have detected is known as "dehydration" melting. Rocks in the transition zone can hold a lot of H2O, but rocks in the top of the lower mantle hold almost none. The water contained in ringwoodite in the transition zone is forced out when it goes deeper (into the lower mantle) and forms a higher-pressure mineral called silicate perovskite, which cannot absorb the water. This causes the rock at the boundary between the transition zone and lower mantle to partially melt.

"When a rock with a lot of H2O moves from the transition zone to the lower mantle, it needs to get rid of the H2O somehow, so it melts a little bit," Schmandt said. "This is called dehydration melting."

"Once the water is released, much of it may become trapped there in the transition zone," Jacobsen added.

The confirmation of water in this transition zone may lend support to alternative theories about the origin of Earth's oceans. Instead of being deposited on the surface by comet impacts, it may have gradually oozed up from the interior. The hidden water could also have acted as a surface "buffer", explaining why the oceans have remained the same size for millions of years.

"We should be grateful for this deep reservoir," says Jacobsen. "If it wasn't there, it would be on the surface of the Earth, and mountain tops would be the only land poking out."

At 400 miles down, this region is far beyond the reach of current drilling technology. The deepest humans have ever drilled is 7.6 miles (12.2 km) at the Kola Superdeep Borehole in Russia. Plans have also been made to obtain samples from the mantle, by probing a particularly thin area of the crust at depths of 5 miles (8 km) in 2018. However, it will likely be many decades or even centuries before we attempt a "Journey to the centre of the Earth".

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