June 13, 2014
An Ocean’s Worth Of Water May Be Sitting In North American Mantle
Brett Smith for redOrbit.com - Your Universe Online
While all of Earth’s oceans may seem like more than enough water for one planet, a new study published in the journal Science has revealed an ocean’s worth of water may be sitting in the mantle, hundreds of miles below North America.
According to the study, molecular water is stored away in a mineral called ringwoodite about 400 miles below the Earth’s surface. The study team said this stored water is sitting in a transition zone, located between the upper and lower mantles.
"We knew about the water cycle, but we didn't know how deep it extended," study author Steve Jacobsen, from Northwestern University, told USA TODAY's Hoai-Tran Bui. "It looks like the same process occurring in the very shallow mantle is occurring at a deeper layer.
"It may be why we have stable oceans on the surface," he added.
The study team said their find was based on the deep pockets of magma beneath North America, a potential sign of the presence of water. The finding suggests water from the Earth's surface can be pushed to such great depths by plate tectonics, ultimately causing incomplete melting of the rocks discovered deep in the mantle.
“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,” Jacobsen said. “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 new study builds on a development reported in March from the journal Nature involving scientists uncovering a piece of the novel mineral ringwoodite inside a diamond raised up from a depth of 400 miles by a volcano in Brazil. The piece of ringwoodite – the only known sample from inside the Earth – held a surprising quantity of water within the mineral.
"The (discovery of the ringwoodite diamond) showed us at least one place there is water, but it didn't tell us how expansive the water would be," Jacobsen said.
Jacobsen has been synthesizing the sapphire-blue ringwoodite in his Northwestern lab by placing the green mineral olivine under high-pressure conditions and exposing it to water. He said over one percent of ringwoodite's weight can contain water – approximately the same quantity of water as was discovered in the Brazilian sample.
"The ringwoodite is like a sponge, soaking up water," Jacobsen said. "There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water."
In the new study, researchers exposed synthetic ringwoodite to conditions about 400 miles below the Earth's surface and discovered it forms small amounts of partial melt when forced to these conditions. The experiments produced the same results of partial melt, or magma, that has been detected beneath North America using seismic waves.
"Seismic data from the USArray are giving us a clearer picture than ever before of the Earth's internal structure beneath North America," said study author Brandon Schmandt, a seismologist at the University of New Mexico. "The melting we see appears to be driven by subduction -- the downwelling of mantle material from the surface."
The researchers said their future work will expand their scope to beyond North America.
"In the future as more seismometers are placed, we will be able to see beneath more places," Jacobsen said. "Looking for this signature of water globally is the next step."