Unique form of quartz may power deep-Earth water cycle

Most of what scientists know about Earth's water cycle involves processes -- evaporation, condensation and precipitation -- happening above the planet's crust. But new research suggests the water cycle may have a deep-Earth component.

In a new paper, published this week in the journal PNAS, scientists have described for the first time the remarkable water-carrying abilities of the mineral stishovite, a unique form of quartz.

Lab tests showed the mineral can transport surprisingly large amounts of water under extreme conditions. It's possible water is moving through Earth's interior, powering a whole-mantle water cycle -- its impacts playing out across geological timescales.

"To get down into the mantle, water must be incorporated into minerals on the surface and then be stably maintained in those structures under the conditions found deep inside the planet," lead study author Yanhao Lin, a researcher at the Carnegie Institution for Science, said in a news release.

Silicate minerals can store significant amounts of water in the upper mantle, located between 62 to 416 miles beneath Earth's surface. The fate of water at greater depths isn't well understood.

In the lab, scientists exposed stishovite to simulated conditions like those found in the lower mantle, between 416 to 1,802 miles below Earth's crust.

"Stishovite is a silica-based mineral and a major component of the oceanic crust." said Kwang "Dave" Mao, of the Center for High Pressure Science and Technology Advanced Research Shanghai. "In plate tectonics, there are areas called subduction zones where an oceanic plate slides beneath a continental plate, sinking from the Earth's surface into its depths. When this happens, stishovite is transported into the mantle."

Water-carrying diamonds have been found at depths of more than 500 miles, but scientists wanted to know if stishovite could take water even deeper. Scientists heated the mineral to between 1,000 and 1,500 degrees Celsius and exposed it to 320,000 to 510,000 times normal atmospheric pressure. Even under such harsh conditions, the mineral was able to carry surprisingly large amounts of water, suggesting stishovite could move water into the lower parts of the deep mantle.

"If water can be stored in minerals at lower mantle pressures and temperatures, it could indicate that there is a global water cycle occurring on very long geologic time scales," said Carnegie researcher Michael Walter. "This could alter our understanding of how deep planetary interiors may influence or control the water content at the surface."

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