Water is fundamental to life on Earth. Yet for all its importance, scientists still do not have a complete picture of where Earth’s water came from in the first place. Now, a new study suggests that the planet’s mantle could be hiding an ancient water reservoir, potentially preserving some of the water that dates back to Earth’s formation billions of years.
A team of scientists identified two previously unknown iron compounds capable of storing water under the extreme pressures and temperatures found deep inside Earth. The findings, published in Nature Geoscience, offer new clues to our understanding of Earth’s water cycle and interior structure.
Hidden reservoir
Water plays a surprisingly important role deep inside Earth; it affects the movement of tectonic plates, helps drive volcanism, and changes how rocks behave. It isn’t necessarily there as liquid water. Instead, much of it is chemically bound inside minerals in the crust and mantle. Until now, however, scientists weren’t sure which minerals can store water over long periods under the extreme conditions of the deep mantle.
To help solve the mystery of Earth’s stored water, the scientists behind the new study recreated the extreme pressure and temperature conditions found deep within the Earth’s interior in the lab. The researchers compressed tiny samples between two diamonds and heated them with lasers, reaching pressures around two million times the atmospheric pressure at Earth’s surface and temperatures of about 4,500 Fahrenheit (2,500 degrees Celsius). Those conditions are similar to ones found around 1,800 miles (2,900 kilometers) below the surface.
Under these extreme conditions, two previously unknown iron oxyhydroxides—Fe5O12Hx and Fe7O12Hx—had formed. The new compounds formed not only from water-rich starting materials but also from samples containing almost no water.
“It appears that even very small amounts of hydrogen are sufficient for these water-rich iron compounds to form,” Leonid Dubrovinsky, a researcher at the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) at the University of Bayreuth and one of the authors of the new study, said in a statement.
Ancient clues
The iron oxyhydroxides are capable of capturing and retaining water across a wide range of lower-mantle conditions and can host both primordial and recycled water, according to the study.
“These minerals could also store water that is transported deep into Earth by the movement of tectonic plates. Because the compounds are very dense, they would likely remain near the core-mantle boundary and retain water there over geological timescales,” Man Lianjie, a former doctoral student at BGI and one of the authors of the study, said in a statement.
While the recent experiment suggests the new compounds are ideal candidates for storing water deep within Earth’s mantle, it doesn’t necessarily demonstrate that they occur naturally in our planet’s interior. Still, the findings offer new clues as to where Earth’s ancient water may be hiding.
