An important reservoir of water could be buried almost at the boundary between Earth's mantle and core.
This water would not, however, form an underground lake or ocean. At such depths, oxygen and hydrogen, the components of water, can enter directly into the structure of certain minerals. Researchers have just produced two iron compounds capable of holding them under the extreme conditions of the deep mantle.
World map of heat flow from Earth's interior.
Credit: Wikimedia Commons.
Wikimedia image
To find out what can exist so far beneath our feet, the team reproduced pressures and temperatures close to those found at great depths in the laboratory. The samples were compressed between diamond tips and then heated with a laser. The experiments reached 78 to 198 gigapascals and approximately 2,127 to 2,527 °C.
Under these conditions, two new iron oxyhydroxides formed. These are minerals containing iron, oxygen and hydrogen. One of the two compounds can hold a particularly large amount of these elements.
Their density also changes the picture. These minerals are heavy enough to sink and accumulate toward the base of the mantle, located nearly 2,900 km beneath the surface. They may therefore have retained oxygen and hydrogen since the earliest days of our planet. They could also trap water carried deep underground by the movements of tectonic plates.
This possibility interests geologists for another reason. Just above the core, seismic waves sometimes travel through regions much more slowly than elsewhere. Researchers call them ultra-low velocity zones. Accumulations of these minerals could contribute to some of these anomalies.
Water stored this deep would not necessarily be locked away forever. If these materials are heated by the core, they could release water and other substances into rocks that slowly rise. This mechanism could create a possible link between a very deep reservoir and certain mantle plumes associated with volcanism.
One major unknown remains: these two minerals were obtained in the laboratory, but their natural presence in the mantle has not yet been demonstrated. Future experiments will have to determine under what conditions they form and how much oxygen and hydrogen they can actually retain over geological timescales.