Geochemical studies of volcanic rocks from Baffin Island (Canada) and Iceland suggest that some of the water dating back to Earth’s formation may still be preserved deep within the planet. However, it has remained unclear which minerals could store such quantities of water under the extreme conditions of Earth’s interior. An international research team co-led by the University of Bayreuth has now synthesised two iron compounds (Fe₅O₁₂Hₓ and Fe₇O₁₂Hₓ) that represent the first realistic candidates for water storage deep inside Earth.
To achieve this, the researchers used so-called diamond anvil cells. In these devices, tiny samples are compressed between two diamonds and then heated with lasers. The resulting conditions resemble those found several thousand kilometres beneath Earth’s surface, with pressures of 78 to 198 gigapascals and temperatures of 2,400 to 2,800 kelvin (approximately 2,100 to 2,500 degrees Celsius).
Using synchrotron X-ray radiation, the researchers were able to identify chemical compositions and crystal structures that formed under these conditions. The new compounds formed not only from deliberately water-rich mixtures, but also from samples containing almost no water. “Apparently, even very small amounts of hydrogen are sufficient to stabilize these highly hydrated iron compounds,” says Professor Leonid Dubrovinsky of the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) at the University of Bayreuth.
A possible reservoir for primordial water
One possible reservoir for water dating back to Earth’s formation is an ancient basal magma ocean that crystallized above the core–mantle boundary. As it cooled, its remaining melt would have become enriched in iron and water, precisely the ingredients required to form the newly identified minerals.
“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,” says Dr. Man Lianjie, one of the authors and former doctoral student at BGI.
The compound Fe₅O₁₂Hₓ is particularly rich in hydrogen. According to the researchers’ calculations, a concentration of only about 1 % of this mineral within the mantle could be sufficient to store a large proportion of the estimated hydrogen inventory of the silicate Earth. The mineral is unlikely to occur uniformly throughout the mantle, however, and would probably be concentrated in localised regions.
Potential implications for seismic observations and volcanism
In the experiments, formation of the oxyhydroxides extracted iron from surrounding silicate minerals. As a result, boundaries between different mantle minerals could shift. This may help explain unusual seismic signals above the core-mantle boundary.
The researchers further propose that these water-rich piles could act as a volatile “fuse” for mantle plumes. If the oxyhydroxides melt more readily than the surrounding silicates during heating from the core, they could release stored water and other volatiles into rising melts, helping connect deep reservoirs with volcanic activity at Earth’s surface, ultimately influencing volcanic activity at Earth’s surface.
The research involved scientists from the Center for High Pressure Science and Technology Advanced Research (HPSTAR), the Bayerisches Geoinstitut at the University of Bayreuth, the European Synchrotron Radiation Facility, ETH Zürich, the Shanghai Institute of Applied Physics, DESY, Goethe University Frankfurt and collaborating institutions.
