This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 synthesized 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 kilometers 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, 3,800 to 4,500 degrees Fahrenheit). 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. The work is published in the journal Nature Geoscience. 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 a 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.
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