Experiments reveal exceptionally water-rich solids stable under pressure–temperature conditions that overlap the deep-mantle geotherm. The experiments took place on three different beamlines at the ESRF. The results are out in Nature Geoscience.
Scientists have synthesized two previously unknown iron oxyhydroxides that remain stable at the immense pressures and temperatures of Earth’s deep mantle. The phases are likely the first experimentally demonstrated water-bearing solids synthesized and stable under conditions overlapping a substantial portion of the mantle geotherm—providing a long-sought mineralogical host for water near the base of the mantle.
The study identifies the hexagonal compounds Fe₅O₁₂Hₓ (x ≥ 9) and Fe₇O₁₂Hₓ (x ≥ 3). They formed in laser-heated diamond anvil cells between 78 and 198 gigapascals and at temperatures of 2,400–2,800 kelvin. These conditions extend across much of the deep lower-mantle pressure range and overlap the temperatures expected along the mantle geotherm.
Water profoundly affects mantle melting, deformation and chemical transport, yet the major minerals of the lower mantle can store only very small amounts of hydrogen. Previously proposed hydrous phases either require unusually cold subducting slabs, restricted chemical compositions or break down at high mantle temperatures. The new iron oxyhydroxides therefore fill an important gap: they are dense, highly hydrated solids that can coexist with the principal minerals of the lower mantle.
Read more on the ESRF website