High-pressure silicates can incorporate water as OH-defects into their crystal structures, with some major consequences for their physical properties. Minerals within the transition region of the mantle from 410-660 km depth could contain the majority of our planet’s water and acted to control surface waters over geologic time.
Researchers have found the first direct evidence for water-bearing fluids in the uppermost lower mantle from natural ferropericlase crystal contained within a diamond from São Luíz, Brazil. The ferropericlase exhibits exsolution of magnesioferrite, which places the origin of this assemblage in the uppermost part of the lower mantle. The presence of brucite–Mg(OH)2 precipitates in the ferropericlase crystal reflects the later-stage quenching of H2O-bearing fluid likely in the transition zone, which has been trapped during the inclusion process in the lower mantle. Dehydration melting may be one of the key processes involved in transporting water across the boundary between the upper and lower mantle.
• Direct evidence for water-bearing fluids in the uppermost lower mantle.
• Exsolution of magnesioferrite from ferropericlase starts in the lower mantle.
• Brucite precipitates reflect the later-stage quenching of H2O-bearing fluids trapped in the lower mantle.
• Dehydration melting is a key process in transport of water across the 660 km discontinuity.
Analysis of a mineral inclusion in a 90-million-years-old diamond revealed that the Earth’s mantle might hide a lot more water than we believed, buried as deep as 1,000 kilometers below the surface.
Water exists far deeper in the Earth than scientists previously thought. Mookherjee and Andreas Hermann from the University of Edinburgh estimate that in the deep Earth—roughly 400 to 600 kilometers into the mantle—water is stored and transported through a high-pressure polymorph of the mineral brucite.
Previously, scientists thought brucite was not thermodynamically stable that deep in the Earth. “This opens up a Pandora’s Box for us,” Mookherjee said. this discovery of a new high-pressure phase of brucite indicates that water could be efficiently transported to far deeper realms without decomposition.
Significance – High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions
Hydrous minerals help transport water deep into Earth’s mantle, and form part of a cycle that regulates the sustained presence of surface water on Earth. To understand the deep-water cycle, it is crucial to study the properties of hydrous minerals under the conditions present in Earth’s mantle. Brucite is one of the simplest hydrous minerals and stores significant amounts of water as hydroxyl groups. It is assumed to decompose in the mantle transition zone, but we show here that a more compact high-pressure phase is stabilized instead that pushes the stability region of brucite into the lower mantle. Brucite might be present in much larger quantities, and play a larger role in water transport and storage, than previously thought.
Abstract – High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions
We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)2, using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth’s mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earth’s interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO6 octahedral units arranged in the anatase–TiO2 structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods.
Water exists one-third of the way to the Earth’s core. A 90 million year old diamond from a volcano near the São Luíz river in Juina, Brazil has a sealed inclusion, an imperfection in the stone. It contains minerals trapped by the forming diamond.
Through infrared microscopy, scientists analyzing the material found it included hydroxyl ions in its chemical make-up, a compound usually formed from water molecules. And there were a lot of these ions present in the inclusion.
They found it was mainly composed of ferropericlase, a mixture of iron and magnesium oxide which can absorb some other metals, such as chromium, aluminum, and titanium, in the extremely hot and pressurized environment of the lower mantle. Jacobsen found that these “extra” metals had separated from the ferropericlase, a phenomena that can only take place in milder conditions as the diamond inches towards the surface. Based on the composition, they estimate the inclusion formed at around 1,000 kilometers deep. The inclusion was sealed in the diamond since the beginning, and for the metals to be present at all, it had to have originated in the lower mantle. That means the water signature can only come from the lower mantle.
Ringwoodite is polymorphous with forsterite, (Mg)2SiO4, and has a spinel structure. Spinel group minerals crystallize in the isometric system with an octahedral habit. Olivine is most abundant in the upper mantle, above about 410 km (250 mi); the olivine polymorphs wadsleyite and ringwoodite are thought to dominate the transition zone of the mantle, a zone present from about 410 to 660 km depth.
Ringwoodite is thought to be the most abundant mineral phase in the lower part of Earth’s transition zone. The physical and chemical property of this mineral partly determine properties of the mantle at those depths. The pressure range for stability of ringwoodite lies in the approximate range from 18 to 23 GPa.
An ultra-deep diamond found in Juína, Mato Grosso in western Brazil, contained inclusions of ringwoodite—the only known sample of natural terrestrial origin—thus providing evidence of significant amounts of water as hydroxide in the Earth’s mantle. The gemstone, about 5mm long, was blasted up from the depths by a diatreme eruption. The ringwoodite inclusion is too small to see with the eye. The mantle reservoir is found to contain about three times more water, in the form of hydroxide contained within the wadsleyite and ringwoodite crystal structure, than the Earth’s oceans combined