Discovery of New Hydrous Minerals in Deep Mantle (Press Release)
- Release Date
- 03 Feb, 2014
- BL04B1 (High Temperature and High Pressure Research)
Ehime University
Tokyo Institute of Technology
Japan Synchrotron Radiation Research Institute
|
A research group led by Masayuki Nishi (research scientist), Tetsuo Irifune (professor), Jun Tsuchiya (associate professor) and Yoshinori Tange (assistant professor) of the Geodynamics Research Center (GRC), Ehime University, who are concurrently affiliated with the Earth-Life Science Institute (ELSI)*10, Tokyo Institute of Technology, discovered new hydrous minerals*1 that are stable in the Earth’s lower mantle for the first time in the world. Part of water existing in abundance near the Earth’s surface is transported to the Earth’s mantle,*3 extending from a depth of 30 to 2900 km, as hydrous minerals by subducting slabs. Hydrous minerals named phase D*2 have been observed in the lower mantle*4 that extends from a depth of 660 to 2900 km. However, it has been reported that phase D dehydrates and decomposes at pressures near 44 GPa, which corresponds to a depth of 1250 km, and that no water can reach the deeper regions of the lower mantle. Jun Tsuchiya predicted that phase D transforms to different crystal structures at pressures near 44 GPa based on theoretical calculations. She published her research achievements in Geophysical Research Letters, the journal of the American Geophysical Union (AGU), in 2013. To verify the prediction of Tsuchiya, the research group carried out experiments using an ultrahigh-pressure apparatus (MADONNA-1500) at the GRC and in situ synchrotron radiation experiments at SPring-8. As a result, they found that phase D transforms to new high-pressure hydrous minerals named phase H near the theoretically predicted pressure. The crystal structure of phase H was slightly different from that predicted by Tsuchiya but very similar. Phase H is likely to be stable even in the mantle-core boundary. It seems to have significant effects on the composition and dynamics of the deep Earth, for example, the general circulation of water in the deep Earth, the generation of upwellings (plumes*5) in the mantle-core boundary, and the penetration of water into the molten iron, which is the main component of the Earth’s core. The achievements of the group were published online in the British scientific journal Nature Geoscience on 2 February 2014 prior to the printed version. Publication: |
<<Glossary>>
Although the highest pressure generated by a typical experiment method using tungsten carbide anvils is approximately 30 GPa, ultrahigh pressures far greater than 50 GPa can be generated by the apparatus developed by the research group of this study utilizing sintered diamond anvils*7.
and its predicted crystal structure
The pressure and temperature ranges of stable phase H are significantly widened by the addition of Al, which is an important component of phase H next to Mg and Si.
water into deep Earth suggested in this study
Phase D seems to transform to the new hydrous mineral phase H within the slabs subducting into the lower mantle and to transport water near the Earth’s core.
<<Figures>>
*1 Hydrous minerals
Hydrous minerals, such as serpentine, contain hydrogen as a major component. The magnesium-rich hydrous minerals formed at high temperatures and pressures in the Earth’s interior are called high-pressure dense hydrous magnesium silicates (DHMSs) or alphabet phases, which are considered to be transported to the deep Earth by subducting slabs.
*2 Phase D
Phase D is the hydrous mineral considered the only DHMS in the lower mantle. Since the discovery of phase D by an Australian researcher in 1986, there have been reports of the discovery of phases E, F and G. However, subsequent studies have revealed that phase E exists only in the upper region of the mantle and that phases F and G are the same as phase D.
*3 Mantle and core
Earth consists of three layers, namely, the thin crust extending to a depth of approximately 30 km, the mantle extending from a depth of 30 to 2900 km, and the core from a depth of 2900 to 6400 km. The mantle mainly consists of rocks such as peridotite, whereas the core mainly consists of iron.
*4 Lower mantle
The mantle is divided into three zones, namely, the upper mantle extending from a depth of 30 to 410 km, the mantle transition zone from a depth of 410 to 660 km, and the lower mantle from a depth of 660 to 2900 km. The largest zone is the lower mantle, which makes up 60% of the volume of the Earth and the lowest part of which is in contact with the Earth’s core.
*5 Plumes
Plumes are high-temperature upwellings rising from the deep mantle in contrast to subducting cold slabs and mantle materials. Seismological studies have shown that huge superplumes originated from the core existing under Africa and the lower part of the Pacific Ocean. Rocks may partially melt in the region where plumes are generated. The presence of water is an important factor in the generation of plumes because the temperature at which rocks melt is decreased with the presence of water.
*6 Ultralow-velocity zone
The region in the lowest part of the mantle near the mantle-core boundary where seismic waves travel at a very low velocity is called the ultralow-velocity zone. The ultralow velocity seems to result from the reaction of the mantle composed of rocks with the molten iron or the partial melt of mantle materials. The presence of water is an important factor in the formation of such a low-velocity zone because the temperature at which rocks melt is decreased with the presence of water.
*7 Sintered diamond anvils
Hard materials used to generate ultrahigh pressures are called anvils. Although cemented carbides such as tungsten carbide are used as anvils in typical high-temperature and high-pressure experiments, efforts are being made to generate higher pressures by using sintered diamonds prepared by sintering diamond powder with metals.
*8 Diamond anvil apparatus
In the diamond anvil apparatus, high pressures are generated on a sample placed between two single-crystal diamond anvils with a flat-polished end by applying load to the anvils. The high pressure of 360 GPa and the high temperature of 6000 °C that correspond to those at the Earth’s core can be produced using a diamond anvil apparatus. However, a multi-anvil apparatus is used in experiments requiring high accuracy or in experiments on materials with complex compositions.
*9 Multi-anvil apparatus
In the multi-anvil apparatus, higher pressures are generated on a sample by applying load to eight cubic anvils using a large press and by focusing the pressure on the sample placed at the center of the anvils. Cemented carbides are typically used as materials of anvils, which can generate a pressure of 30 GPa at maximum. In this study, as well as that performed by a research group of Okayama University, the generation of pressures of 100 GPa was possible using sintered diamond anvils. A multi-anvil apparatus is superior to a diamond anvil apparatus*8 for use in experiments on large samples and in experiments requiring high accuracy.
*10 Earth-Life Science Institute (ELSI)
Founded in 2012, ELSI is a new research unit of the Tokyo Institute of Technology. It was founded on the basis of the World Premier International Research Center Initiative (WPI) project in the field of earth and life sciences, which is led by Professor Kei Hirose. The GRC of Ehime University to which the research group of this study belongs is the only satellite facility of ELSI in Japan.
|
For more information, please contact: Prof. Tetsuo Irifune (Ehime University, Geodynamics Research Center) Associate Prof. Jun Tsuchiya (Ehime University, Geodynamics Research Center) Dr. Yuji Higo (JASRI) Prof. Hiroyuki Kagi (School of Science, The University of Tokyo) Prof. Kei Hirose (Tokyo Institute of Technology, Earth-Life Science Institute) |
- Previous Article
- Successful Synthesis of Artificial Rhodium (Press Release)
- Current article
- Discovery of New Hydrous Minerals in Deep Mantle (Press Release)