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Grain boundary sliding as the major flow mechanism of Earth’s mantle (Press Release)

Release Date
03 Oct, 2015
  • BL04B1 (High Temperature and High Pressure Research)

Ehime University
Japan Synchrotron Radiation Research Institute (JASRI)

A research group led by Tomohiro Ohuchi (assistant professor) and Tetsuo Irifune (professor, also a member of the Earth-Life Science Institute*1 of Tokyo Institute of Technology) of the Geodynamics Research Center (GRC) of Ehime University and Yuji Higo (research scientist) of JASRI clarified that the flow of Earth’s upper mantle is controlled by the grain boundary sliding*8 of olivine*2.
The temperature of the upper mantle (~60–410 km in depth)*3 is extremely high, reaching 1400 oC, and thus, on a geological timescale, rocks flow as if they were made from starch syrup. The surface plates*4 (thickness, ~60 km), on which we live, move or subduct along with the flow of the mantle because they float on the viscous mantle. Thus, natural phenomena, such as earthquakes and volcanic eruptions that may cause disasters, occur. Over the 45 years since 1970, a theoretical model of the flow in the upper mantle being controlled by dislocation creep*5 has been generally accepted. According to this model, the mantle flow proceeds via the deformation of individual olivine grains. However, the model cannot explain the geophysical observations*6 that the viscosity in the upper mantle is almost constant regardless of depth.

The research group carried out experiments at SPring-8*7 to reexamine the mechanism behind the flow in the mantle. The group clarified that the flow of the upper mantle follows the theoretical model based on grain boundary sliding*8 of olivine. With this model, the nearly constant viscosity in the upper mantle regardless of depth can also be explained, which means that this model is now the most appropriate model for the evaluation of the flow behavior of the upper mantle.

The flow in the upper mantle causes the movement and subduction of plates. Along with the motion of plates, natural phenomena, such as earthquakes and volcanic eruptions, occur. The flow law of grain boundary sliding obtained in this study should be used to reexamine our understandings on the dynamics of Earth’s interior and the evolution of solid Earth.

Publication
Title: Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle
Science Advances

[Figures]

Fig. 1 D-DIA apparatus (SPEED Mk-II) and high-pressure field in the apparatus
Fig. 1 D-DIA*9 apparatus (SPEED Mk-II) and high-pressure field in the apparatus

A total of six tungsten carbide alloy anvils are arranged in six places around a sample, i.e., in front, behind, on the right, on the left, above, and below, to apply high pressure to the sample placed at the center of a high-pressure field. The anvils above and below the sample are advanced independently of the anvils on the right and left of the sample to deform the sample placed at the center in the high-pressure field.

Fig. 2 Comparison of measured viscosity in the upper mantle with that simulated using theoretical model
Fig. 2 Comparison of measured viscosity in the upper mantle with that simulated using theoretical model

Among the calculations based on the theory of grain boundary sliding (red-hatched area between thick red lines: anhydrous mantle; thin red lines: hydrous mantle), those for the anhydrous mantle agree fairly well with the geophysical observations of postglacial rebound (gray region). In contrast, the calculations based on the theory of dislocation creep are not in good agreement with the observations because of the strong dependence of viscosity on depth (the slope of viscosity deeper than 100 km is too steep). In the calculations, it is assumed that the stress is constant regardless of depth and the size of mineral grains is 1–10 mm (typical grain sizes in the upper mantle).

[Glossary]
*1 Earth-Life Science Institute (ELSI)
　The ELSI of Tokyo Institute of Technology was launched in 2012 as part of the World Premier International Research Center Initiative (WPI) for research in the fields of earth and life sciences under the leadership of Professor Kei Hirose. The GRC of Ehime University is the only satellite center of the ELSI in Japan.

*2 Olivine
　Olivine is the dominant mineral of the upper mantle and constitutes 60–70% of the volume of the mantle. It is also called peridot. It is a green transparent jewel stone and the birthstone for August. The upper mantle is an aggregate of olivine grains.

*3 Upper mantle
　The mantle consists of three regions each with different main constituent minerals: upper mantle (60–410 km in depth), mantle transition zone (410–660 km), and lower mantle (660–2900 km). The upper mantle flow causes various phenomena that occur on the surface (e.g., earthquakes, volcanic eruptions, and other phenomena of plate tectonics) because the upper mantle is the layer just below the plates on which we live.

*4 Plate
　Plates make up the uppermost layer of the solid Earth. A plate behaves as a hard rigid body because the temperature of the plate is low. In contrast, the temperature of the mantle is high and on a geological timescale, the mantle behaves as a viscous fluid as if it were made from starch syrup.

*5 Dislocation creep
　Dislocation creep is the most common theoretical model that explains the deformation mechanism of crystals under high temperatures. This theoretical model can be applied when the deformation of crystals is controlled by the motion of nanometer-scale dislocations (linear defect).

*6 Geophysical observations
　The viscosity of the mantle is estimated from the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period (i.e., postglacial rebound). On the basis of the measurement of the rise of land, the viscosity of the upper mantle is estimated to be almost constant regardless of depth.

*7 SPring-8
　SPring-8, located in Harima Science Park City, Hyogo Prefecture, Japan, is owned by RIKEN and managed by JASRI. It is a large synchrotron radiation facility and delivers the most powerful synchrotron radiation currently available. The name “SPring-8” is derived from Super Photon ring-8 GeV (8 GeV, being the power output of the ring). Consisting of narrow, powerful beams of electromagnetic radiation, synchrotron radiation is produced when electron beams, accelerated to nearly the speed of light, are forced to travel in a curved path by a magnetic field.

*8 Grain boundary sliding
　The model of grain boundary sliding is the most recent theoretical model among those that explain the deformation of crystals under high temperatures. Polycrystals (aggregates of single crystals, synonymous to rock) are deformed by the micrometer-scale sliding of a grain boundary between two grains. This model has been treated as unimportant compared with preceding models such as models of dislocation creep and diffusion creep.

*9 Deformation-DIA (D-DIA) apparatus
　The D-DIA apparatus is an advanced type of multianvil apparatus designed to generate high pressure, high temperature, and differential stress. Pressure is generated by advancing six anvils, and the motion of the upper and bottom anvils is controlled to deform a sample placed at the center of the apparatus.

For more information, please contact:
Geodynamics Research Center (GRC) of Ehime University
Tomohiro Ohuchi (assistant professor)
E-mail：mail1