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The Earth’s Mantle Has a Two-Layer Structure with Different Chemical Compositions! (Press Release)

Release Date
03 May, 2012
  • BL10XU (High Pressure Research)
—A call for a reappraisal of conventional concepts in earth science—

Graduate School of Science and Faculty of Science, Tohoku University
Japan Synchrotron Radiation Research Institute

Associate Prof. Motohiko Murakami at Tohoku University (Earth and Planetary Material Physics Research Group, Graduate School of Science),in collaboration with Dr. Yasuo Ohishi (principal researcher, Japan Synchrotron Radiation Research Institute), Dr. Naohisa Hirao (Japan Synchrotron Radiation Research Institute), Tokyo Institute of Technology, and Prof. Kei Hirose (Japan Agency fro Marine-Earth Science and Technology),conducted research on the lower mantle deep inside the earth’s interior using elastic wave*1 velocity measurement. They succeeded, for the first time in the world, in precision measurement of elastic wave velocity propagating through molten earth-mantle minerals under the most extreme conditions in the Earth’s deep crust—under pressure of one million atmospheres and at temperatures exceeding 2500°C—and discovered that the earth’s mantle has a two-layered structure with different chemical compositions. The finding holds utmost significance in that it disproves the accepted theory of earth science, i.e. the earth’s mantle has uniform chemical composition throughout, and calls for a fundamental reappraisal of the conventional concepts of the internal structure of Earth, as well as the history of its formation and evolution. The results of this research were accepted by the British scientific journal “Nature,” and issued in its online publication on May 3, 2012.

Publication:
"A perovskitic lower mantle inferred from high pressure, high-temperature sound velocity data"
Motohiko Murakami, Yasuo Ohishi, Naohisa Hirao, Kei Hirose
Nature 485, 90-94 (2012) Published online 02 May 2012

<<Figures>>

Fig.1.
Fig.1.

Seismic waves (longitudinal and transversal waves) generally tend to gain velocity as they propagate deeper into the mantle. Local crystal structure and the chemical composition of the minerals may create abrupt changes in propagation velocity (seismic discontinuities).

Fig.2.
Fig.2.

A sample is contained between a pair of single diamond crystals flattened out at their apexes and rammed down from both sides, thereby producing extremely high pressure. High temperature can be generated by irradiating the sample with a laser via heating through the diamonds. Velocities of the elastic wave propagating in the sample and its density—held at an extremely high temperature and under extremely high pressure—were determined by the irradiating laser (for elastic wave velocity) and an X-ray (for density).

Fig. 3.
Fig. 3.

A new earth interior model inferred from the results of the research: the earth’s mantle is divided into two layers with different chemical compositions. The inner layer was found to be richer in silicon than the outer layer. The results suggest that the overall chemical composition throughout the earth is similar to that of primogenial meteorites (i.e. richer in silicon than previously expected).

Fig.4.
Fig.4.

It is generally considered that the earth was covered with a sea of molten magma 4 billion or more years ago, and that an internal layered structure developed gradually over time through the process of cooling down and solidifying (crystallizing). After solidification, the mantle is thought to have undergone agitation through convective flows until a uniform distribution was achieved. This research made it clear that two distinctive layers (inner and outer) remain in the mantle and that chemical composition may differ between them. This finding indicates that mantle agitations failed to encompass the whole of the earth’s interior throughout its history, and that a two-layered convection that hindered material transport between the outer and inner layers dominated instead.

<<Glossary>>
*1 Elastic wave
A wave that propagates through a solid (an elastic body). Seismic waves are a type of elastic wave. Ultrasonic waves, with their very high frequency, propagate through elastic media as P and S waves. Because the propagation velocity does not, in principle, depend on the frequency, an ultrasonic wave can be considered to have the same nature as a seismic wave.


For more information, please contact:
　 Ph.D. Motohiko Murakami (Department of Earth Science, Tohoku University)
　　　　　E-mail : mail1