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Vol. 83 No. 3 (2007)

Vol. 83 No. 3 (2007)

Discovery of post-perovskite phase and the Earth's lowermost mantle

  The upper left photo shows the diamond-anvil cell (DAC, 3-mm in width), a high-pressure device. Combined with heating by a laser, one can generate ultrahigh-pressure and high-temperature conditions corresponding to the deep interior of the Earth. Recently, Prof. Hirose's group at the Tokyo Institute of Technology discovered a MgSiO3 post-perovskite phase using the DAC and synchrotron X-rays at SPring-8, above 125 GPa and 2500 K corresponding to the Earth's lowermost mantle. MgSiO3 post-perovskite has a sheet-stacking structure (lower left image); the yellow octahedra and white balls represent SiO6 polyhedra and the blue balls indicate Mg ions. The discovery of this novel phase transition from MgSiO3 perovskite to post-perovskite in the D'' layer of the Earth's mantle has profound geophysical implications (right panel). The boundary between the core and mantle has long been the most enigmatic region inside the Earth. Seismological observations identified large anomalies in D'', but their origins were difficult to explain with the known properties of perovskite. Many seismological features observed in the D'' layer can be explained by the post-perovskite phase without chemical heterogeneities. In addition, the transition boundary has a large positive Clapeyron-slope, suggesting that this phase transformation promotes upwelling of high-temperature plumes from the bottom of the mantle.

Eiichi Takahashi, Professor, Department of Earth and Planetary Sciences, Tokyo Institute of Technology

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