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Published online on January 9, 2006, 10.1073/pnas.0506879103
PNAS | January 17, 2006 | vol. 103 | no. 3 | 543-546


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GEOPHYSICS
MgSiO3 postperovskite at D'' conditions

Renata M. Wentzcovitch *, Taku Tsuchiya {dagger}, and Jun Tsuchiya {dagger}

Department of Chemical Engineering and Materials Science, Minnesota Supercomputing Institute for Digital Technology and Advanced Computation, University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, MN 55455

Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved November 14, 2005 (received for review August 9, 2005)

The postperovskite transition in MgSiO3 at conditions similar to those expected at the D'' discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D'' region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are (i) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D'' discontinuity and (ii) the anticorrelation between shear and bulk velocity anomalies as detected within the D'' region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.

Earth's mantle | elasticity | D'' layer | core–mantle boundary

Conflict of interest statement: No conflicts declared.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: LM, lower mantle; PREM, Preliminary Reference Earth Model.

{dagger} Present address: Geodynamics Research Center, Ehime University, Bunkyo-cho 2–5, Matsuyama 790-8577, Japan.

{ddagger} The molecular dynamics simulation in ref. 13 used a supercell of 60 atoms and the GGA approximation for exchange correlation. The latter is the likely source of the discrepancy in BS, the adiabatic bulk modulus, displayed in Fig. 1.

§ Ref. 22 indicates the existence of excess iron in the mantle beneath Hawaii, presumably originating at the core–mantle boundary. This chemical heterogeneity should result in negative R{rho}/S (23) as appears to be observed (24).

The high P, T elasticity of MgO was presented in ref. 31.

* To whom correspondence should be addressed. E-mail: wentzcov{at}cems.umn.edu.

© 2006 by The National Academy of Sciences of the USA


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