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    Solid earth sciences

    201604201604

    Sound velocities of bridgmanite from density of states determined by nuclear inelastic scattering and first-principles calculations

    McCammon C, Caracas R, Glazyrin K, Potapkin V, Kantor A, Sinmyo R, Prescher C, Kupenko I, Chumakov A, Dubrovinsky L

    elasticity, perovskite, nuclear resonance, sound velocity, lower mantle, ab initio calculations

    Influence of pressure on VD for bridgmanite. Experimental values from the present study are indicated by large colored circles, while Brillouin scattering data from the literature are indicated by small black triangles (Jackson et al. 2005) and small black circles (Murakami et al. 2007). Values from a previous computational determination of the elastic tensor from finite differences (Caracas and Cohen 2005) are indicated as red triangles for MgSiO3 bridgmanite and red squares for antiferromagnetic FeSiO3 bridgmanite

    Sound velocities of bridgmanite measured in the laboratory are a key to deciphering the composition of the lower mantle. Here, we report Debye sound velocities determined using nuclear inelastic scattering (NIS) for one majorite composition (Mg0.82Fe0.18SiO3) and five bridgmanite compositions (Mg0.82Fe0.18SiO3, Mg0.86Fe0.14Si0.98Al0.02O3, Mg0.88Fe0.12SiO3, Mg0.6Fe0.4Si0.63Al0.37O3, Mg0.83Fe0.15Si0.98Al0.04O3) measured in a diamond anvil cell at pressures up to 89 GPa at room temperature. Debye sound velocities for majorite determined from NIS are consistent with literature data from Brillouin scattering and ultrasonics, while Debye sound velocities for bridgmanite are significantly lower than literature values from the same methods. We calculated partial and total density of states (DOS) for MgSiO3 and FeSiO3 bridgmanite using density functional theory and demonstrate that Debye sound velocities calculated from the reduced DOS using the same approach as for the experimental data (i.e., the limit of D(E)/E2 as energy goes to zero) give the same sound velocities for each phase irrespective of which partial DOS is used. In addition, we show that Debye sound velocities calculated using this approach are consistent with values obtained from the calculation of the full elastic tensor. Comparison of the calculated DOS with the one obtained from NIS indicates that the experimental DOS has enhanced intensity at low energies that leads to a different slope of the DOS and hence a lower sound velocity. This effect is present in all of the bridgmanite samples examined in this study.