Hydrogen Disorder and Elasticity of Phase D at High Pressures

Details Hydrogen Disorder and Elasticity of Phase D at High Pressures Hydrogen Disorder and Elasticity of Phase D at High Pressures

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmdi42a..03k&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #DI42A-03

Physics

1025 Composition Of The Mantle, 3621 Mantle Processes (1038), 3909 Elasticity And Anelasticity, 7208 Mantle (1212, 1213, 8124)

Scientific paper

One of the major goals of Earth's sciences is to develop models for the evolution of our planet. This goal is directly linked to our understanding of the dynamics within the Earth's interior. It has long been recognized that small amounts of volatiles such as hydrogen can have a disproportionately large effect on viscosity. This implies less resistance to convection and shorter overturn times. Thus knowing the abundance of hydrogen and its distribution in the mantle has important implications for the evolution of our planet. However, one of the remaining question is provide observables that may aid constraining the presence of volatiles. Phase D, MgSi2O6H2, is of particular interest since it is the only hydrogen bearing phase whose stability field extends into the lower mantle. Thus, phase D is a prime candidate for hydrogen transport into the lower mantle along subduction zones. In order to investigate the elasticity of phase D, we performed static (0 K) first-principle calculations. All calculations were performed with a plane-wave basis-set using GGA-PAW potentials. We find that long H-H bonds are energetically favorable which is likely due to H-H repulsion. The equation of state of the energetically most favorable structures are consistent with previous and experimental and theoretical studies of Phase D. Our preliminary results show that hydrogen bond symmetrization occurs in the pressure range of 40-50 GPa in agreement with previous theoretical studies. The elastic constant tensors of the relevant hydrogen distributions agree to within ~0.5% at least up 70 GPa. We also find that the largest change in elasticity with hydrogen symmetrization is an increase of C33 by ~ 10%. The induced changes of compressional and shear wave speeds are similar, ~ 1% at 40 GPa. The predicted azimuthal and polarization anisotropies of shear waves decrease by only ~ 1% with symmetrization. In contrast, the azimuthal P-wave anisotropy decreases from ~ 18% to ~ 7% with hydrogen bond symmetrization. Thus, changes in azimuthal P-wave anisotropy with depth along subduction zones may allow to constrain the presence of Phase D and the transport of hydrogen into the Earth's lower mantle along subduction zones.

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