The Seismic anisotropy of the Earth's mantle: From single crystal to polycrystal

Abstract

The anisotropic single crystal seismic properties are reviewed in the light of recent experimental and theoretical determinations. Although considerable progress has been made on the determination of single crystal properties, data are still lacking, particularly for the temperature derivatives of transition zone and lower mantle phases. The common types of LPO of olivine, opx and cpx are presented together with their associated seismic properties. It is emphasized that simple seismic symmetry pattern of upper mantle rocks are a direct result of the interaction of olivine, opx and cpx. Using the standard structural frame (X lineation, Z pole to foliation) for typical LPOs of olivine, opx and cpx have the maximum Vp parallel to X, in the XZ plane and parallel to Y respectively. Destructive interference occurs between these minerals and hence P-wave anisotropy should be sensitive to the aggregate composition. For shear wave splitting (dVs) typical olivine and opx LPOs result in similar patterns with the maximum dVs in the YZ plane and the fast split shear wave (Vs1) polarized parallel to the foliation. A typical cpx LPO on the other hand produces destructive interference as the max dVs is close to X. By comparison with experiments and numerical simulations, it is estimated that upper mantle samples have an olivine LPO strength which recorded shear strain gamma of between 0.25 and 2.0. Pyrolite and piclogite models are compared with global transverse isotropic models. The slowly reducing P-wave anisotropy in the first 200 km can be explained by a model with constant composition and LPO strength. The sharp decrease in the observed anisotropy in the global models cannot be explained by the transformation of opx to cpx at 300 km, it is proposed that this decrease is due to a reduction in LPO strength from 200 to 350 km at the base of the lithosphere.

Elasticity of (K,Na)AlSi3O8 hollandite from lattice dynamics calculations

“…Even if it would not be seismically visible by itself, the presence of hollandite in subducting slabs would release some of the constraints that are imposed on the lattice-preferred orientation of olivine and pyroxene crystals (Mainprice et al, 2000, Karato et al, 2008 to explain the seismic observations. Thus our results show that further mineralogical and seismic models of K-rich regions of the upper mantle should also consider the seismic signature of hollandite and of its ferroelastic transition.…”

Structural and elastic anisotropy of crystals at high pressures and temperatures from quantum mechanical methods: The case of Mg2SiO4 forsterite

“…Among other fields, geophysics would particularly benefit from the knowledge of structural and elastic response of minerals at earth's mantle conditions (pressures up to 140 GPa and temperatures up to 2000 K), 20 as only such a full characterization of all the major constituents of the mantle could finally allow for a correct interpretation of seismological data and for the validation of different compositional models for the earth's deep interior which have been proposed so far. [21][22][23][24] From an experimental point of view, a large amount of X-ray diffraction studies were performed in past years in order to measure the isothermal equations of state of several rock-forming minerals to determine their isotropic equilibrium bulk modulus and its pressure and temperature derivatives.…”

“…27,28 Most crystals of the earth's mantle are structurally and elastically anisotropic; rocks from the mantle do show mineral preferred orientation and a large degree of alignment due to stress, flow fields, and high temperatures which make recrystallization a likely process. 20,[29][30][31] It is now accepted that the top 200 km layer of the mantle behaves as an anisotropic medium on a global scale. 32 Elastic anisotropy gives rise to several subtle features such as the azimuthal anisotropy of both longitudinal and transverse seismic wave velocities (i.e., elastic wave velocities depend on propagation direction) and the shear-wave birefringence, that is, the two polarizations of transverse seismic waves do travel with different velocities with respect to each other.…”

Single-crystal elasticity of SrCO3 by Brillouin spectroscopy

“…The v p anisotropy can be analyzed using the anisotropy factor A p [30] which is defined as: [31] in which the full tensorial character of the elastic stiffness is used:…”