Grain-scale fabrics in a two-phase matrix-inclusion system. Right-lateral shear deformation is imposed by rigid plates (shown in red) that slide in opposite directions. Lineated fabrics occurs when inclusions are harder than the matrix, whereas foliated structures appear when inclusions are weaker than the matrix. The latter structures may be particularly important in a basalt-rich transition zone in generating seismic anisotropy. Credit: Faccenda et al. [2019], Figure 2g and 2c
Source: Journal of Geophysical Research: Solid Earth

Directional dependence of seismic wave speed, so called seismic anisotropy, has potential to reveal important information on Earth’s interior structure and state of deformation. The sources that generate seismic anisotropy are generally divided into intrinsic and extrinsic sources. The former refers to mineral intrinsic properties, where crystal preferred orientation is mainly of importance. Extrinsic sources result from various factors, notably layering of minerals and rocks but also lenses of melt, cracks and grain boundaries.

Previous seismological studies have noted the difficulty in resolving intrinsic from extrinsic sources of anisotropy. Faccenda et al. [2019] focus on extrinsic anisotropy arising from layering that occurs both on the scale of grains, smaller than one centimeter in size, and rocks that consists in aggregates of minerals with specific composition that occur in layers ranging from centimeters to kilometers in thickness. They present model results of seismic anisotropy for different settings of the whole mantle, based on predictions of thermodynamically consistent phase equilibria [Stixrude and Lithgow-Bertelloni, 2011] and modelled grain-scale fabrics.

Although the predicted seismic anisotropy from compositional layering is generally small, the new modelling results predicts notable (several percent) seismic anisotropy when foliated fabrics are present in a basalt-rich transition zone. These results may provide an explanation for the observed seismic anisotropy in this region that separates the upper and lower mantle, in particular for the lower transition zone, which is considered intrinsically isotropic.

Citation: Faccenda, M., Ferreira, A. M. G., Tisato, N., Lithgow‐Bertelloni, C., Stixrude, L., & Pennacchioni, G. [2019]. Extrinsic elastic anisotropy in a compositionally heterogeneous Earth’s mantle. Journal of Geophysical Research: Solid Earth, 124.

—Bjarne Almqvist, Associate Editor, JGR: Solid Earth

Text © 2019. The authors. CC BY-NC-ND 3.0
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