Exploring Deformation Mechanisms at the Base of the Earth's Mantle with Combined Constraints from Geodynamics, Mineral Physics and Seismic Anisotropy.
Abstract
While the bulk of the lower mantle appears seismically fairly isotropic, the lowermost 200 - 300 km, the D'' region, is characterized by significant seismic anisotropy, particularly beneath the circum-Pacific rim. This has been attributed to crystallographic preferred orientation of MgSiO3 post-perovskite (pPv) within deforming subducting slabs or possible shape preferred orientation such as layering or melt inclusions. On average, seismological observations of radial anisotropy in D" suggest faster shear-wave velocity for shear waves polarized parallel to the core mantle boundary, except within the large low shear velocity provinces.
Here we start from a 3D geodynamic model in spherical geometry to explore deformation during mantle convection. Assuming deformation mechanisms derived from high pressure experiments and first principles calculations, we can then predict macroscopic elastic properties and anisotropy based on polycrystal plasticity models. We consider a composition of bridgmanite+ferropericlase+Ca-perovskite with bridgmanite transforming to pPv at high pressures. Compared with earlier studies (Wenk et al., 2011, EPSL; Cottaar et al., 2014, GJI), we (1) use a 3-phase system and (2) investigate the anisotropy changes during phase transitions, including topotactic relationships. We assume that half of the deformation is accommodated by dislocation glide, leading to preferred orientation, and half by diffusion creep, climb, or other non-texture forming mechanisms. In addition, we also explore the effects of partial melting that may occur in areas of high temperature upwelling. The dominant contributor to anisotropy within a slab impinging and spreading on the CMB appears to be pPv, in part due to its high single crystal anisotropy. It appears that dominant (001) slip, as observed experimentally, fits the radial and azimuthal anisotropy patterns observed by seismologists well. Moreover, complicated deformation paths predicted by the geodynamic model leads to rapidly changing anisotropy in areas of upwelling. By exploring different dominant slip systems in pPv, more complicated deformation geometries, and accounting for other contributors to anisotropy, we provide further constraints on the dynamic processes at play in the deep mantle.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI53A0040C
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRYDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 7208 Mantle;
- SEISMOLOGYDE: 8125 Evolution of the Earth;
- TECTONOPHYSICS