Sanne Cottaar, Allen McNamara, Barbara Romanowicz and Rudy Wenk
The D" zone at the base of the mantle is a boundary layer, both chemically and thermally. Complex dynamic processes are the cause of lateral heterogeneities, with sharp boundaries at the edge of superplumes (e.g. Toh et al., 2005). Another characteristic of D" is the presence of strong and laterally varying anisotropy. A global long-wavelength model for S-wave velocity and radial anisotropy shows that, in general, SH phases are faster than SV phases (Panning and Romanowicz, 2006). This transverse isotropy is strongest in a ring around the Pacific, a.k.a. ``the slab graveyard,'' and confirmed by a large number of regional studies. These observations lead to the idea that flow causes anisotropy by alignment of anisotropic minerals. This study combines geodynamics and mineral physics to investigate seismic heterogeneities and anisotropy in the D". Different seismic velocity models are created to constrain possible microscopic and macroscopic processes.
Best fitting isotropic and transversely isotropic models for the D". Figures are strongly vertically exaggerated: dimensions are 72 degrees horizontally to 300 km vertically above the core-mantle boundary (CMB). Slab subduction takes place on the left, and upwelling on the right. A. Isotropic velocities for perovskite and postperovskite plotted relative to PREM. B. Transversely isotropic models in which blue is SH faster than SV and red the opposite.
The two-dimensional isochemical convection model is refined to emphasize deformation in D" (McNamara and Zhong, 2004). Lagrangian tracers travel through the lowermost part of the model providing strain information. Deformation is strong in the lowermost mantle, and if dislocation creep occurs, preferred orientation of minerals can cause seismic anisotropy. The exact resulting anisotropy depends on dominant deformation mechanisms and single crystal elastic constants. Using results from different experimental studies, we calculate the texturing of assemblages of perovskite and post-perovskite with periclase using VPSC (Lebensohn and Tom, 1993). Averaging the single crystal elastic constants (Stackhouse et al., 2005, Wentzcovitch et al., 2007) over the orientation of the textured grains results in the prediction of a 2D, fully anisotropic model. Here we present a least squares fit of isotropic and transverse isotropic models to the fully anisotropic models.
Results for the isotropic velocity models are presented in Figure 2.27A for 75% perovskite or 75% post-perovksite with 25% periclase. Variations are mainly due to variations in pressure and temperature and are stronger than in typical inverted models. The temperature sensitivity of the shear modulus causes strong lateral S-wave velocity variations, while the bulk modulus, and thus the bulk sound velocity, have more sensitivity to vertical pressure variations. S-wave velocity variations are predicted to be stronger for post-perovskite than for perovskite. The occurrence of post-perovskite can thus explain the increase in in the lowermost mantle in inverted models.
Transversally isotropic models are shown in Figure 2.27B, in which the case for post-perovskite is expanded for three different dominant slip planes. Although all slip planes are activated during dislocation creep, different mineral physics experiments have concluded that different slip planes are dominant (Merkel et al., 2007, Miyagi et al., 2009, 2010). As the figures show, the different assemblages and deformation regimes result in different anisotropic signatures. The post-perovskite assemblage for dominant slip planes of  and  corresponds with these observations, while the perovskite assemblage has an opposite signature.
The results for dominant slip planes of  and  have opposite signatures for P-wave anisotropy. Beghein et al. (2006) find a likely anti-correlation between P and S wave anisotropy using normal modes, similar to the  case here. A number of local studies find anti-correlation between P and S wave velocities (Wysession et al., 1999, Tkalcic and Romanowicz, 2002), as well as global studies, which find an anti-correlation between shear wave and bulk sound velocities (Su and Dziewonski, 1997). Possibly these measurements reveal the anti-correlation between SH and PH (or horizontal bulk sound velocity) caused by anisotropy, as isotropic variations (Figure 2.27A) cannot explain them.
The plumes on the right in the figures are more difficult to interpret as we use a 2D model for these 3D features, and it is more implausible for dislocation creep to occur at these higher temperatures.
From a mineral physics point of view, the occurrence of the perovskite to post-perovksite transition a couple hundred kilometers above the D" is the subject of strong debate. Shim et al. (2009) show that adding Al and Fe causes the phase transition to broaden and shift to higher pressures.
From a seismological point of view, postperovskite can explain observations of strong S-wave heterogeneities, SH faster than SV in subduction regions, and anti-correlation between S and P wave anisotropy. Additionally, seismic observations can constrain postperovskite to have a dominant slip plane along  as measured by Miyagi et al. (2010).
This project is funded by NSF's CSEDI program under grant number NSF EAR-0757608.
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