Can global or regional scale seismic anisotropy in D″

<p>Slides for an invited talk at the 2011 Australian Academy of Science Elizabeth and Frederick White Research Conference "<strong>Minerals at extreme conditions – Integrating theory and experiment</strong>".</p

D" anisotropy derived from models of mantle flow: Predictions of elasticity and comparisons with seismic observations

<p><strong>Slides for invited talk at AGU 2011 Fall Meeting.</strong></p> <p><strong><br>Abstract number DI32A-02: <br></strong>We have developed an integrated model of the development of elastic anisotropy in the lowermost mantle by assuming mantle flow and deformation leads to the development of a lattice preferred orientation in post-perovskite. This texture development is simulated by tracing particles through a range of plausible mantle flow fields derived from the joint inversion of body wave travel times and geodynamic observations. Strain histories for the particles are extracted from the particle paths and these are used as the boundary conditions for polycrystalline models of texture development based on the visco-plastic self-constant approach. The resulting models of elastic anisotropy have been compared to global and regional scale observations of seismic anisotropy attributed to post-perovskite in D". At one extreme, we reduce the modelled anisotropy to one displaying vertical transverse isotropy. This is incompatible with some splitting measurements from ScS and SKS phases but permits a direct comparison with the results of global scale anisotropic tomography using S-waves. In order to assess the extent of azimuthal anisotropy in D" we make use of a global database of S-wave splitting measurements. These are also compared to the models of elastic anisotropy and provide further constraints on the degree to which lattice preferred orientation can be the explanation for anisotropy in D". On the regional scale we make comparisons with recent multi-azimuthal measurements of ScS splitting where the source side and receiver side upper-mantle anisotropy has been measured and removed. These measurements offer the most rigorous test of our predictions for the anisotropy of the lowermost mantle but also lead to difficulties in describing the frequency dependent effect of rapidly varying elasticity on the simulated seismic waveforms. Key findings are that uncertainty in the flow model is relatively insignificant compared to the current uncertainty in the single crystal plasticity, which can lead to model results which are anti-correlated to each other. In models where post-perovskite deformation is accommodated by dislocations moving on (010) or (100), patterns of anisotropy are approximately correlated with the results of tomographic inversions. On the other hand, in models where dislocations move on (001) patterns of anisotropy are nearly anti-correlated with tomographic inversions. If all the seismic anisotropy extracted from global anisotropic inversions is due to the presence of a lattice preferred orientation in post-perovskite in the lowermost mantle, and if the results of the tomographic inversions are not strongly biased by the sampling geometries, these results suggest that deformation of post-perovskite in the lowermost mantle may be accommodated by dislocations moving on (010) or (100)</p

Simulating the seismic signal of phase transtions in the deepest mantle

<p><strong>Slides for invited talk at AGU 2013 Fall Meeting.</strong></p> <p><strong>Abstract number DI34A-02:</strong> The discovery of the perovskite to post­perovskite phase transition in (Mg,Fe)SiO3 explains many of the seismic observations of the lowermost mantle including the presence of multiple seismic discontinuities and significant seismic anisotropy. However, the explanations of many detailed features remain elusive. The recent discovery of a topotactic relationship between the orientation of perovskite and post­perovskite crystals in a partially transformed analogue opens the possibility of texture inheritance through the phase transition [1]. This effect must be captured in simulations designed to explain the seismic anisotropy of the lowermost mantle, especially those which link mantle dynamics with seismic observations.</p> <p><br>We have extended our previous work linking models of flow in the lowermost mantle with simulations of texture development and predictions of seismic anisotropy [2] in order to account for the topotaxy between perovskite and post­perovskite. In particular, we compare four cases: (1) As in [2], anisotropy is only generated in post­perovskite by dislocation mediated deformation dominated by one of a number of slip systems, phase transitions destroy texture and ferropericlase and perovskite dominated rocks are isotropic. (2) Although phase transitions destroy texture, ferropericlase and/or perovskite deform by dislocation motion permitting the generation of seismic anisotropy in warmer regions of the mantle where post­perovskite is unstable. We account for the possibility of the inversion of slip­system activities in ferropericlase at high pressure as suggested by models of dislocation motion based on atomic scale simulations [3]. (3) Allow texture development by dislocation motion in perovskite and post­perovskite and texture inheritance through phase transitions by the mechanism described in [1]. However, we assume that the bulk of the lower mantle deforms by a mechanism that does not lead to the development of texture and so begin the simulation from a random distribution of crystal orientations the first time the post­perovskite stability field is encountered for downward migrating packages of mantle material. (4) Allow the bulk of the lower mantle to deform by dislocation creep such that material entering the lowermost mantle for the first time is already textured, allow this texture to be inherited and further modified by strain and phase transitions.</p> <p><br>These calculations show clear differences in global and local scale elastic anisotropy in the lowermost mantle between cases where texture is allowed to persist through the phase transitions and those where it is not. On a global scale and when radial anisotropy is imposed<br>the inclusion of topotaxy results in a dramatic decrease in the strength of the degree two signal and better agreement between observations and the model for post­perovskite deformation where dislocations moving on (001) dominate. On a smaller scale we see potential signs of reflectors generated by a change in anisotropy between perovskite that has inherited a strong starting texture from post­perovskite and overlaying perovskite that has never undergone the phase transition. These observations suggest that the incorporation of texture inheritance will be an important feature of future models of anisotropy in the lowermost mantle.</p> <p><br>[1] Dobson et al. 2013 Nature Geosci. 6:575–578 [2] Walker et al. 2011 Geochem. Geophys. Geosys. 12:Q10006 [3] Cordier et al. 2012 Nature 481:177­180</p