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Abstract:
Seismic anisotropy in Earth’s lowermost mantle, resulting from crystallographic preferred orientation (CPO) of elastically anisotropic minerals, is the most promising observable to map mantle flow patterns. The shear wave anisotropy observed in the lowermost mantle might be caused by CPO of (Mg,Fe)O ferropericlase that is characterized by large elastic anisotropy in the deep lower mantle. However, our understanding of the slip system activities of ferropericlase at conditions of the lowermost mantle is still incomplete. Here, we present results of an experimental study designed to determine slip system activities in (Mg,Fe)O at P-T conditions of the lower mantle. In-situ deformation experiments on powders of (Mg0.8Fe0.2)O were conducted in a graphite heated diamond anvil cell (DAC) up to a temperature of 1400K. Synchrotron x-ray diffraction data were fit with the program MAUD (Materials Analysing Using Diffraction) to extract textures and lattice strains. The experimental results were modelled using the Elasto-Viscoplastic Self Consistent (EVPSC) code. Our data indicate a change in slip system activities from dominant {110} to increasing {100} slip at temperatures above 1150 K and pressures corresponding to the mid-lower mantle. Our findings indicate an effect of both pressure and temperature on the plasticity of (Mg,Fe)O and, hence, pave the way to a better understanding of with a potential change of dominant slip system between 40-60 GPa in MgO predicted from numerical models (Amodeo et al., 2012). We use the results to model the possible contribution of ferropericlase CPO to observed seismic anisotropy in the D’’ layer in the lowermost mantle.
