Lattice preferred orientation in deformed polycrystalline (Mg,Fe)O and implications for seismic anisotropy in D″

Abstract

Magnesiowüstite [(Mg,Fe)O] is an important constituent of the lower mantle, probably occupying about 20–25% of its volume. Laboratory and theoretical studies have shown this mineral to be highly elastically anisotropic at lower mantle pressures and temperatures. Thus, strain-induced formation of lattice preferred orientation (LPO) in magnesiowüstite is a candidate mechanism for the origin of anisotropic structure in D″. Although observations of seismic anisotropy within D″ are robust, both the occurrence and the style of that anisotropy are spatially variable. Two hypotheses have been offered to explain the observations of D″ anisotropy: LPO of intrinsically anisotropic minerals, or shape preferred orientation (SPO), perhaps in the form of horizontal layering or oriented inclusions. To investigate the first hypothesis, we performed confined simple shear deformation experiments in the dislocation creep regime using a gas-medium deformation apparatus over a range of compositions: the MgO and FeO endmembers and three intermediate compositions. Samples were deformed at 1273–1473 K, at a confining pressure of 300 MPa, to large shear strains ( γ = 3.5–4.5) using deformation pistons cut at 45°. After deformation, the LPO was measured by electron backscatter diffraction (EBSD). The LPO produced varied for differing compositions, indicating the activity on individual slip systems and/or the nature of grain boundary migration in (Mg,Fe)O are affected by composition and/or homologous temperature. We predicted seismic anisotropy from the measured LPOs and theoretically determined single-crystal elastic constants. Anisotropic behavior predicted from LPO agrees well with observations of D″ anisotropy, so the LPO hypothesis appears to satisfy the seismological constraints. Our calculated anisotropy patterns suggest that if D″ anisotropy is due to LPO of (Mg,Fe)O, then azimuthal variations in anisotropy in the horizontal plane should be present. Such azimuthal variations are not generally predicted for SPO-type hypotheses, and this may provide a means for distinguishing the cause of D″ anisotropy.