Fabric development in (Mg,Fe)O during large strain, shear deformation: implications for seismic anisotropy in Earth’s lower mantle

Abstract

Large strain, shear deformation experiments were performed on (Mg 1− x ,Fe x )O ( x=0.25, 1.0), one of the important minerals in Earth’s lower mantle. Deformation experiments were made on coarse-grained (∼15–20 μm grain-size) hot-pressed aggregates at conditions of T/ T m∼0.46–0.65 ( T: temperature, T m: melting temperature) and σ/ μ∼0.4×10 −3 to 0.9×10 −3 ( σ: differential stress, μ: shear modulus) up to the shear strain of ∼7.8. Under these conditions, deformation occurs by dislocation creep. The microstructural development in (Mg,Fe)O is found to be sluggish and the complete dynamic recrystallization and nearly steady-state fabric (lattice preferred orientation) are achieved only after shear strains of γ∼4. At nearly steady-state, (Mg,Fe)O shows strong fabrics characterized by the 〈1 1 0〉 axes being parallel to the shear direction and the poles of the {1 0 0} planes (and to a lesser extent the poles of the {1 1 1} planes) normal to the shear plane. The seismic anisotropy corresponding to the deformation fabrics in (Mg,Fe)O was calculated. The nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution. Anisotropy changes also with depth (pressure) due to the large variation of elastic anisotropy of (Mg,Fe)O with depth. Seismic anisotropy caused by the deformation fabric of (Mg,Fe)O is very weak in the shallow lower mantle (<0.1%), but it becomes strong in the deeper portions due to the high elastic anisotropy of (Mg,Fe)O. Near the bottom of the mantle, the steady-state fabric of (Mg,Fe)O corresponding to the horizontal shear will result in ∼1–2% V SH> V SV anisotropy (assuming that (Mg,Fe)O occupies ∼20% volume fraction of the lower mantle) and little shear wave splitting of vertically travelling waves, a result that is consistent with the seismological observations in the D′′ layer of the circum-Pacific regions. Thus, the deformation fabric of (Mg,Fe)O is a vital candidate of the cause of seismic anisotropy in these regions. Anisotropy caused by the lattice preferred orientation of (Mg,Fe)O has a distinct azimuthal anisotropy with a strong 4 θ term ( θ: azimuth): the direction of propagation of the fastest (slowest) SH (or P) wave is parallel (perpendicular) to the flow direction and the slowest (fastest) SH (or P) wave is at 45° from these two directions.