LABORATORY AND SEISMOLOGICAL OBSERVATIONS OF LOWER MANTLE ISOTROPY

We have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle. Based on observations of shear wave splitting, we conclude that the lower mantle is effectively isotropic. This result is surprising since the lower mantle is composed of elastically anisotropic minerals (silicate perovskite, MgO, SiO2) that have been extensively strained by geologic processes. To reconcile the seismic observations, we have measured texture development during deformation and high temperature recrystallization of mantle silicates. We show that these experiments provide a direct explanation for the shear wave splitting measurements: the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization.