LABORATORY AND SEISMOLOGICAL OBSERVATIONS OF LOWER MANTLE ISOTROPY

We have carried out seismological and mineral physics investigations t o identify the extent and origin of elastic anisotropy in the lower ma ntle. Based on observations of shear wave splitting, we conclude that the lower mantle is effectively isotropic. This result is surprising s ince the lower mantle is composed of elastically anisotropic minerals (silicate perovskite, MgO, SiO2) that have been extensively strained b y geologic processes. To reconcile the seismic observations, we have m easured texture development during deformation and high temperature re crystallization of mantle silicates. We show that these experiments pr ovide a direct explanation for the shear wave splitting measurements: the lower mantle appears isotropic because silicate perovskite maintai ns an isotropic texture during deformation and recrystallization.