Large regions of the earth are anisotropic for propagation of seismic waves. This is best understood for the upper mantle where anisotropy is attributed to preferred orientation of olivine crystals, attained during convection. Anisotropy is also present in the crust, where macroscopic and microscopic fractures have a significant influence, and in the lower mantle (particularly the lowermost D? zone), and the inner core. While there is general agreement that anisotropy is caused by deformation, the detailed mechanisms in the deep earth are poorly understood. Deformation experiments at ultrahigh pressure and temperature are the focus of this project to explore deformation mechanisms in the principal lower mantle minerals perovskite, postperovskite, magnesiowuestite and ringwoodite. Diamond anvil cells can be used not only to produce pressure but also stress, and in radial diffraction geometry changes in the preferred orientation pattern can be observed in situ. Most recently these experiments can be performed both at pressure and temperature and we propose to study variations of orientation patterns and thus deformation mechanisms with pressure and temperature. Such data can then be used to infer the development of anisotropy in the mantle, combined with geodynamic models. Plasticity models will be modified to take very large strain into account and then applied it to mantle conditions.
While emphasis is on the lower mantle, we also plan to pursue recent work on quartz where the role of mechanical twinning remains enigmatic. From our latest phase transformation experiments it appears that the trigonality of quartz textures develops early during deformation and recrystallization and is only indirectly affected by twinning. One example where twins can be used as a paleopiezometer is during shock by meteorite impact or conceivably seismic rupture.
