The strength of mantle silicates at high pressures and room temperature: implications for the viscosity of the mantle

Abstract

THE rheological properties of the upper mantle, important in convective and tectonic processes, have long been studied through low-pressure creep experiments on olivine1–5. In comparison, our understanding of deformation in the deep Earth is incomplete, because there are few experimental constraints on the rheology of the transition zone (400–670 km depth) and the lower mantle. Estimates of the viscosity at these depths have had to rely on geophysical models of, for example, post-glacial uplift, the geoid and changes in the Earth's rate of rotation6–13. Recently we developed techniques for measuring solid-state creep at mantle pressures14, and here we use these methods to make direct laboratory measurements of the strength of key mantle minerals. We study (Mg, Fe)2SiO4 olivine (representative of about 60% of the upper mantle), (Mg, Fe)2SiO4 γ-spinel (which constitutes a large fraction of the transition zone) and (Mg, Fe)SiO3 perovskite and perovskite + (Mg, Fe)O magnesiowiistite assemblages (which comprise most of the lower mantle), all at room temperature and pressures of up to 60 GPa. Our results suggest that the transition zone may form a layer of relatively high viscosity or strength between the upper and lower mantle. A simple Theological model shows that this behaviour may be compatible with current geophysical observations.