Collaborative Research: Rheology of the Earth's Transition Zone - An Integrated Approach

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Heat convection in the Earth is controlled by flow of the hot, but solid mantle. This convection drives plate tectonics, generating major societal hazards (e.g., earthquakes, volcanic eruptions, tsunamis, etc.) and controls the compositional and thermal evolution of the planet. Since the early 1960s, quantifying the deformation properties of mantle rocks has been a major goal in experimental Earth Sciences. Advancement has been limited by technology, owing to serious difficulties in conducting experiments involving deformation of minerals at the extreme pressures and temperatures prevailing in Earth's deep interior. The upper-mantle (top 410 km) consists of peridotites, rocks comprised dominantly of olivine, i.e., the semi-precious gem known as peridot. In the transition zone (410-670 km depth) at pressure in excess of 140,000 atm, olivine is no longer stable and transforms into high-pressure minerals, wadsleyite and ringwoodite, which have comparable compositions but denser structures. Decades of experimental work have provided strong constraints on olivine plasticity, yet little is known about the viscosity of minerals in the transition-zone. The aim of the present project is to provide accurate computational models for the viscosity of Earth's transition zone, which will integrate new experimental data on wadsleyite and ringwoodite obtained in state-of-the art high-pressure deformation devices set at synchrotron facilities. These experiments, involving newly developed devices and analytical techniques, are at the forefront of research on the mechanical behavior of materials at high pressure. Besides advancing our understanding of mantle convection, this program will provide support and training in modern experimental science to one graduate student as well as undergraduate students. All the new experimental and analytical tools will become available to other scientists, advancing our general knowledge in high-pressure research. The team's results will find direct applications in Geophysics and Seismology, and broader applications in Materials Science. Flow laws for Earth materials provide vital constraints on mantle dynamics, while knowledge of deformation mechanisms at the atomic scale provides insights into crucial observables such as seismic anisotropy. The complexity of the stress field within deforming rocks, which varies from grain to grain as plastic properties are anisotropic, can now be observed in situ using new high-pressure devices coupled with X-ray synchrotron radiation, and addressed by self-consistent mean-field modeling. In this project, the investigators will take advantage of these recent developments to address the plasticity of the transition zone. Specifically, they will study the flow properties of wadsleyite and ringwoodite as a function of iron and water contents, and constrain strength contrasts with olivine, using the Deformation-DIA apparatus and the newly developed D-TCup and DT-25. In situ X-ray radiography and diffraction will be used to measure strain, stress and texture (i.e., lattice preferred orientation). The new flow laws will be integrated into models for the effective viscosity and seismic anisotropy of the transition-zone. Modeling efforts will benefit from the second-order (SO) method, a recent improvement in mean-field schemes which describes accurately highly non-Newtonian materials, such as silicates. The models will account for stress-field heterogeneities due to crystal orientations, complex deformations mechanisms (dislocation glide and diffusion), incorporate several minerals, and improve confidence for extrapolation of results to geologic strain rates. The model construction will be flexible, allowing integration of additional phases and flow law parameters as they become available in the future. The outcome will give crucial insights on transition-zone viscosity, and the crystal preferred orientations that produce seismic anisotropy.