The main goal of this joint project is to further develop the experimental techniques of studying plastic deformation under deep Earth conditions. When a large force (stress) is applied to minerals or rocks under shallow Earth conditions, they will be deformed by brittle fracture. In the deep interior of Earth, temperature is higher and then plastic deformation becomes possible. This plastic deformation helps material circulation by convection that cools Earth and causes most of geological activities including mountain building and deep circulation of water and other materials. However, to date very little is known on the plastic flow properties of materials under deep Earth conditions due mainly to the technical difficulties. For example, in the deep interior of Earth, not only is temperature high, but also pressure is high. Usually pressure suppresses atomic motion and hence plastic deformation becomes difficult under high-pressure conditions. Does the role of pressure become more important than temperature and hence the viscosity of materials increases with depth? Also most of minerals undergo a series of phase transformations. How do these phase transformations affect the plastic properties? These issues are critical to our understanding of the dynamics and evolution of Earth and other terrestrial planets.
Despite its importance, almost nothing was known about these deep earth deformation as recently as ~ten years ago. Recognizing this need, the investigators started a group effort to develop new techniques of plastic deformation under deep Earth conditions in 2002. Based on the studies during the previous funding periods, they have made major progress including the development of new types of deformation apparatus and the improvements to the stress (and strain) measurements using synchrotron x-ray sources. As a result, we can now conduct quantitative deformation experiments to ~20 GPa and ~2000 K. However, these conditions correspond only to the depth of ~500 km. Earth's mantle extends to ~2900 km. Also, there has been very poor control of water content in materials previously studied. In this new phase of technical development, the team of investigators will focus on (i) extending the maximum pressure to ~30 GPa and higher (~1000 km depth), (ii) improving the control of chemical environment (such as water fugacity) under high-pressure conditions, and (iii) improving the stress measurements through the use of new hardware and theory. These developments will allow investigation of the plastic properties of Earth materials to the conditions equivalent to the shallow part of the lower mantle under well-controlled chemical environment. Applications of these techniques will shed important new light into our understanding of dynamics of whole Earth. The project is a collaboration among teams at four institutions, and will provide enhanced infrastructure to the experimental geophysics community, including new facilities at national synchrotron beamlines that will be available to the broader community. The developments will include training and mentoring of graduate students and post doctoral scholars.
This collaborative program includes scientists from Yale, Stony Brook University, Massachusetts Institute of Technology, and University of Chicago. The underlying issue is that non-hydrostatic stress within the Earth provides the driving force for all dynamic processes within the Earth from earthquakes and volcanoes to mountain building and plate tectonics. Our quest is to understand how these non-hydrostatic stresses affect the properties of rocks from the surface to the center of the Earth. The frontier of these studies in the community focuses on increasing the pressure and temperature (hence the depth) for such measurements. We have harnessed synchrotron radiation for interrogating the stress and strain state of the sample, thereby increasing the pressure range for such measurements by about two orders of magnitude. For this research grant the following specific results were obtained: (1) The pressure range for quantitative deformation experiments has been extended, with RDA, to ~28 GPa (at ~2000 K). This will allow us to conduct quantitative deformation experiments on mineral assemblages that are stable in the lower mantle. (2) Rheological properties of wadsleyite and ringwoodite were studied under the transition zone conditions. Evidence for the Peierls mechanism, power-law dislocation creep and grain-size sensitive (diffusion) creep is found. (3) Crystalline anisotropy and lattice preferred orientation (LPO) in phyllosilicates play an important role in defining the rheology. Phyllosilicate minerals on top of a subducting slab acts as a lubrication and essentially controls the stress build-up in the slab. This information should help provide constraints on the process responsible for seismicity in the ‘ring of fire’ region of the Earth. (4) Deformation mechanisms in olivine change with pressure and temperature, thereby affecting creep law parameters in the upper mantle, especially the activation volume. With improved x-ray optics and increased speed of data reduction the widely recognized change on the olivine (010) plane has been directly observed in quasi real time. This provides an explanation for the wide range of activation volumes reported in previous studies. These observations also have an effect on the relationship between the stress field and the induced lattice preferred orientation for mineral flow as a function of depth. These data will be required to determine the ancient flow field from the seismic anisotropy. (5) Effects of texture and fabric development in multiphase assemblies of the lower mantle (silicate perovskite + ferropericlase) strongly affect rheology. Perovskite dominates the rheology of the two-phase composite when shear strains are low; at high shear strains, the two phase composite develops strong fabric, with the weaker phase forming interconnected weak layers, significantly weaken the rheology of the bulk. Pressures and temperatures up to ~40 GPa and 2000 K have been simultaneously generated in DDIA-30. (6) Elastic/anelastic properties of a peridotite with partial melting have been studied at Low Velocity Zone conditions. The elastic moduli are significantly lowered. Models that include the thermodynamics of melting need to be included in order to explain this data. We assert that this may be key in defining the seismic properties of partially molten regions. These outcomes have resulted as we design, build, and implement new facilities and software. In this process, several graduate students and post docs have been trained and moved into a career in Earth sciences. All of the development has been done at national facilities (the National Synchrotron Light Source at Brookhaven National Laboratories and the Advanced Photon Source at Argonne National Laboratories). The facilities, experimental protocol, and software remain at these facilities for all scientists to use for studying the mechanical properties of materials. They have access to these tools through the General User program at these facilities and can be awarded access time on the basis of proposals that are reviewed and awarded by the national facility.
