Grand Challenge for Experimental Study of Plastic Deformation Under Deep Earth Conditions

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.