Rheology of Lower Mantle Perovskites

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The Earth is a dynamic evolving body, driven by heat sources deep within and enabled by the plastic character of the materials that make up the Earth. As observers on the outer skin of the Earth, even after four and a half billion years, we see constant reminders of this evolution in the form of earthquakes and volcanoes. Even the distinction between ocean and continent owes its origin to this dynamic process. This study is focused on the enabler of this process - the plastic nature of what we consider hard rocks. Over long times at elevated pressure and temperature, rocks behave as liquids. In recent years, tools have evolved that allow us to measure the viscous character of these hard objects at pressures and temperatures that one finds deep in the Earth. These experiments are built around a synchrotron facility that has been built at a national lab. The investigators will use both the National Synchrotron Light Source II at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory. This study will help our understanding of this dynamic evolution of the Earth. A graduate student will be supported and will participate in this research.MgSiO3 in the perovskite crystal structure (bridgmanite) is the most abundant mineral in the Earth and it's calcium counterpart (CaSiO3) is among the five most abundant minerals. The plastic nature of these minerals defines the fluid character of the deep Earth. These properties are both pressure and temperature dependent, and as of yet, unknown. Knowledge of these properties along with proposed temperature models can help constrain dynamics of the Earth evolution and current state. The plastic properties of these minerals is particularly interesting because these minerals are composed of silicon in six coordination with oxygen, while the bulk of materials studied to date have silicon in four coordination with oxygen. The investigating team proposes to study the mechanical properties of these minerals at conditions of pressure and temperature appropriate to the deep Earth. They will focus on defining a quantitative flow law that expresses the plastic properties of these materials. The plasticity of lower mantle minerals at lower mantle conditions of pressure and temperature has been beyond the experimental reach. This has left us with little mineral-based knowledge of the effective viscosity of the lower mantle at the conditions of the anticipated geotherm. Development of new deformation equipment enables a new effort to characterize the quantitative flow law of the high pressure phases at the extremes of pressure and temperature of the top of the lower mantle. This proposal is to study both the Mg and the Ca end-member silicate perovskite at these conditions. This is made possible by a new facility being installed at the new synchrotron, NSLS II, at the Brookhaven National Laboratories. The new facility will provide world class synchrotron X-ray light for these experiments with a new DT25 guideblock in a 1000 ton hydraulic press with deformation capabilities. Using standard X-ray diffraction and imaging tools, the team will be able to define stress and strain rate in the sample at the extreme conditions.