Rheology of Multi-Phase Mantle Rocks to 800 km Depth

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A quantitative description of the plastic properties of mantle rocks at mantle conditions has long been the pursuit of geoscientists, as these properties control the motion of plates, define the nature of earthquakes, and constrain the evolution of the Earth. Over the last decade, a series of experimental technique breakthroughs have enabled the team to deform rock samples with controlled flow rates and at the pressure and temperature relevant to the Earth’s interior. However, even with increased knowledge on the rheological properties of minerals such as olivine, pyroxene, garnet, periclase, perovskite, etc., the flow law for a real rock sample is still unknown. An establishment of flow laws for composite rock requires not only the properties of its constituents, but also the understanding of interactions among them. Synchrotron X-ray radiation coupled with multi-anvil high pressure deformation apparatus, the current state of the art technique for rheological experiments, is the team's tool to probe these interactions inside the rock under mantle conditions. In this project, the goal is to (1) define the flow law of the rocks at conditions spanning the region from the upper mantle through the top of the lower mantle using peridotite as the sample. (2) Pursue specific mixing scenarios that will give added insights into the rheological properties of regions actively undergoing phase transitions and to better understand regions, such as the lower mantle, where there is a mixture of a very strong phase and a very weak phase. This project will support the training of a graduate student, and develop and extend novel experimental tools on national X-ray synchrotron beamline facilities that are open to the scientific community.The theme of this project is to provide an experimentally based reference for the viscosity of the mantle as a function of depth to 800 km. The data will naturally include the effect of mineralogical and compositional changes with depth due to pressure-induced phase transitions. By measuring rheological flow laws of a model mantle rock, the investigators expect to see the total effect of depth on the rheological properties. By comparing the rock data with olivine data, one can identify the contribution of the multi-phase aggregate properties to the effective flow law. The effect of mixing will be addressed not only with the mechanical deformation data but also on a microstructure level with post-experiment study of the final sample. This study will use multi-anvil high pressure deformation equipment at synchrotron facilities that have already provided deformation conditions to pressures equivalent to 600 km depth. This project will further develop the capabilities to achieve 800 km conditions for the highest-pressure measurements. The synchrotron X-ray provides stress and strain data in situ allowing documentation of sample evolution during deformation. In addition, stress is determined from the analysis of each diffraction peak yielding information about the stress partitioning between the minerals, and for each mineral, between sub-populations of grains that share a common crystallographic orientation relative to the stress field. All of this information constrains the deformation process in each mineral and the relative strengths of each mineral. This project will develop techniques suitable for synchrotron high-pressure research. The proposed research has potential to open new frontiers by combining different techniques and applying them to high pressure in situ studies. In particular, the experimental protocol that enables these experiments to 400 km depth is currently developed. This research program will refine the experimental tools to enable the experiments to pressures equivalent to 800 km depth. This will expand the national research base for high pressure deformation studies. The techniques that evolve from this project will be open to all users of the beamline through a proposal system to all scientists interested in carrying out these high-pressure experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.