Rheology of Mantle Rocks at High Pressure

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The Earth is a dynamic body, constantly evolving, changing the shapes of oceans, building mountains. Our understanding of these phenomena includes a recognition of the plastic behavior of minerals at high temperature. Plate tectonics asserts that stable surface plates are mechanically isolated from the deep Earth. Thermal evolutionary models transport heat by vertical flow of minerals. The rebound from glaciers is testimony to the viscous flow beneath. This project is to study the effects of pressure on the plastic character of the major minerals of the Earth's transition zone. While we have vastly improved our understanding of the flow properties of minerals over the past several years, the effects of pressure induced phase transitions are still poorly constrained. The flow characteristics of the high-pressure forms of olivine and pyroxene, the phases that control flow in the deep Earth, are virtually unknown.New tools have been developed that are capable of attacking this problem. Using synchrotron x-ray sources, it is possible to measure both diffraction spectra and direct images of samples in a multi-anvil press while the sample is at high pressure and temperature. It is now possible to determine the stress field from the x-ray diffraction signal and strain from shadow-graph images. One complete data collection cycle that yields values of stress and strain takes about five minutes. Thus, time resolution of 300 seconds is possible and time derivatives of these parameters can be obtained. Stress accuracy is a few tens of MPa, and strain rates of 10-7 s-1 can be resolved. The DDIA has proven successful in generating a steady state flow condition for 10's of per cent strain. Thus, texture and defect structures can equilibrate during the experiment and quantitative flow laws obtained.This program is investigating the quantitative flow properties of wadsleyite, ringwoodite, and high-pressure garnet at high pressure. Samples are synthesized in the high-pressure laboratory at Stony Brook. The synchrotron at Brookhaven National Labs is used for the rheology study. The results are interpreted in terms of their implications on deep focus earthquakes and mantle viscosity structure. The rheology of olivine at subduction zone conditions with large strains is also being studied. The results are being quantified as a constraint on deep focus earthquakes. This research is providing a learning experience for students including graduate students in the preparation of their PhD and undergraduates students in the focus of a summer research project. The results will be used throughout the Earth sciences as fundamental information on the behavior of the Earth's interior. The tools that are developed will be made available to other researchers for studying strength related properties of materials. These tools (software and hardware) will immediately become part of the COMPRES operated synchrotron facility and available to all in the community that wish to use them.