Rheology of Mantle Minerals at High Pressure

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EAR-0229260WeidnerThe 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 is a proposal to study the effects of pressure on the plastic character of the major minerals of the Earth's mantle. While we have vastly improved our understanding of the flow properties of minerals over the past several years, the effects of pressure are still poorly constrained. Estimates of activation volume for olivine flow in the power law creep regime give rise to an uncertainty in viscosity at the base of the lithosphere of 1000 fold. This uncertainty is greater that the total effect of water content or partial melting. Furthermore, the flow characteristics of the high-pressure forms of olivine, the phases that control flow in the deep Earth, are virtually unknown.The investigators have been developing new tools 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. They can now determine the stress field from the x-ray diffraction signal and strain from shadow-graph images. One complete data collection cycle takes about three minutes. Thus, time resolution of 200 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 investigators have demonstrated that they can create a stress field in the sample during each of several heating/compression cycles. Thus, texture and defect structures can equilibrate during the experiment.With this award, the researchers will investigate the flow properties of olivine, wadsleyite, ringwoodite, garnet, and perovskite at high pressure. They will synthesize the samples in the high-pressure laboratory at Stony Brook, and will use the synchrotrons at Brookhaven National Labs and at Argonne National Labs for the rheology study. Results will be interpreted in terms of their implications on deep focus earthquakes and mantle viscosity structure. --