Partial Melting at Mantle Conditions: Effect on Elastic, Anelastic, and Plastic Behavior

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The temperature within the Earth is high enough to melt rocks as evidenced by volcanoes and volcanic rocks. Here we wish to address the question of how important partial melting is in defining the mechanical properties of regions within the mantle. The association of low shear wave velocity and low viscosity with partial melting has been part of the lore since the early days of plate tectonic theory. The strong rigid lithosphere defined the plates that were lubricated from below by the plastically weak aesthenosphere. The seismic verification came from the ubiquitous low velocity zone (at least under oceanic plates). The implicit assertions of this model are 1) velocity and viscosity are both affected by small amounts of partial melting, 2) shear modulus is more affected than bulk modulus, and 3) the top of the low velocity zone marks the onset of melting and the bottom of the low velocity zone is thus defined by the disappearance of partial melting and the increase of both viscosity and velocity. Laboratory studies, however, have not fully endorsed this view of the material response to partial melting. While partial melting is often associated with low seismic shear wave velocity zone (LVZ); laboratory studies have suggested that partial melting may not be effective in creating the large velocity variation in the upper mantle. Our main potential contribution to this problem comes from our capability to assess these questions over the pressure range of 0 - 15 GPa and 700 - 2000K that are relevant to the conditions of the low velocity zone. Previous studies have been capable of pressures of up to 0.3 GPa. The low velocity zone in the upper mantle is found at 3 - 7 GPa and the upper mantle itself extends to about 14 GPa. Thus, our study will be the first that can analyze these properties at the conditions where the Earth phenomena are found. We will use the facilities at the National Synchrotron Light Source where we have developed high-pressure equipment capable of achieving these goals. The use of X-rays to probe the sample provides a piezometer to measure stress and images to measure strain. Our system can operate in a DC mode to generate steady state flow and AC at frequencies of 0.1 - 0.001 Hz. This research will provide 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. The tools developed here will be useful for studying material properties under extreme environments and will be useful to develop better industrial materials.