Effect of Partial Melting on Elastic Properties of Rocks at Mantle Conditions

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The temperature within the Earth is high enough to melt rocks as evidenced by volcanoes and volcanic rocks. It is thereby probable that parts of the Earth's crust and mantle are partially molten even though they are dominantly solid. The goal of this proposal is to improve the understanding of the relationship between the state of partial melting and changes in elastic properties of the media. The focus will be the upper mantle of the Earth. Since elastic properties quantitatively define the speed that seismic waves propagate through the media, the results of this research will enable better associations between maps of seismic speed variations and degrees of partial melting. The approach of this program is to probe the physics and chemistry basics of melting and mechanics yielding a deeper understanding of the underlying principles in order to provide a robust model that can then be applied to a wide variety of environments. While the focus of the proposal is the upper mantle region of the Earth, an improved model will also be relevant to the crust. Indeed, the results could also have applications in defining this relationship in industrial settings in detecting incipient stages of melting of various materials. This program provides a cutting-edge learning experience for students including graduate students in the preparation of their PhD and undergraduate 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.The authors have recently demonstrated that the current models of the mechanical properties of solid/melt combinations are inadequate and have proposed that the interaction between the solid and melt has a significant effect on the elastic properties. The new model, "dynamic melting," changes the predicted effect of melting on the relationship between the bulk modulus and the shear modulus. In the dynamic melting model, the kinetics of melting introduces a change in the frequency dependence of the elastic properties. The current proposal is to carefully test the dynamic melting model by studying a broader range of chemistry of the material and alter the experimental conditions and protocols in order to compare the predicted changes with the experimentally observed changes. The experiments are made possible by new high pressure capabilities on multi-anvil high pressure apparatus installed at synchrotron light sources. These systems enable the generation of pressure and temperature of the upper mantle on samples that can then be axially compressed, with the synchrotron yielding images of the sample with very high precision measurements of length, and diffraction profiles that yield state of stress.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.