Collaborative Research: Experimental determination of the influence of water on the viscosity of rocks

点击翻译按钮获取中文摘要

At the high pressures and temperatures prevailing within the Earth, and over geological times, solid rocks can flow like viscous fluids. The flow of rocks is involved in important geological processes such as the movement of tectonic plates. It also controls Earth’s surface rebounds after the melting of large ice sheets, and the way stresses buildup on earthquake-producing faults. Models of these processes, which inform past and future events, rely on our understanding of how rocks flow. Estimates of rock viscosity - that is, their resistance to flow - come primarily from laboratory experiments. Much effort has been directed towards measuring and predicting rock viscosity in various conditions. The amount of water present in minerals has been shown to have a critical effect on rocks viscosity at high temperature. Yet, there are essentially no data on this effect at low temperature. This limits our ability to understand processes within tectonics plates and assess the corresponding hazards. Here the researchers fill this data gap by carrying out low-temperature deformation experiments on wet rocks. They study two important minerals, olivine and quartz, which are major constituents of the Earth's mantle and continental crust. They use state-of-the-art high-pressure devices set at a national synchrotron facility. There, powerful x-rays allow measuring the viscosity of cold rocks at the extreme conditions of Earth’s interior. Coupled with deformation tests at room pressure and theoretical modeling, these data gradually unveil the effect of water on the flow of cold rocks. This project also provides support for two female early-career scientists, as well as training for undergraduate students, notably from groups underrepresented in Science. Here, the team aims to determine the microphysical mechanism(s) of water weakening at low to moderate temperatures. Several theoretical models exist to describe how water affects mineral flow. Yet, their predictions vary greatly which allows testing them experimentally. The team carry out experiments to determine whether dislocation velocity is controlled by the concentration of kinks or the velocity of kinks. Samples are hydrated during synthesis at high temperature in a Paterson rig at the University of Minnesota. Deformation experiments are conducted with instrumented nanoindentation at the University of Pennsylvania, and with the Deformation-DIA at the Advanced Photon Source (Argonne National Laboratory). Run-product microstructures are characterized by electron backscatter diffraction (EBSD). Nanoindentation experiments are conducted at room temperature and allow material behavior to be evaluated for different components of the microstructure. Deformation-DIA experiments, conducted at temperatures ranging from room temperature to 1000°C, allow the bulk material behavior to be evaluated. The project’s ultimate goal is to evaluate the effect of water - i.e., hydroxyls dissolved in minerals - on important geological phenomena. These include the flexing of plates at subduction zones, the relaxation of stresses at the base of fault zones, and the evolution of roughness on frictional fault surfaces.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.