Heat dissipation by convection of the Earth's mantle drives plate tectonics and deformations in the Earth's crust. These deformations cause earthquakes and fertile mantle to conditions which cause melting and, therefore, volcanoes, both of which have a significant impact on society. Therefore, it is important to understand the mechanisms that enable heat dissipation of the Earth's mantle. The Earth's upper mantle is a polyphase system, composed in bulk of approximately 55% olivine, 25% orthopyroxene, 15% clinopyroxene and 5% aluminous phases (Ringwood, 1975). However, secondary phases (primarily clinopyroxene and aluminous phases) are preferentially removed during partial melting of the lithospheric mantle, leaving a residue of olivine and orthopyroxene (e.g. dunites and harzburgites). As a result, understanding the deformation mechanisms of the most dominant mineral, olivine, and how its rheology is affected by various environmental conditions (pressure, temperature and chemical environment) and physical factors (grain size and distribution) has been a priority of field-based, experimental and modeling studies. However, to date, little work has been performed to determine the strength of orthopyroxene, which if significantly stronger or weaker than olivine, would impact the viscosity of the mantle, and thereby affect rates of convection and plate displacement. This project will use an experimental approach to investigate how strain rate, water content, temperature and pressure affect the strength of orthopyroxene aggregates. The experiments will be performed in two different high pressure rock deformation apparatus, one located at Texas A&M University and another capable of higher confining pressures located at the National Synchrotron Light Source at Brookhaven National Laboratory. Data from these experiments can be used by field-based and numerical simulations of the viscosity of the mantle at plate boundaries and may aid in our understanding of the processes that cause the deformations in the mantle that causes earthquakes and volcanic activity. This study will also provide salary and mentoring for an early-career scientist and provide at least one undergraduate student with a hands-on research experience.
The overall goals of this project are to use experimental techniques to investigate the rheology of orthopyroxene aggregates deforming by dislocation creep and how this rheology is affected by the chemical environment (water fugacity or hydroxyl content and activity of silica), pressure and temperature in an axial compression deformation geometry. We will also investigate how microstructures evolve as a function of strain by performing experiments using a shear deformation geometry. These goals will be accomplished by performing experiments in both water-present and dry conditions over a wide range of temperatures, pressures and strain rates in a Griggs-type piston-cylinder rock deformation apparatus using a molten salt cell for precise determination of mechanical data and the deformation DIA operated by COMPRES located at the National Synchrotron Light Source at Brookhaven National Laboratory.
Convection of the Earth’s mantle drives plate tectonics and the deformations of the Earth’s crust. The viscosity of the Earth’s mantle is controlled by the viscosity of the dominant mineral in the mantle, olivine. However, the Earth’s mantle may contain significant amounts (up to ~40%) of other minerals such as orthopyroxene, clinopyroxenes and garnet, of which orthopyroxene is the most common. These minerals may affect the viscosity of the upper mantle if they are significantly stronger or weaker than olivine, which could affect rates of post-seismic rebound and processes at plate boundaries. We performed an experimental investigation in order to determine how pressure, temperature and strain rate affects the strength of polycrystalline orthopyroxene and determine how its viscosity could affect the viscosity of the Earth’s upper mantle. Orthopyroxene deformed plastically by dislocation creep at all conditions of our experiments, consistent with observations of naturally deformed orthopyroxene. By performing experiments over a wide range of temperatures, pressures and strain rates, we were able to determine parameters that describe the rheology of orthopyroxene. These results indicate that orthopyroxene is stronger than olivine at the conditions of the Earth’s upper mantle and if present in significant quantities, will cause the mantle to be more viscous than if it contained olivine alone. These results will help develop more accurate models of the mantle’s viscosity and its effects on plate tectonics. We also tested two older high pressure/temperature experimental techniques and developed a new technique for deforming materials at high temperatures and pressures. We performed experiments in order to determine if it was possible to use old experimental data collected using techniques used in early high pressure and temperature experiments. These experiments demonstrated that the old techniques produced erroneous mechanical data and microstructures. We also performed experiments to determine the temperature gradients along samples deformed in the DDIA, which will help scientists interpret the results of DDIA experiments. Lastly, we developed a new experimental assembly that is stable for long periods of time at high pressure and temperature that was used in the orthopyroxene experiments described above. This assembly will be useful for exploring the deformation mechanisms of other phases stable at the high temperatures and pressures of the Earth’s mantle. In addition to the scientific results, this project supported the professional development of a post-doctoral scholar. The post-doctoral scholar had the opportunity to mentor two undergraduate students who worked on different portions of this project. The mentoring experience helped the post-doctoral scholar develop mentoring skills, helping the undergraduates develop as scientists and helping guide the students in their careers after completing their undergraduate degrees. The post-doctoral scholar also helped students taking a course in rock mechanics perform high pressure and temperature experiments that were relevant to this project.
