Rheology of Orthopyroxene

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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.