Experimental Deformation of Olivine Aggregates

Abstract

Mechanical modeling of deformation in the Earth’s upper mantle relies on flow laws derived from deformation experiments on olivine aggregates. Olivine is the primary phase in Earth’s upper mantle and is thought to control its bulk mechanical behavior. In particular, it is critical to understand the dependence of strain rate on grain size, temperature, and stress. Additionally, as olivine rocks are deformed their microstructures evolve by dynamic recrystallization and formation of crystallographic preferred orientation (CPO), which influence their mechanical properties. In this thesis, we add to the large body of work attempting to understand the physics of high-temperature deformation of olivine aggregates by presenting new experimental data, using both hot-pressed olivine aggregates and naturally sourced dunite rocks as starting material. In Chapter 1, the thesis is broadly summarized in an introductory chapter. This is followed by the description and characterization of a new method for synthesizing fine-grained, nearly pore-free olivine aggregates by evacuated hot pressing of naturally sourced olivine powders in Chapter 2. In this chapter, the material is thoroughly characterized, densification kinetics are examined, and differences in grain growth kinetics between this new method and conventional methods are demonstrated. In Chapter 3, we examine data from experiments on evacuated hot-pressed aggregates that were deformed in torsion in a gas-medium deformation apparatus. These data are used to understand the microstructural evolution of olivine aggregates deformed to high strain. Microstructures were characterized by electron backscatter diffraction (EBSD) to investigate grain size and CPO evolution. This is followed, in Chapter 4, by the presentation of an experimental, wherein evacuated hot-pressed olivine aggregates were deformed in a deadweight creep apparatus at 1 atm confining pressure at conditions where diffusion creep was active. The data are examined to evaluate the diffusion creep flow law for olivine. Finally, in Chapter 5, we present an experimental study aimed at measuring the anisotropy in viscosity of naturally sourced dunite rocks with a preexisting CPO. These studies, together, represent a significant contribution to our understanding of the physics of olivine deformation, and in turn, mantle dynamics.