Plate tectonics on Earth are linked to solid-state convective flow in the mantle. Olivine is the primary mineral phase in the upper mantle. Its physical properties are critical to modeling mantle processes and, therefore, understanding plate tectonics. Laboratory-synthesized, olivine-rich rocks form the starting material for many experimental investigations of upper mantle processes (e.g., creep behavior, ionic diffusion and grain growth). We developed an improved method for synthesizing highly dense olivine aggregates. Subsequently, we ran a series of deformation and static annealing experiments to examine grain-scale microstructural changes under variable stress conditions.
Hot-pressed olivine aggregates have been used as the starting material for many experimental studies. In previous research, olivine powders were sealed in a metal can and compacted at high temperatures under isostatic confining pressure, at roughly 1250°C and 300 MPa. These samples are opaque and retain a small amount of porosity. To limit trapping of pores, we developed an improved hot-pressing method wherein the pore space is evacuated during densification. This method produced translucent ceramics that are nearly pore-free. Static annealing experiments showed dramatically enhanced grain-growth rates in evacuated hot-pressed olivine relative to conventional methods.
Further, we used evacuated hot-pressed olivine as the starting material for a series of deformation and static annealing experiments designed to understand transient microstructural evolution in the Earth's upper mantle. Olivine aggregates were deformed to high shear strains in torsion in a gas-medium, high-resolution deformation apparatus at 1250°C and 300 MPa confining pressure. The deformed samples were then statically annealed in the same apparatus. Sections of the deformed and annealed samples were analyzed by electron backscatter diffraction. Olivine aggregates deformed to high shear strains showed grain-size refinement, the formation of strong crystallographic preferred orientation (CPO), and intragranular crystallographic distortion. Subsequent static annealing exhibited two regimes of microstructural recovery, labeled the continuous and discontinuous recovery regimes.
