UC Berkeley

Large scale deformation in the Earth linked to mantle convection and plate tectonics causes plastic deformation of minerals in the lower crust and the mantle. During plastic deformation dislocation slip may occur, causing crystallographic planes to reorient. Observations of seismic anisotropy have been linked to crystallographic preferred orientation, but many questions and details remain unresolved.

Mineral physics aims to better constrain composition and evolution of the interior of the Earth with deformation experiments of Earth materials at high pressure and temperature. This thesis aims to provide further insight into crystallographic preferred orientation (CPO) and deformation mechanisms active at high pressure. Preferred orientation of iron-rich magnesiowustite (Mg,Fe)O, a major mantle mineral phase, stishovite (SiO₂), the high pressure polymorph of quartz that is likely present in the lower crust and mantle, and in NaMgF₃ + NaCl, an analog system to lower mantle minerals MgSiO₃ + MgO, have been examined with synchrotron X-ray diffraction while at high pressure in either a diamond anvil cell or a multianvil press.

Magnesiowüstite, (Mg_0.08Fe_0.88)O, and wüstite, Fe_0.94O, were compressed up to 37 GPa at ambient temperature in diamond anvil cells (DAC) at the Advanced Light Source (ALS). X-ray diffraction patterns were taken in situ in radial geometry in order to study the evolution of CPO through the cubic-to-rhombohedral phase transition. Under uniaxial stress in the DAC, cubic texture developed (i.e. {100}_c planes aligned perpendicular to the compression direction). Variant selection of preferred orientation was observed immediately following the transition to the rhombohedral phase. Specifically, the {100}_c in cubic became {01-12}_r in rhombohedral and remained aligned perpendicular to the compression direction. However, the {101}_c and {111}_c planes in the cubic phase split into {10-14}_r and {11-20}_r and (0001)_r and {10-11}_r, respectively, in the rhombohedral phase. The {11-20}_r planes preferentially aligned perpendicular to the compression direction while {10-14}_r oriented at a low angle to the compression direction. Similarly, {10-11}_r showed a slight preference to align more closely perpendicular to the compression direction than (0001)_r. This variant selection may occur because the <10-14>_r and [0001]_r directions are the softer of the two sets of directions. The rhombohedral texture distortion may also be due to subsequent deformation. Indeed, polycrystal plasticity simulations indicate that for preferred {10-14}<1-210>_r and {11-20}<-1101>_r slip and slightly less active {10-11}<-12-10>_r slip, the observed texture pattern can be obtained. Upon decompression in the DAC, FeO reverted back to cubic symmetry and the cubic texture reappeared, demonstrating that the transition is reversible and has texture memory.

The crystal structure of the high pressure SiO₂ polymorph stishovite has been studied in detail, but little is known about texture development during deformation, which provides information for understanding subduction of quartz-bearing crustal rocks into the mantle. Radial DAC experiments were done at the ALS and the Advanced Photon Source (APS) while collecting X-ray diffraction patterns in radial geometry to examine in situ development of CPO. Starting pressure in the sample chamber was still in the quartz stability field, and compression of quartz produced a weak texture, likely due to Dauphiné twinning. Following compression of quartz into the stishovite stability field, near 13-16 GPa, the sample was laser heated to activate kinetics and transition to stishovite. Stishovite nucleated with (001) planes preferentially aligned perpendicular to compression. Increased preferred orientation during further compression up to 38 GPa is attributed to slip. Slip systems responsible were inferred from visco-plastic self-consistent modeling and are most likely basal and pyramidal slip at experimental conditions.

While much is known about preferred orientation in single phase rocks, deformation of polyphase rocks is largely unexplored. Nearly all of the Earth is composed of polymineralic aggregates, including the lower mantle, which is of critical importance for understanding the geodynamic evolution of the planet. Geodynamic models predict large strains due to convection in the mantle, and polycrystal plasticity simulations suggest strong preferred orientation. However, these models ignore interaction among phases, which is important for the lower mantle, estimated to be composed of ∼25% soft magnesiowüstite (Mg,Fe)O and ∼70% harder Mg-perovskite (MgSiO₃). How much preferred orientation develops as a result of large strains in the lower mantle depends on the volume percent ratios and arrangement of the two phases. If grains of the softer phase, MgO, become interconnected, they may act as a lubricant between grains of the harder phase, thereby absorbing most of the deformation. Alternatively, the soft phase may sit in pockets in between harder MgSiO₃ grains, and thus not be interconnected, leaving MgSiO₃ to bear the load. In the former case, MgO will control the deformation, and in the later, MgSiO₃, and the development of CPO in these two cases may greatly differ.

To study CPO development in a two-phase system, deformation experiments were performed in the deformation-DIA (D-DIA, DIA being shortening of "diamond") at the APS while collecting X-ray diffraction patterns in situ. While the D-DIA offers more control over deformation conditions and temperature and can deform larger samples, resulting in better counting statistics than for DAC samples, it cannot reach pressures beyond 12 GPa or temperatures higher than 2000 K. Thus neighborite (NaMgF₃) and halite (NaCl), which have the same structures and relative strengths as mantle minerals silicate-magnesium pervoskite (MgSiO₃) and periclase (MgO) but deform more easily, were chosen as analogs. Information on grain structure and distribution before and after deformation was collected using X-ray microtomography, both at the APS and the ALS.

Results from D-DIA experiments show that when present in as little as 15%, the soft phase absorbs much of the deformation, greatly reducing CPO of the harder phase. Conversely, CPO in NaCl is highest for the sample with highest NaCl content. This suggests that CPO develops and evolves best in rocks largely composed of one mineral phase, and the presence of a second phase greatly hinders CPO, even at high strain, likely due to greater activity of deformation mechanisms which do not produce CPO. In addition, microtomography data shows that soft NaCl is largely connected and surrounds the harder grains of NaMgF₃. Deformation of NaMgF₃ and NaCl was simulated with both a self-consistent polycrystal plasticity model and with another polycrystal plasticity model which uses fast Fourier transform that takes into account interactions among grains and is better suited to study deformation in two-phase materials. Implications for the lower mantle are discussed.