ANISOTROPY CHANGES DURING DEFORMATION AND PHASE TRANSFORMATIONS

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Large regions of the Earth are anisotropic for propagation of seismic waves. This is best understood for the upper mantle where anisotropy is attributed to preferred orientation of olivine crystals, attained during convection. Anisotropy is extreme in sheet-silicate-rich rocks in the crust such as shales and slates, especially important because of the significance for hydrocarbon exploration. It also is present in the lower mantle, particularly the lowermost D" zone at the core-mantle boundary. Much of the anisotropy is caused by deformation and associated alignment of crystals, but the detailed mechanisms in the deep Earth are still poorly understood. This project will explore macroscopic deformation and associated anisotropy development in the Earth by investigating the underlying microscopic mechanisms with novel methods that have become available at National facilities, such as synchrotron X-ray and neutron diffraction, and then applying physical models to project the large picture. The project will provide data that are critical for interpreting macroscopic seismic observations. But research methods that will be advanced are relevant for a broad range of physical sciences as well as engineering. In situ observation of changes during phase transformations has direct application in materials science (e.g. transition metals, high temperature superconductors) and civil engineering (e.g. cement minerals). The experimental methods, data analysis, as well as modeling capabilities will become available to other researchers through interdisciplinary collaborations and disseminated to the scientific community with workshops and training. Participation of students will continue to be an important part of this project. During the last years the PI and collaborators have performed in situ deformation experiments with diamond anvil cells (DAC) and multi anvil apparatus at the Advanced Light Source (LBL) and the Advanced Photon Source (ANL) on systems such as perovskite, postperovskite and magnesiowuestite, and were able to infer deformation mechanisms by comparing experimental preferred orientation patterns with predictions from polycrystal plasticity theory. When applying the microscopic mechanisms they could propose an explanation for macroscopic seismic anisotropy in D". However, conditions of experiments are still far from the real Earth. They have initiated a program to work at higher temperature and slower strain rates. A major focus is to develop reliable heating techniques for the deformation DAC and apply them to study plastic deformation, recrystallization and phase transitions of high pressure phases. A second focus are polyphase materials whose deformation behavior is still largely enigmatic, for example perovskite (strong)-magnesiowuestite (weak) mixtures. Also here experimental results will be compared with plasticity models. The new full-field fast Fourier transform formulation developed at Los Alamos which takes local interactions between grains into account has great possibilities. While emphasis is on the deep Earth, the PI and his team also plan to pursue recent work on quartz where mechanical twinning and residual stresses are potential paleopiezometers. Preliminary deformation, X-ray microdiffraction and EBSD experiments are promising for not only estimating magnitudes but more importantly the directionality of the stress field.