Anisotropy Changes during Phase Transformation

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When rocks are deformed or subjected to temperature and pressure changes in the earth, the orientation of crystals that compose the rock changes. These orientation changes, known as texture, are recorded by seismologists in the form of directionality (anisotropy) of seismic wave speeds and have been documented for large parts of the earth. This proposal addresses the changes in macroscopic anisotropic properties and tries to explain them based on microscopic mechanisms, both through experiments at high temperature and pressure and by modeling. Of particular importance are phase changes when minerals transform into different structures. With the advent of new instrumentation such as the new neutron diffractometers at Los Alamos and diamond anvil cells at synchrotron sources, it has become possible to investigate changes in anisotropy of bulk samples during deformation, recrystallization and phase transformations in real time, at a wide range of temperature and pressure conditions. Preliminary studies revealed that often a close orientation relationship exists between parent and daughter phases that depends on structural similarities and the stress state imposed by neighboring grains. Materials of primary interest in this proposal are quartz (and implications for metamorphic rocks to interpret the ancient stress field), fcc-bcc metals (as a complex model system to investigate detailed mechanisms, with relevance for the core) and the olivine-ringwoodite-perovskite+periclase transformations (essential to explain anisotropy in the mantle). The first two will be investigated with neutron diffraction at high temperature at LANSCE (and later perhaps at SNS). The last system will be studied with diamond anvil cells at APS and ALS. Results from these experiments, together with other available data, will be used to model the development of texture and anisotropy in the lower mantle. Anisotropy, texture and phase transformations are issues that go far beyond geophysics and results from this research will have direct impact on materials science. Methods of quantitative macroscopic texture analysis with neutrons and X-rays as well as microscopic analysis with synchrotron beams and electron microscopy that are being developed can be applied to such diverse materials as metals, high temperature superconductors and bones where crystal orientation is critical for functionality.