The solid portion of the Earth is extremely dynamic on a million year time scale. Plastic flow of the solid leaves a history in the form of alignment of the crystalline building blocks of rocks. Unraveling this history requires understanding the processes that create this fabric through experiments at the pressure and temperature conditions of the Earth's interior. Here we capitalize on new facilities that have been developed at the National Synchrotron Light Source to probe mineral systems subjected to the deforming conditions of the deep Earth. The goal is to better understand these processes, to define the time scale for changing the induced fabric, and to define the control of the pressure and temperature conditions on the efficiency of producing these fabrics.
The dynamic history of the Earth can now be constrained by the anisotropy of seismic wave velocities. Texture in the rocks of the deep Earth is understood to give rise to this anisotropy and it is the plastic flow of the rocks that creates the texture. This breakthrough in understanding was enabled by laboratory investigations of rock deformation at mantle conditions. The former approaches generally resolve the end-product of large deformation. The efficiency of fabric production as a function of the environmental variables is still not well defined. The degree to which small strains will create elastic anisotropies has not been experimentally quantified. We lack a clear understanding of the interactions of the grains during the texture formation such as the relative roles of recrystallization and dislocation glide. New studies need to occur at mantle pressures and temperatures to assure that the proper processes are active. The research of this proposal will provide important information about plastic deformation of solids in general. It will provide metrics that can be used to evaluate the amount of deformation and the mechanism of deformation. This provides potential tools for assessing the failure state of structural materials in engineering applications. Preliminary experiments with sinusoidal stress fields indicate that the techniques proposed here have a strong probability of success and are well defined by theoretical models.
Plastic flow of materials often has the consequence of producing a lattice preferred orientation (LPO) in the fabric of the grains that make up the material. This phenomena gives us the possibility to define 'paleo piezometers' for the Earth by relating the texture of the rock with known flow properties of the minerals. Our project was to study this process and improve our knowledge of the relationship between plastic strain and the induced LPO. We use a synchrotron x-ray source with a DDIA high pressure apparatus. The DDIA can create a deformation of a sample at high pressure and temperature. The x-rays can define the texture by sampling diffraction peaks as a function of orientation. In particular, the amplitude of the diffraction peak is proportional to the number of grains aligned with the detector. We use the DDIA to apply an oscillating stress field to define the amplitude of the diffraction peaks with the oscillation. In this study, we looked at a number of materials, namely MgO, olivine, garnet and NaMgF3 perovskite. We conclude that the LPO is reversible with oscillation. We see very strong LPO in the perovskite sample as a twinning mechanism is sensitive to stress. The MgO exhibits strong variation of LPO in agreement with models of dislocation produced LPO. We find some experiments with olivine demonstrate very strong oscillations of the LPO, while others show a much weaker variation. We have not defined the significant variables that favor one state over the other, but this will be the fodder for future studies. The goal will be to define where in the Earth, the LPO is put into place and what conditions govern the magnitude of the LPO. The final product will allow us to understand the boundaries of elastic anisotropy in the mantle, the relationship to the plate tectonic process, and time scale for a particular signature to be produced. This will help unravel the tectonic history or various parts of the Earth.
