In Situ Observation of Anisotropy Development With Pressure to 50 GPa in Olivine, Ringwoodite, Magnesiowuestite and Silicate Perovskite

Magnesium silicates are the dominant minerals in the earth's mantle. Constraining the development of lattice preferred orientation in these materials is important for understanding anisotropy in the deep earth. Here we report results on deformation of San Carlos olivine in axial compression studied in-situ in the diamond anvil (DAC) using synchrotron radial x-ray diffraction. In order to minimize extraneous diffraction, gaskets of amorphous boron were used. Experiments were performed at room temperature, except for brief periods of in-situ laser heating to induce the phase transformations. At all conditions, high stresses and development of preferred orientation were documented. With increasing pressure (between 9 and 43GPa) olivine [001] axes align perpendicular to the compression direction. Ringwoodite (between 25 and 50GPa), develops moderate preferred orientation with a (111) compression component. After the phase transformation (at 50GPa), perovskite and magnesiowuestite display strong transformation textures. Magnesiowuestite has a [100] maximum parallel to compression, consistent with earlier observations. Patterns in perovskite, due to peak overlaps, have not yet been deconvoluted. These preliminary results indicate that significant anisotropy changes occur during phase transformations, e.g. by topotaxial growth, nucleation under stress, or martensitic mechanisms, and accompanied by deformation. All phases appear to be ductile at the high confining pressures in the DAC, even at room temperature. This is the first evidence for preferred orientation in ringwoodite and perovskite. These results will help in establishing deformation mechanisms of these minerals and are significant for interpreting seismic anisotropy in the transition zone. In the future, to be closer to earth conditions, we plan to conduct similar experiments at temperature and pressure.