This project focuses on study of mechanical properties of olivine, a dominant mineral in Earth's upper mantle. The objectives of this project include studies of the flow properties of individual slip system under different pressures and water containing environments. Success of the project will help to explain formation/variation of lattice preferred orientation (LPO) of this major constitution (olivine) of the upper mantle under the conditions of Earth's interior. As the LPO is believed to give rise to seismic anisotropy, this study will supply experimental evidence, from the mineral physics point of view, to understand the attenuation of seismic anisotropy observed at about 200km depth in the upper mantle, the cause of this attenuation has been one of the long standing issues for Earth science community.
The project will take advantage of state-of-the-art technical developments in high-pressure deformation experiments at synchrotron facilities of national laboratories. The deformation-DIA (D-DIA) apparatus in conjunction with a synchrotron x-ray source will be the major instrument for the experimental investigation. This apparatus has extended the pressure range for studying aggregate samples beyond 10 GPa. To study properties of individual slip systems, this project will use single crystals as the deformation specimens in a modified cell assembly. Flow properties of a-slip system, (010)[100], and c-slip system, (010)[001] will be studied individually and comparatively. In the comparative study, two samples oriented in such directions that active (010)[100] and (010)[001] slip, respectively, will be deformed simultaneously in sequence during the experiment. This will minimize the experimental error when determination of which of these two slip systems is activated at given pressure/temperature/chemical conditions. Samples recovered after deformation experiments will be examined using FTIR and TEM to determine the water concentration and microstructure of the sample.
The project will enhance the experimental capability of the high pressure facility at the X17B2 beamline of the National Synchrotron Light Source. This facility serves a large number of general users from Earth science community. The project will promote collaborations with experts/pioneers in the traditional deformation field to valid the new technology. In addition, students from underrepresented groups (women/Hispanics) will be attracted, engaged, trained during the project.
The project established experimental protocol for single crystal deformation using D-DIA at high pressure up to 10 GPa, high temperature up to 1300oC and hydrated conditions in conjunction with brilliant synchrotron x-ray source. The sample differential stress is derived from multiple x-ray diffraction measurement through detectors at different azimuth angles; sample strain rate under such stress is measured through time-resolved x-ray radiograph imaging of strain marks at each end of the sample. Sample rheological properties are therefore characterized through the stress – strain rate relation. Deformation experiments along different orientations of San Carlos olivine single crystals indicate that structural H2O in olivine influences different dislocation slip system very differently, and therefore alters the pressure of active slip system switchover with respect to "dry" sample from 8 GPa (dry condition) to 6 GPa (wet condition). To date, most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure (and low water fugacities). For the first time, the influence of water on the single crystal deformation of San Carlos olivine is characterized at the pressure and temperature conditions equivalent to the depth down to 260 km. The experimental result explains the attenuation of seismic anisotropy at about 200-220 km through the lattice preferred orientation of olivine crystals in the upper mantle and the depth variation for such seismic anisotropy attenuation through water concentration. The project trained a number of undergraduate and graduate students, and produced one complete PhD dissertation.
