HR: 1330h
AN: V42A-0327    [PDF]
TI: Deformation of olivine at high pressures using the Deformation-DIA
AU: * Mei, S
EM: shenghua@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550 United States
AU: Durham, W B
EM: durham1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550 United States
AU: Wang, Y
EM:
AF: GeoSoilEnviroCARS, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
AB: The rheological behavior of olivine, the most abundant component of the Earth's upper mantle, under high pressures is essential for understanding the dynamic processes occurring within the Earth's interior. Conventional gas- and solid-medium experiments to date have been limited to pressures of about 3 GPa. We report here results from recent tests on olivine using the Deformation-DIA (D-DIA). The D-DIA is capable of constant-pressure deformation tests at pressures to 15 GPa and is configured to allow operation at a synchrotron x-ray beam line in order to provide in-situ measurement of pressure, differential stress, and sample length as a function of time. Experiments have been conducted on polycrystalline olivine samples cold-pressed from mixtures of olivine plus 5$%$ enstatite powder. A 1 mm long $\times$ 1.1 mm diameter sample is encapsulated with 0.025-mm thick Ni foil, and assembled along with fully-densified Al$_{2}$O$_{3}$ or MgO pistons, a boron nitride sleeve, and graphite resistance heater into a 6-mm edge length cubic pressure medium of boron-epoxy resin. During experiments, the cell is first pressurized isotropically to desired levels and then deformed in compression at constant pressure. Experiments have been conducted at constant displacement rates of $\sim$ 0.5 - 8 $\times$ 10$^{-5}$ s$^{-1}$ over axial strains of 10 -20$%$ at temperatures of 773 -1473 K and pressures of $\sim$ 5 - 6 GPa. The oxygen fugacity and silica activity of the olivine sample are buffered by Ni/NiO and the presence of enstatite, respectively. Using x-ray diffraction, we determine pressure (i.e., mean stress) and differential stress from the strain of various lattice planes measured as a function of orientation with respect to the stress field. At this point we are able to measure elastic strains from several prominent reflections in the olivine, and they indicate qualitatively that the in situ environment is significantly nonhydrostatic. For polycrystalline olivine deformed at high temperature and a constant rate of deformation, the differential lattice strains first increase steadily and then level off as deformation proceeds, indicating that deformation begins with a transient stage before reaching a quasi-steady state. In one test at 1273 K and a pressure of $\sim$ 5 GPa, it took about two hours to reach the quasi-steady state at a deformation rate of 6.5 $\times 10^{-6}$s$^{-1}$. Such observations are helpful in understanding the details of deformation behavior not normally observable in conventional deformation rigs. In addition, the length change of deforming samples is precisely measured from periodic x-radiographic images. In-situ measurement at high pressure of stress, pressure, and plastic strain is a distinct feature of this study, and opens new ground for studying other minerals at high pressures.
DE: 3630 Experimental mineralogy and petrology
DE: 3900 MINERAL PHYSICS
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting