HR: 15:25h
AN: GP13A-07    [Abstracts]
TI: Anisotropy of Point Defect Mechanisms and Electrical Conduction in Single Crystal Olivine
AU: * Du Frane, W D
EM: Wyatt.Dufrane@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287 United States
AU: Roberts, J J
EM: roberts17@llnl.gov
AF: Lawrence Livermore National Laboratory, Experimental Geophysics Group 7000 East Avenue, Livermore, CA 94551 United States
AU: Constable, S
EM: sconstable@ucsd.edu
AF: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography University of California San Diego, La Jolla, CA 92093-0225 United States
AU: Toffelmier, D A
EM: dan.toffelmier@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287 United States
AU: Tyburczy, J A
EM: jim.tyburczy@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287 United States
AB: Understanding how the relative mobility of major electrically conducting point defects in olivine varies with orientation is crucial in interpreting conductivity-depth profiles of anisotropic regions of the upper mantle. We performed electrical conductivity and thermopower measurements, over mantle temperatures (1100-$1400\deg$C) and controlled oxygen fugacity, on San Carlos olivine samples, oriented along the three principle crystallographic directions cut from a single crystal. Fe-soaked Pt electrodes were used to prevent significant Fe exchange or loss in the samples; electron probe measurements before and after experimental runs prove the effectiveness of this method. Conductivity results, for various f$_{O2}$ values at $1200\deg$C, range between 1.49x10$^{-3}$ to 6.63x10$^{-3}$ S/m, which is about 2.5 times higher than previous studies on single crystal San Carlos olivine in which Ir or pure Pt electrodes were used. In accordance with previous studies, our measurements generally indicate that \sigma$$_{[001]}$ $>$ \sigma$$_{{[100]}$ $>$ $\sigma$$_{[010]}$. At a given set of conditions, conductivity varied 50-100% higher in $\sigma$$_{[001]}$ than for $\sigma$$_{[010]}$. Our thermopower measurements, ranging between 3.06x10$^{-4}$ to -0.79x10$^{-4}$ V/K, can be modeled to obtain valuable information about point defect mechanisms. Thermopower of olivine along the [001] direction decreases with increasing temperature more quickly than for other directions, crossing to negative values at log(f$_{O2}$) between -6.3 to -5.9 log units (atm) for $1315\deg$C. Negative thermopower indicates that a negatively charged defect dominates electrical conduction, which confirms previous predictions of mixed-conduction models of polaron and magnesium vacancies. This work was supported by Laboratory Directed Research and Development funding, and was performed by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 5139 Transport properties
DE: 3904 Defects
DE: 3914 Electrical properties
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting