HR: 0800h
AN: MR41A-0894    [Abstracts]
TI: Effect of Water on Electrical Conductivity in Olivine
AU: * Wang, D
EM: duojun.wang@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06511 United States
AU: * Wang, D
EM: duojun.wang@yale.edu
AF: Institute of Geology,China Earthquake Administration, State Key Laboratory of Earthquake Dyanmics, Beijing, 100029 China
AU: Xu, Y
EM: yousheng.xu@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06511 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06511 United States
AB: Water (hydrogen) likely enhances electrical conductivity in olivine either directly through charge transport by proton or indirectly through enhancement of ionic conductivity by M-site vacancies (Karato, 1990). Although this hypothesis has been used to interpret results of geophysical measurements of electrical conductivity, there has been no experimental data to test it. In fact, the recent study on wadsleyite and ringwoodite (Huang et al. 2005) did not entirely validate Karato's hypothesis: electrical conduction in these minerals (with water) occurs through the motion of free proton and not by all protons (protons at M-site do not contribute to conductivity directly). Here we report the results of laboratory measurements of electrical conductivity in olivine as a function of water content (and other factors). Polycrystalline samples of olivine (with ~5% of opx) were hot-pressed with or without the addition of water. We control water content, grain-size and oxygen fugacity in addition to temperature (and pressure). We have evidence that a large amount of water is present in polycrystalline olivine at grain-boundaries and therefore conductivity is measured for samples with different grain-sizes. Grain-size of samples range is ~ 2 to ~ 20 microns (except for a dunite for which grain-size is ~2mm). The electrical conductivity was measured under high-pressure (2~3GPa) and temperature (900~1600K) using an impedance spectroscopy within a frequency range of 10²~10⁶Hz. Electrode or "capsule (shield)" materials are Mo, or Fe or Ni that define oxygen fugacity. Water content and grain-size of all samples were measured both before and after conductivity measurements. We minimize the water loss by choosing low voltage and high frequency, but in all cases, there is some water loss during a measurement. Our current results show clear evidence of enhanced conductivity by water, and weak dependence of conductivity on grain-size. The results will be compared with various models to identify the microscopic mechanisms of conduction and with geophysical measurements to infer the water contents in the upper mantle
DE: 3904 Defects
DE: 3914 Electrical properties
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005