HR: 09:15h
AN: T31F-06    [Abstracts]
TI: The Effects of Hydrogen on Electrical Conductivity in Wadsleyite and Ringwoodite: Implications for Hydrogen Content in the Mantle Transition Zone
AU: Huang, X
EM: xghuang@mail.igcas.ac.cn
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
AU: Huang, X
EM: xghuang@mail.igcas.ac.cn
AF: Yale University, 210 Whitney Ave, New Haven, CT 06511 United States
AU: * Xu, Y
EM: yousheng.xu@yale.edu
AF: Yale University, 210 Whitney Ave, New Haven, CT 06511 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, 210 Whitney Ave, New Haven, CT 06511 United States
AB: The transition zone of Earth's mantle ($\sim$410 to 660 km depth) can contain a large amount of hydrogen (water) but the exact amount of hydrogen content in the mantle transition zone is unknown. Here we determined the effects of hydrogen on electrical conductivity in wadsleyite and ringwoodite under controlled chemical environment to infer the water content in the mantle transition zone. Synthetic polycrystalline samples with varying hydrogen contents were prepared and their electrical conductivity was measured under the transition conditions using an AC impedance method for 10$^{2}$ to 10$^{6}$ Hz. Hydrogen content of each sample was measured both before and after the conductivity measurement. The change in hydrogen content was relatively small (less than $\sim$20%). The electrical conductivity in wadsleyite is similar to that of ringwoodite and is given by $\sigma$=A$\cdot$C$^{r}_{H}$$\cdot$ exp(-H$^{*}$/RT) with A=0.1$\pm$0.1 (S/m), r=0.68$\pm$0.05 and H$^{*}$=87$\pm$3 (kJ/mol) for wadsleyite and with A=1.0$\pm$0.3 (S/m), r=0.69$\pm$0.03 and H$^{*}$=104$\pm$2 (kJ/mol) for ringwoodite, where T is temperature (K), R the gas constant, $\sigma$ the electrical conductivity (S/m) and C$_{H}$ the molar concentration of hydrogen (H/10$^{6}$Si). This relation suggests that the dominant charge carrier in these minerals under the experimental conditions is free proton. The activation enthalpy determined is relatively small and consequently the hydrogen content can be well constrained with relatively small uncertainties associated with the uncertainties in temperatures. A comparison with the geophysically inferred conductivity values shows that the water content in the mantle transition zone to be $\sim$0.2$\pm$0.1 wt$%$ for the acceptable range of temperatures from 1750 to 1950 K. These values of water content significantly exceed those in the upper mantle, suggesting that not all of the hydrogen is transported with upwelling current across the 410 km discontinuity.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3904 Defects
DE: 3914 Electrical properties
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting