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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2145
Electrical conductivity of wadsleyite
Mr. M.A. Geeth Mahinda Manthilake
Institute for Study of the Earth's Interior Okayama University, Japan
T. Matsuzaki, T. Yoshino, S. Yamashita, E. Ito, T. Katsura
Electrical conductivity of wadsleyite was measured in its stability field in order to examine the
conductivity variation associated with the olivine-wadsleyite phase transition. All high pressure
experiments were carried out at 16 GPa using KAWAI-type multi-anvil apparatus under controlled
oxygen fugacity. Water-doped and undoped samples were used to examine the effect of water on
conductivity. Two water-doped wadsleyite samples with initial water contents of 0.3 0.01 and 1.2 0.02
wt % were used for the conductivity measurements. The temperature ranges were 500-2000 K for
water-undoped samples and 350-1000 K for water-doped samples. All recovered samples from the
water-undoped experiments show a few hundred ppm of water (maximum 910 ppm) incorporated into
the sample. Above 1500 K, the electrical conductivity values of water-undoped wadsleyite show good
agreement among different runs. The average activation enthalpy is about 1.7 eV. We consider that the
small polaron conduction dominates above this temperature. Below 1000 K, the conductivity
systematically increases with increasing water content. Activation enthalpy of each conductivity path
show systematic decrease from 1.0 to 0.55 with the increasing water contents from 0.008 to 1.2 wt %.
This observation suggests the proton conduction as a dominant mechanism at the low temperatures.
Electrical conductivity of anhydrous wadsleyite in the mantle transition zone is about 310-2 S/m.
Hydration enhances the conductivity of wadsleyite, by containing 0.1 % of water, the conductivity of
wadsleyite increases by 0.3 log units. At 1.0 wt % of water, the conductivity becomes 110-2. The
conductivity jump associated with the dry olivine-wadsleyite transition is only 0.7 log units. The
hydration of olivine and wadsleyite decreases conductivity jump if water is distributed homogeneously at
the bottom of the upper mantle and top of the transition zone, and it should become ~0.5 log units by
0.1 % hydration. If water is partitioned between olivine and wadsleyite, the water content of wadsleyite
should be 5 times higher than the olivine, resulting in the conductivity jump of ~0.9 log units. This
study shows the contribution of water to the conductivity is smaller at the mantle transition zone
temperatures and it is difficult to detect water less than 0.1 wt % from electrical conductivity data. Our
dry wadsleyite data well explain the recent semi-global conductivity depth profiles obtained from the
electromagnetic studies.
Keywords: electrical conductivity, wadsleyite, transition zone