Comments on Electrical conductivity of wadsleyite as a function of temperature and water content by Manthilake et al.

In a recent paper, Manthilake et al. [Manthilake, M.A.G.M., et al. Electrical conductivity of wadsleyite as a function of temperature and water content. Physics of the Earth and Planetary Interiors, in press] presented the results of experimental study on the electrical conductivity of wadsleyite and concluded that the influence of water is small at transition zone temperatures and that a high concentration of water (hydrogen) cannot explain the observed conductivity in the transition zone as oppose to the conclusion originally obtained by Huang et al. [Huang, X., Xu, Y., Karato, S., 2005. Water content of the mantle transition zone from the electrical conductivity of wadsleyite and ringwoodite. Nature 434, 746-749) from a similar experimental study. In this note, we discuss the causes of discrepancies between the results by two groups and show that almost all the differences are due to the experimental artifacts in the studies by Manthilake et al. and Yoshino et al. [Manthilake, M.A.G.M., et al. Electrical conductivity of wadsleyite as a function of temperature and water content. Physics of the Earth and Planetary Interiors, in press; Yoshino, T., Manthilake, G., Matsuzaki, T., Katsura, T., 2008a. Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite. Nature 451, 326-329] namely (i) the use of inappropriate method of determining electrical conductivity and (ii) the use of the data from a sample of wadsleyite with a substantial amount of water as a dry conductivity. A comparison of electrical conductivity of truly dry wadsleyite and olivine shows that the conductivity is similar at the same pressure and temperature. We also show that the use of one frequency method results in systematic errors in the conductivity measurements that explains the discrepancies in the results by two sets of studies. When an appropriate method for determining electrical conductivity (i.e., the impedance spectroscopy) is used and when the results of truly dry sample are used for the background dry conductivity, we find that the influence of water (hydrogen) is large enough to explain a majority of variation of electrical conductivity by the regional variation in water content. (C) 2009 Elsevier B.V. All rights reserved.