HR: 0800h
AN: T41B-1194    [Abstracts]
TI: Water in Olivine - not a Simple Question
AU: * Bansal, S A
EM: sbansal@ices.utexas.edu
AF: University of Texas, Computational and Applied Mathematics, Austin, TX 78712 United States
AU: Dickinson, J T
EM: jtd@wsu.edu
AF: Washington State University, Department of Physics, Pullman, WA 99164 United States
AU: Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Geodynamics Branch MS 921, Greenbelt, MD 20771 United States
AU: Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AB: How much "water" is present in olivine is of great interest for the upper mantle. Thermodynamics mandates that small amounts of H$_{2}$O become structurally incorporated, when olivine crystallizes in an H$_{2}$O-laden environment. It is widely believed that the formation of hydroxyl (OH$^{-}$ or Si-OH) is the end point of the H$_{2}$O dissolution mechanism. Therefore it is also believed that quantitative solute H$_{2}$O contents can be obtained by measuring the O-H stretching bands in olivine crystals by infrared (IR) spectroscopy. However, we can take olivine crystals, which are devoid of OH$^{-}$ according to the IR criteria, and fracture them in front of a mass spectrometer. Instantly upon fracture, H$_{2}$ molecules evolve from the freshly formed surfaces, followed by H$_{2}$O and then again by H$_{2}$. This observation indicates that an apparently "dry" olivine can contain dissolved H$_{2}$O, though not in form of IR-recognizable OH$^{-}$ or Si-OH. When we measure the effective dielectric constant \epsilon$_{eff}$ at the limit of 0 Hz, we observe that \epsilon$_{eff}$ changes reversibly in two steps, around 200øC and 400øC. At about 400øC the olivine turns into a p-type semiconductor. Our observations point (i) at the conversion of hydroxyl, probably Si-OH pairs, into H$_{2}$ plus peroxy links, Si-OO-Si, (ii) at a two-step dissociation of the peroxy links. We conclude that, to determine the amount of solute "water" in olivine, it is not enough to measure its hydroxyl content by IR spectroscopy. Other methods have to be used or developed to provide information about the number of H$_{2}$ molecules and peroxy links in the olivine matrix.
DE: 8045 Role of fluids
DE: 3904 Defects
DE: 3914 Electrical properties
DE: 1655 Water cycles (1836)
DE: 1645 Solid Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting