Single-crystal elasticity of wadsleyites, β-Mg 2 SiO 4 , containing 0.37-1.66 wt.% H 2 O

Z. Mao, S. D. Jacobsen, F. Jiang, J. R. Smyth, C. M. Holl, D. J. Frost, Thomas S. Duffy

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Abstract

The presence of hydrogen can affect elastic properties and seismic velocities of minerals in the Earth's upper mantle. In this study, the second-order elastic constants of hydrous wadsleyites containing 0.37, 0.84, and 1.66 wt.% H 2 O were determined by Brillouin scattering at ambient conditions. Measurements were performed on at least three independent crystal planes for each composition. The aggregate bulk modulus, K S0 , and shear modulus, G 0 , were calculated using VRH (Voigt-Reuss-Hill) averages. The results are: K S0 = 165.4(9) GPa, G 0 = 108.6(6) GPa for wadsleyite with 0.37 wt.% H 2 O;K S0 = 160.3(7) GPa, G 0 = 105.3(6) GPa for 0.84 wt.% H 2 O; K S0 = 149.2(6) GPa, G 0 = 98.6(4) GPa for 1.66 wt.% H 2 O. We find that the bulk and shear moduli of hydrous wadsleyites decrease linearly with water content according to the following relations (in GPa): K S0 = 170.9(9)-13.0(8)C H2O , G 0 = 111.7(6)-7.8(4)C H2O , where C H2O is the H 2 O weight percentage. Compared with anhydrous wadsleyite, addition of 1 wt.% H 2 O will lead to a 7.6% decrease in the bulk modulus, and a 7.0% decrease in the shear modulus. Using these results, we examine the velocity contrast between hydrous olivine and wadsleyite at ambient conditions for an Fe-free system assuming an H 2 O partition coefficient between wadsleyite and olivine of 3. The velocity contrast in compressional and shear velocity between wadsleyite and olivine ranges from 12 to 13% for an H 2 O-free system to 7-8% for wadsleyite containing 1.5 wt.% H 2 O. Thus, the magnitude of the seismic velocity change at 410-km depth can be expected to be sensitive to the presence of H 2 O in olivine polymorphs.

Original languageEnglish (US)
Pages (from-to)78-89
Number of pages12
JournalEarth and Planetary Science Letters
Volume266
Issue number1-2
DOIs
StatePublished - Feb 1 2008

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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