High-pressure elasticity of Fo90 hydrous ringwoodite

Details High-pressure elasticity of Fo90 hydrous ringwoodite High-pressure elasticity of Fo90 hydrous ringwoodite

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.t41b1188j&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #T41B-1188

Physics

8124 Earth'S Interior: Composition And State (Old 8105), 3900 Mineral Physics, 3909 Elasticity And Anelasticity, 3924 High-Pressure Behavior, 1025 Composition Of The Mantle

Scientific paper

If the Earth accreted with chondritic amounts of hydrogen, three quarters of the planet's water has either been lost to space, or is now tied up in minerals of the interior. The nominally anhydrous polymorphs of (Mg,Fe)2SiO4 (olivine, wadsleyite, and ringwoodite) in the upper mantle constitute one of the largest potential reservoirs of water (as hydroxyl) in the planet. New constraints on the hydrogen content of Earth's potentially hydrous transition zone are emerging from seismology and electrical conductivity profiles, but these geophysical observations require quantitative laboratory measurements on the effects of hydration on e.g. velocities and conductivity. We have measured the sound velocities and elastic constants of Fo90 ringwoodite containing about 1 wt% H2O to 10 GPa using gigahertz ultrasonic interferometry in the diamond anvil cell. The aggregate bulk and shear moduli of hydrous ringwoodite are 176(7) and 103(5) GPa, respectively, with pressure derivatives of 4.8(6) and 1.8(3). The elastic constants of hydrous ringwoodite represented by C11, C44, and C12 are 298(13), 112(6), and 115(6) GPa, respectively, with pressure derivatives of 8.2(6), 1.4(1), 3.2(4). We calculate that compressional and shear-wave velocities are reduced by about 40 m/s for every 0.1 wt% H2O added to ringwoodite. The Vp/Vs ratio of ringwoodite increases by roughly 2% upon hydration to 1 wt% H2O, and this difference does not appear to change significantly with pressure. At ambient pressure, we observe a reduction of P-wave velocity equivalent to an increase in temperature of ˜600° C and on S-wave velocity of ˜1000° C. Hydration is therefore expected to have a much larger effect on velocity than does temperature or variations in Fe/Mg within reasonable ranges of these parameters in the transition zone, and may be recognized by elevated Vp/Vs ratios.

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