HR: 1330h
AN: T32B-0931    [PDF]
TI: Hydration and Velocity Heterogeneity in the Mantle
AU: * Smyth, J R
EM: joseph.smyth@colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309 United States
AU: Holl, C M
EM: christopher.holl@colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309 United States
AU: Jacobsen, S D
EM: steven.jacobsen@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D95440 Germany
AU: Manghnani, M H
EM: murli@soest.hawaii.edu
AF: Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822 United States
AU: Amulele, G
EM: gamulele@soest.hawaii.edu
AF: Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822 United States
AB: Olivine, wadsleyite, and ringwoodite are the mineral phases generally believed to compose the majority of the upper mantle and transition zone. Although nominally anhydrous, these phases can incorporate enough hydroxyl to significantly affect their P and S seismic velocities and to compose the planet's largest reservoir of water. Using single crystal X-ray diffraction, we have measured the effect of hydration on compression of Fo$_{90}$ ringwoodite to 12 GPa. Using powder diffraction of synchrotron radiation we have measured compression to 50 GPa. Using GHz ultrasonic measurements on single crystals containing about one percent water by weight, we observe a reduction of P-wave velocity equivalent to an increase in temperature of $600\deg$C and on S-wave velocity of $1000\deg$C at ambient pressure. Single-crystal velocity measurements at pressure are in progress. The data obtained to date indicate that hydration of ringwoodite will have a larger effect on velocities in the Transition Zone (TZ) than does temperature within the uncertainties of each. We have measured the isothermal bulk moduli of hydrous wadsleyite by single-crystal X-ray diffraction and find a similar effect of hydration on the bulk modulus. Lateral velocity variations in the TZ are therefore more likely to reflect variations in hydration than variations in temperature, at least in regions distant from subduction zones. In tomographic images of the TZ in regions distant from active subduction, red is more likely to mean `wet' than it is to mean `hot'. Observed seismic velocities in the TZ are consistent with a pyrolite composition with 0.5 to 1.0 percent by weight H$_{2}$O, but are not consistent with dry pyrolite compositions. This degree of hydration would allow for TZ storage of two to three times the amount of water currently in the hydrosphere.
UR: http://ruby.colorado.edu/~smyth/Research/Puball.html
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 7207 Core and mantle
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8180 Tomography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting