Hydration and Velocity Heterogeneity in the Mantle

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 Fo₉₀ 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° C and on S-wave velocity of 1000° 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 H₂O, 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.