Seismic Velocity and Density Jumps Across the 410- and 660-Kilometer Discontinuities

The average seismic velocity and density jumps across the 410- and 660-kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long-period seismic wave arrivals that reflect off of these interfaces. The preliminary reference Earth model (PREM) is within the computed 95 percent confidence ellipse for the 410-km discontinuity but outside the allowed jumps across the 660-kilometer discontinuity. Current pyrolite mantle models appear consistent with the constraints for the 410-kilometer discontinuity but overpredict amplitudes for the 660-kilometer reflections. The density jump across the 660-kilometer discontinuity is between 4 and 6 percent, below the PREM value of 9.3 percent commonly used in mantle convection calculations.

Elastic moduli of wadsleyite (beta-Mg2SiO4) to 7 gigapascals and 873 kelvin

Simultaneous sound velocity measurements and x-ray diffraction studies were made on wadsleyite (beta-Mg2SiO4) to 7 gigapascals and 873 kelvin. The calculated adiabatic bulk (K) and shear (G) moduli yield K (at room conditions) = 172(2) gigapascals, dK/dP = 4.2(1), and dK/dT = -0.012(1) gigapascals per kelvin, and G (at room conditions) = 113(1) gigapascals, dG/dP = 1.5(1), and dG/dT = -0. 017(1) gigapascals per kelvin, respectively. The data imply that the P and S wave velocity contrasts between olivine and wadsleyite require an olivine amount of 38 to 39 percent in the upper mantle to satisfy the observed 410-kilometer discontinuity, but 55 to 60 percent to account for the velocity increase through the transition zone.