Elastic wave velocities of forsterite and its β‐spinel form and chemical boundary hypothesis for the 410‐km discontinuity

Abstract

A method for measuring elastic wave velocities at pressures up to 15 GPa has been developed. Velocities of forsterite and β‐spinel were measured using a piston‐cylinder apparatus with a pulse‐echo‐overlap method for pressures up to 4 GPa and using a split‐sphere apparatus with a pulse‐transmission technique for higher pressures. To validate the results, volume compression, incompressibility, and rigidity were estimated from measured travel times, and comparison was made between the present estimates and data from X ray, Brillouin, and acoustic measurements. The Vp and K values for β‐spinel are 3 and 8% higher than previous measurements at 10 GPa, respectively. Velocity jumps in Vp and Vs due to olivine to β‐spinel transition were about 1.34 and 0.80 km s−1, respectively, very different from seismological observations of velocity jumps at the 410‐km discontinuity, which were concentrated around 0.45 km s−1 for the P wave and 0.22 km s−1 for the S wave. Velocity jumps in mantle peridotite were calculated using the experimental results, and comparison was made between these estimates and seismological observations at the 410‐km discontinuity. The evidence does not support the phase‐change hypothesis, and a chemical boundary was proposed as the origin of the 410‐km discontinuity. An eclogitic mineral assemblage might be an alternative for the composition of the transition layer.