The relationship between the physical properties of the assumed pyrolite composition and depth distributions of seismic velocities in the upper mantle

Abstract

Assuming that the upper mantle consists of pyrolite composition, we determined values of derivatives of bulk modului and shear modului with respect to pressure and temperature and values of the Grüneisen–Anderson parameter for composite minerals of pyrolite composition, by comparing depth distributions of P- and S-wave velocities calculated from the values of physical properties obtained from high pressure and high temperature experiments with those of IASP91. For this purpose, we determined the above-described values, which have not been determined well in high pressure and high temperature experiments, within a certain range for each parameter, using genetic algorithm (GA). As a result, the values of physical properties for wadsleyite were determined so that seismic velocities can decrease. The values of derivative of bulk modulus for perovskite and magnesiowüstite were determined so that seismic velocities can increase. On the other hand, the values of derivative of shear modulus for perovskite and magnesiowüstite were determined to be the same values as standard values, or the values were determined so that seismic velocities can decrease. The values of physical properties for pyroxene, garnet and majorite, except the values of derivative of bulk modulus with respect to pressure for orthopyroxene and garnet, were determined so that seismic velocities can increase. Depth distributions of seismic velocities calculated from the determined values of physical properties indicate that the velocity jump, corresponding to the 410 km seismic discontinuity, occurs at a depth shallower than that for IASP91. The amount of velocity jump at the 660 km seismic discontinuity obtained in this study reaches 3.5% for P-wave, which is smaller than that (5.8%) for IASP91. We also investigated the effect of mantle geotherm on depth distributions of seismic velocities. The results show that the seismic velocities increase 0.5% when the mantle geotherm becomes low by 100 K for all the depths concerned. However, the amount of P-wave velocity jump at the 660 km seismic discontinuity obtained in this study is too small to explain that for IASP91 for any mantle geotherms. If we assume that the upper mantle consists of pyrolite composition, these suggest that the amount of P-wave velocity jump at the 660 km seismic discontinuity for IASP91 is overestimated.