Bina, C. R., Mineralogic transformations and seismic velocity variations in the upper mantle of the Earth, Ph.D. dissertation, Northwestern University, Evanston, Illinois, U.S.A., 1987.
Abstract. Radial variations in seismic velocities in the Earth's upper mantle are indicative of changes in the elastic properties of the materials comprising the planet's interior. These variations may be due to changes in bulk chemical composition or to the occurrence of structural transformations in material of uniform composition. Here we investigate whether seismic velocity variations in the 200-650 km depth interval may be ascribed to the occurrence of isochemical phase transformations in material of uniform bulk composition or whether it is necessary to invoke changes in bulk composition. We also seek to constrain the bulk composition of the upper mantle by comparing seismic velocities predicted for model compositions with those actually observed for the Earth. We first combine available thermodynamic and thermoelastic data on low- and high-pressure mineral phases with data from high-pressure phase equilibrium experiments to produce phase diagrams for simple systems which are internally consistent thermodynamically. We then use the thermodynamic parameters which define these phase diagrams to calculate stable phase assemblages and associated elastic properties as functions of pressure and temperature for a variety of model bulk chemical compositions.
We find that phase transitions from pyroxene to garnet-majorite do not produce a sharp increase in seismic velocity such as that observed at approximately 400 km depth in the Earth and that seismic velocities predicted for model eclogitic bulk compositions are a poor match to velocities observed for the upper mantle. Phase transitions from alpha-olivine to beta-modified-spinel, however, do produce an increase in seismic velocity of appropriate magnitude and sharpness to that observed at 400 km depth, and seismic velocities predicted for model peridotitic bulk compositions are in agreement with observed upper mantle velocities. Upon comparing calculated seismic velocity profiles for a variety of model bulk compositions with velocity profiles derived from seismic observations, we conclude that the upper mantle is composed of a uniform peridotitic material containing 70-75% olivine by volume and that there is no need to invoke changes in bulk chemical composition to explain upper mantle seismic velocities.Copyright © 1987 Craig R. Bina