PETROLOGY, ELASTICITY, AND COMPOSITION OF THE MANTLE TRANSITION ZONE

We compare the predictions of compositional models of the mantle transition zone to observed seismic properties by constructing phase diagrams in the MgO-FeO-CaO-Al2O3-SiO2 system and estimating the elasticity of the relevant minerals. Mie-Gruneisen and Birch-Murnaghan finite strain theory are combined with ideal solution theory to extrapolate experimental measurements of thermal and elastic properties to high pressures and temperatures. The resulting thermodynamic potentials are combined with the estimated phase diagrams to predict the density, seismic parameter, and mantle adiabats for a given compositional model. We find that the properties of pyrolite agree well with the observed density and bulk sound velocity of the upper mantle and transition zone. Piclogite significantly underestimates the magnitude of the 400-m velocity discontinuity and overestimates the velocity gradient in the transition zone. Substantially enriching piclogite in Al provides an acceptable fit to the observations. Invoking a chemical boundary layer between the uppermost mantle and transition zone leads to poor agreement with observed seismic properties for the compositions considered. Within the transition zone, the dissolution of garnet to Ca-perovskite near 18 GPa may explain the proposed 520-km seismic discontinuity. Below 700 km depth, all compositions disagree with observed bulk sound velocities, implying that the lower mantle is chemically distinct from the upper mantle.