Reconciling Global Stacks of PP and SS Waveforms With Mechanically Mixed Mantle Models

The opposite signs of the Clapeyron slopes render temperature dependent depths of the olivine to wadsleyite (at ~410 km) and ringwoodite to perovskite and ferropericlase transitions (at ~660 km). Seismological analysis of phase conversions and reflections demonstrate thicker and thinner transition zones in 'downwelling' and 'upwelling' regions, respectively. New thermo-dynamic calculations of phase equillibria in a mechanically mixed mantle (Xu et al., 2007) suggest that an upper mantle with increased basalt fractions renders diminished velocity jumps at the 410-km and 660-km discontinuities without affecting discontinuity depths. Using new global waveform stacks, we explore whether combined measurements of the reflection coefficient and traveltime of underside reflections at phase transitions in the upper mantle (i.e., PP and SS precursors) may enable us to constrain both compositional and ambient temperature variations in the upper mantle.