The effect of bulk composition and temperature on mantle seismic structure

We use a self-consistent thermodynamic formalism to compute the phase equilibria and physical properties of model mantle compositions, demonstrating that the seismological properties of a mantle comprised of (1) an equilibrium assemblage (EA) of pyrolitic composition and (2) a mechanical mixture (MM) of basalt and harzburgite with identical bulk composition differ significantly. We calculate the density, compressional wave velocity (VP), and shear wave velocity (VS) of EA and MM with basalt fraction varying from 0% to 100% and along adiabats with potential temperatures ranging from 1000 K to 2000 K. For MM, VS in the transition zone is greater, increases more rapidly with depth, and is insensitive to basalt fraction, while for EA VS decreases by 2.5% with increasing basalt fraction for fractions < 70%. The magnitude and sharpness of the 520-km discontinuity depends strongly on temperature in both EA and MM, which may explain lateral variations in its seismic detection. Both MM and EA feature complex structure in the depth range 640–750 km due to the transformations of akimotoite, ringwoodite, and garnet to denser assemblages. MM is faster than most seismological models in the upper mantle, and slower in the lower mantle, suggesting an increase of basalt fraction with depth in the mantle.