The Geotherm, Composition and Structure of the Upper Mantle and Transition Zone

Abstract

We use a self-consistent thermodynamic formalism to compute phase equilibria and physical properties and demonstrate that the seismological properties of a mantle comprised of (1) an equilibrium assemblage of pyrolitic composition and (2) a mechanical mixture of basalt and harzburgite with identical bulk composition are different. We calculate the shear wave velocity (VS) for both compositional models in a mantle with basalt fractions that vary from 0% to 100% and along adiabats with potential temperatures ranging from 1000 to 2000K. For the mechanical mixture, VS in the transition zone is higher. It increases more rapidly with depth, and it is virtually insensitive to basalt fraction, while for the equilibrium assemblage VS decreases by 3.5% with increasing basalt fraction from 0% to 60%. The magnitude of the 520-km discontinuity depends strongly on temperature in both models, which may explain lateral variations in its seismic detection. Both compositional models feature “double-step” discontinuities in the range of 660-750 km due to the ringwoodite-perovskite transition and the gradual dissolution of garnet into perovskite between 665 km and ~725 km depth. The mechanical mixture 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. In addition, the geotherm of Western North America was inverted from two regional sheae wave velocity models. It strongly indicates a super-adiabatic geotherm in the low velocity zone and an asthenosphere fed by plumes rising from the core-mantle boundary. The inverted geotherm from both global and regional models suggests a sub-adiabatic temperature gradient in the transition zone, which can be explained by a slab accumulation of 30% by mass or volume in this region of the mantle. The latter is not only the consequence of phase transitions but of a large viscosity increase between upper and lower mantles.