Present-day Three-Dimensional Temperature Distribution From a Mantle Flow Model.

In an attempt to understand the global temperature distribution in the mantle and its consequences for Earth structure we construct a model for the instantaneous temperature field of the mantle, assuming downwellings slabs to be the most important source of density heterogeneity and temperature variations. We neglect the contributions due to active upwellings. We use a model for the history of subduction derived from tectonic reconstructions and compute the 3-D velocity field for an incompressible Newtonian fluid. We solve the advection- diffusion equation in steady state for a spherical shell using the finite element package ABAQUS. We choose free-slip, 3000 K velocity-temperature boundary conditions at the core-mantle boundary, and at the surface we constrain velocities to be plate velocities and temperatures to be 300 K. The vertical resolution is on the order of ~5 km at the top and bottom boundary layers, and ~200 x 200 km horizontally. We recover the half-space cooling behavior in the lithosphere and obtain reasonable values of the heat flow, indicating that our predicted temperature field behaves as expected. Not surprisingly, the 3-D variations in the entire mantle are dominated by the presence of slabs in regions of long-lived subduction We use our predicted temperature fields to compute the expected phase assemblage for a mantle of constant bulk composition. We will focus our discussion on the expected topography on major seismic discontinuities (410 and 660) and comparisons to three-dimensional seismological observations.