A possible origin for upper mantle plumes

Global and regional seismological observations indicate hot up welling plumes limited to the upper mantle in numerous locations around the earth. In contrast to whole mantle plumes, upper mantle plumes are associated with a flat 660 km depth seismic discontinuity and a thermal anomaly of less than 200 C at depths shallower than 400 km. In many cases these shallow plumes are associated with continental rift zones or divergent plate motion in mid-oceanic ridges. Although studied extensively, there is no clear explanation for the origin of upper mantle plumes. Here we use 2D plain strain thermo-mechanical models to study controls on upper mantle plumes. The numerical model follows the extended Boussinesq approximation and explicitly includes adiabatic heating/cooling, viscous heating, and latent heat and associated buoyancy from phase changes at 410, 520, and 670 km depth. Model materials follow non-linear temperature and strain rate dependent flow laws. The numerical models indicate that thermal buoyancy resulting from upward passive flow through the 410 km depth exothermic olivine-spinel phase change boundary can explain both magnitude and depth extent of shallow upper mantle plumes. Upward passive flow may result from continental extension or from divergent motion at mid ocean ridges. Modeled thermal anomalies range from 100-150C depending on Clapeyron slope and density jump, consistent with seismic velocity contrasts inferred from regional and global tomographic studies.