Oscillating and stagnating plumes in the Earth's lower mantle

We investigate the coupled effects of mineralogy and pressure on the dynamics of axisymmetric thermochemical plumes in the lower mantle, using both high resolution numerical experiments and simple analytical theory. We focus on the effect of composition on the compressibility which has not been studied before. Our results show that the effect of mineralogy is considerable: For relatively low Si enrichment, the compressibility of the plume and of the surrounding mantle is similar therefore the compositional density excess of the plume is constant with depth. This leads to an oscillatory behavior in plume head vertical position with time, similar to previous work [A. Davaille. Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle. Nature, 402 (1999) 756–760.]. Si-enriched compositions instead induce a lower compressibility for the thermochemical plume with respect to the surrounding mantle, implying an increase of the compositional density excess with decreasing depth. Therefore, although thermochemical plumes can fully develop and rise toward the surface, their ascent may be impeded by the chemical density excess, leading to the stagnation of large plume heads at various depths in the lower mantle. As a consequence, Si-enriched thermochemical plumes may display broad (∼ 1200 km wide) negative seismic velocity anomalies at various lower mantle depths, which may not necessarily be associated with upwelling currents. In addition, the coupling between mineralogy and dynamics may provide an efficient mechanism for the long term survival of compositional heterogeneity in the lower mantle.