SUPERPILES: A DYNAMIC EXPLANATION FOR THE SHAPE OF LARGE LOW SHEAR VELOCITY PROVINCES

One hypothesis for the formation of Large Low Shear Velocity Provinces (LLSVPs) is that they are an intrinsically more-dense compositional reservoir present in their lower mantle. The possibility that mantle plumes could originate from the tops of these more-dense compositional reservoirs aligns with the observed differences in trace element geochemistry between mid-ocean ridge basalts (MORBs) and ocean island ballasts (OIBs). Geodynamic models show such compositional reservoirs may have one of two dynamical styles: active superplumes of passive piles. The formation of these features is largely controlled by the upwards buoyancy of the compositional reservoirs which is dependent on the intrinsic density and temperature of the compositional reservoir which respect to the background mantle. We perform geodynamical experiments that investigate the initial temperature of thermochemical convection systems. We find two scenarios in which compositional reservoirs can form a morphology similar to that of the plume-resembling LLSVPs observed in recent tomography models (French and Romanowicz 2015). One scenario is already well known from past geodynamical studies, active thermochemical superplumes. Another scenario is that they are passive piles with low buoyancy numbers, which we refer to as superpiles. Superpiles are tall structures that resemble superplumes, but have a slight negative to neutral buoyancy. If superpiles explain the morphology of LLSVPs, it would imply that compositional reservoirs have a density similar to the surrounding mantle.