SUPERPILES: A LOW DENSITY EXPLANATION FOR LLSVPS

One hypothesis for the formation of Large Low Shear Velocity Provinces (LLSVPs) is that they formed from an intrinsically more dense compositional reservoir in the lower mantle. The possibility of mantle plumes originating from this compositional reservoir could provide an explanation for the observed trace element geochemistry differences between mid ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Our geodynamic models have shown compositional reservoirs tend to form either active superplume, or passive pile morphology. Tomography models indicate the LLSVPs have a morphology which resembles superplumes. The thermal initial condition of a geodynamic model greatly impacts the morphology of these compositional reservoirs. A hot thermal initial condition increases the upwards buoyancy or effective density of the surrounding mantle. This causes the compositional reservoir to form a new type of pile morphology at smaller than expected density differences, a superpile. Superpiles are tall features resembling superplumes, but they are passive features formed by mantle convection the same as a pile morphology. If LLSVPs have a superpile morphology they could have a density similar to the surrounding mantle while explaining the tall LLSVP morphology.