On the Fixity of the Thermo-Chemical Piles at the Base of Mantle

The two Large Low Velocity Provinces (LLVPs) at the base of lower mantle are prominent features in all shear wave tomographic models. Their shear wave anomaly and bulk-sound speed anomaly are anti-correlated. They form a degree-2 pattern. The reconstruction of the eruption sites of Large Igneous Provinces since ∼250 Ma coincide with LLVP margins spatially. This suggests that the LLVPs are not short-term transient features, but long-term stable, chemical structures. In this study, we investigate whether dense chemical piles can remain laterally fixed in a strongly convecting mantle. We use CIG’s CitcomS v3.1 with one modification: the chemical density anomaly is allowed to change radially to simulate the effect of the bulk modulus anomaly. The model has a Rayleigh number 1.4e9, free-slip boundary conditions, no internal heating, a strong lithosphere, a weak asthenosphere, temperature dependent viscosity, and an initially flat chemical layer at the base of mantle. We found that with reasonable values of bulk modulus and chemical density anomalies, the chemical structures are dome-like and survive for hundreds of million years. Warm upwellings rise from the top and margins of the structures, while cold downwellings occur near the perimeter of the structures or where the structures are absent. The structures have a degree-2 pattern in most models, and a degree-3 pattern in a few models. This is likely controlled by the initial conditions. Nonetheless, the chemical piles are laterally fixed. The location of upwellings and downwellings do not change substantially with time in a no-net-angular-momentum reference frame. The intensity of upwellings vs downwellings in our models is stronger than that of Earth. Adding plate motion and internal heating will strengthen the intensity of downwellings and weaken the intensity of upwellings, which might affect the fixity of the piles.