How increased intrinsic viscosity of lower mantle compositional reservoirs affects their spatial and morphological stability

The mobility of two Large Low Shear Velocity Provinces (LLSVPs), often hypothesized to be thermochemical piles with higher density in the Earth’s lowermost mantle, is important for paleogeographic reconstructions and understanding the Earth’s mantle dynamics. Recent paleomagnetic studies have implied that the LLSVPs may remain stationary over geological time, while Flament et al. (2022) showed the paleomagnetic data could also be explained by mobile LLSVPs. Traditional thermochemical models employing the same rheological formulation for piles and the background mantle consistently show mobile piles easily deformed by changing subduction patterns; however, piles may have a higher intrinsic viscosity due to a larger grain size or other compositional differences. Here, we investigate whether and how composition-dependent rheology can explain the possible lateral fixity of thermochemical piles. We performed effectively 2D geodynamical calculations in a spherical annulus geometry to investigate how thermochemical piles with increased intrinsic viscosity respond to slabs and plumes. As pile viscosity increases, we find that the pile morphology is more stable against changing upwelling flow. However, piles remain laterally mobile even when their viscosity is 5000 times higher than the ambient mantle. In summary, we find that increased thermochemical pile viscosity stabilizes pile morphology yet does not make them less laterally mobile. If LLSVPs are laterally fixed, another mechanism must be found.