INFLUENCE OF COMPOSITION-DEPENDENT VISCOSITY ON THE LATERAL SPATIAL AND MORPHOLOGIC STABILITY OF PRIMORDIAL THERMOCHEMICAL PILES IN THE EARTH’S LOWER MANTLE

Global seismic tomography models reveal two Large Low Shear Velocity Provinces (LLSVPs) that are hypothesized to be caused by large-scale compositionally distinct reservoirs (e.g., thermochemical piles) in the Earth’s lowermost mantle. Whether the LLSVPs are fixed is important in paleogeographic reconstructions and our understanding towards the dynamic and nature of the Earth’s lower mantle. Recent paleomagnetic studies have implied that the LLSVPs may remain stationary for several hundred million years, while another recent study³ showed that the paleomagnetic data could non-uniquely be explained by mobile LLSVPs. Although geodynamical studies reveal that thermochemical piles are easily swept around by changing subduction patterns, there are other parameters such as compositional viscosity contrast between piles and the ambient mantle that have not been sufficiently explored in these models. Conventionally, thermochemical calculations employ the same rheological formulation for piles and the background mantle; however, piles may possibly have an increased intrinsic viscosity due to compositional differences. In this study, we investigate whether and how the composition-dependent rheology could possibly cause lateral fixity of thermochemical piles against changing subduction patterns. We performed high resolution 2D annulus geodynamical calculations to explore how increased intrinsic viscosity of thermochemical piles affects the morphology and lateral fixity of LLSVPs in response to changing subduction and upwelling flow patterns. We introduce isolated cold and hot anomalies in the Earth’s mantle to generate changing subduction and upwelling flow, respectively. We then observe how piles with varied intrinsic viscosity react to them. We find that increased thermochemical pile intrinsic viscosity stabilizes pile morphology, yet does not make them less laterally mobile, even if with 5,000x of pile intrinsic viscosity. If LLSVPs are indeed laterally fixed, another mechanism must be found.