An investigation into compositionally heterogeneous plume clusters in 3D spherical geometry

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Observations from seismology reveal two large regions of the lowermost mantle, beneath Africa and the Pacific, that exhibit slower-than-average seismic wavespeeds. Additional observations infer the cause of these anomalies to include both thermal and compositional heterogeneity. A few, competing hypotheses exist to explain them, each of which have significant consequencestoward our understanding of mantle convection patterns, thermal transport, geochemical evolution, and cooling of the Earth?s interior. Here, we test the hypothesis that the large seismic anomalies beneath Africa and the Pacific are due to the interaction of ancient, subducted oceanic crust with plumes of hot, upwelling mantle that are expected to form in these regions. By performing 3D numerical modeling studies of mantle convection, combined with processing our results in a manner that best reproduces the distortion inherent to seismological techniques, we will examine whether this hypothesis is dynamically feasible. If so, we will investigate the important thermal and chemical transport properties associated with it, comparing and contrasting them to those of competing ideas. We will investigate how well this hypothesis fits numerous geological, geochemical, and seismological constraints, with the ultimate goal of understanding the nature of large-scale mantle convection. Combined with advances from other research, this research will work toward the larger goal of understanding how mantle convection causes plate tectonics. The first part of this project is to perform a comprehensive fluid dynamical study to better understand the fundamental dynamics associated with multiple plumes in the presence of compositional heterogeneity. Using numerical finite-element modeling in 3D, spherical geometry, we will investigate the morphology, time-dependent dynamics, heat transport properties, and expected surface expression of mantle plume clusters in an environment dominated by external stress forces (i.e., background convection) and various degrees of compositional heterogeneity. Secondly, we will investigate whether compositionally-heterogeneous plume clusters are consistent with observations from seismology and geology. Employing Earth?s plate history to guide subduction patterns in thermochemical numerical models, we will investigate how subducted oceanic crust is segregated into upwelling mantle regions. Through tomographic filtering of geodynamical model results, we will investigate how well plume clusters, combined with compositional heterogeneity, fit observations of seismic tomography, compared to other conceptual thermochemical mantle models. Furthermore, we will investigate the expected surface expression of plume clusters, determining their consistency with characteristic patterns of hotspot islands observed on Earth?s surface.