Geodynamics of Earth's Deep Mantle.

摘要

Seismic tomography and waveform modeling reveal several prominent structures in the Earth's lower mantle: (1) the D'' discontinuity, defined by a seismic velocity increase of 1--3% about 250 km above the core-mantle boundary (CMB), (2) Ultralow-velocity zones (ULVZs), which are thin, isolated patches with anomalously low seismic wavespeed at the CMB, and (3) two large, low-shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean. The geodynamics of these structures are investigated using numerical convection models that include new discoveries in mineral physics and recent insight from seismology. In addition, I assess the influence of an iron spin transition in a major lower mantle mineral (ferropericlase) on the style and vigor of mantle convection.;A phase change model for the D'' discontinuity produces significant thermal and phase heterogeneity over small distances due to the interaction of slabs, plumes, and a phase transition. Perturbations to seismic arrivals are linked to the evolutionary stage of slabs and plumes and can be used to determine phase boundary properties, volumetric wavespeed anomaly beneath the discontinuity, and possibly the lengthscale of slab folding near the CMB.;I simulate convection within D'' to deduce the stability and morphology of a chemically distinct iron-enriched ULVZ. The chemical density anomaly largely dictates ULVZ shape, and the prescribed initial thickness (proxy for volume) of the chemically distinct layer controls its size. I synthesize the dynamic results with a Voigt-Reuss-Hill mixing model to provide insight into the inherent seismic trade-off between ULVZ thickness and wavespeed reduction.;The dynamics of the LLSVPs are investigated using global 3-D models of thermochemical structures that incorporate paleogeographic constraints from 250 Ma to present day. The structures deform and migrate along the CMB, either by coupling to plate motions or in response to slab stresses. Slabs from Paleo-Tethys and Tethys Ocean subduction push the African structure further to the southwest than inferred from tomography. Dense and viscous slabs can severely compromise the stability of thermochemical structures with a high bulk modulus at the CMB.;Finally, I explore the consequences of the intrinsic density change caused by the Fe2+ spin transition in ferropericlase on the style and vigor of mantle convection. The transition generates a net driving density difference for both upwellings and downwellings that dominantly enhances the positive thermal buoyancy of plumes in 2-D cylindrical geometry. Although the additional buoyancy does not fundamentally alter large-scale dynamics, the Nusselt number increases by 5--10%, and vertical velocities increase by 10--40% in the lower mantle. Advective heat transport is more effective and temperatures in the CMB region are reduced by up to 12%.