CSEDI Collaborative Research: A Multidisciplinary Approach to Investigate the Origin of Anisotropy at the Base of the Mantle

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The lowermost part of the earth's mantle, referred to as the D" region, is a dynamic region that is both a thermal and chemical boundary layer between the solid, silicate mantle and the fluid, mostly iron outer core. A better understanding of the deformation processes that occur in this region would provide important constraints on the current dynamics of the entire mantle, the processes of heat transfer from the core to the mantle, the thermal evolution of our planet, and the existence and extent of geochemical heterogeneity. To study deformation processes in the deepest mantle, the investigators combine expertise from several disciplines: seismology, mineral physics and geodynamical modeling of mantle convection, linked together around a common object of study: seismic anisotropy, i.e. the difference in propagation speeds of seismic waves depending on the orientation of the path travelled, a proxy for macroscopic deformation. The latter's characteristics reflect mineral properties as well as flow strength and geometry. In this project, the team will apply the multi-disciplinary tools developed in a previous joint study funded by the CSEDI program of NSF to further characterize the possible causes of seismic anisotropy in the earth's deep mantle. They will begin with 3D fluid dynamical modeling of mantle convection, in which they will model the deformation of descending tectonic plates as they come in contact with Earth's core-mantle boundary. It is unclear how strong these plates are, and their strength will control how they deform. Strong plates will buckle and bend, whereas weak plates will deform is a more ductile fashion. The team will examine numerous scenarios, each assuming a different strength for descending plates. They will also examine scenarios in which descending plates interact with hypothesized compositional heterogeneity in the deep mantle. Deformation data from the dynamical calculations will be used as input for mineral physics calculations to predict the alignment of minerals which will control the nature of seismic anisotropy. By comparing predicted seismic anisotropy from various models to that observed by seismic studies, they will constrain deformation characteristics of descending plates and compositional characteristics of the D" zone. This will provide important information on how sinking plates drive larger-scale mantle convection.The presence of anisotropy in the D" region of the earth's mantle is now well established, although its cause remains unclear. Much progress was recently achieved in mineral physics, to characterize elastic and deformation properties of lowermost mantle minerals including the post-perovskite (pPv) phase, as well as in geodynamics, tracking strain evolution in mantle convection modeling. There are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies. This study will advance our understanding of the structure and dynamics of an important boundary layer region in the earth. In previous collaborative work funded by CSEDI, the investigators developed a multi-disciplinary approach combining elements from geodynamic modeling, mineral physics and material science experiments and computations, to perform forward modeling of crystal preferred orientation (CPO) anisotropy in a 3D spherical earth, in the deep mantle part of a subducted slab, under different starting assumptions, and compared them with seismic observations. The ultimate goal is to gain better understanding of the origin of seismic anisotropy in D", and determine which microscopic and macroscopic processes may or may not be at play. So far, they investigated the case of a 3D geodynamical model under rather simple rheological assumptions. Now, they will explore varying rheologies producing slabs of variable strength, including the effect of the pPv phase-change, and how slabs will deform in the presence of hypothetical thermochemical piles. For each of these different calculations, they will provide the deformation information to serve as input for the mineralogical texture development within a polycrystalline mineral aggregate. While the team will still focus on three phases, perovskite, pPv and ferropericlase, they will also explore the effect of a variety of Fe and Al substitution mechanisms, both theoretically and experimentally. The predicted seismic anisotropy from these models will be confronted with seismological observations of radial and azimuthal anisotropy both acquired during this project and from the literature.