This file contain data for Figures 1-19. After unzip the file, you will see 19 folders and 19 Figures in pdf format. Wihtin each folder, there is a Figure*.sh file, which is a bash script that uses GMT version 6.1.0 to make plots for each figure. The GMT software is publically available at https://www.generic-mapping-tools.org.
The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core–mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperature-dependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]—resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core–mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.
Post-collisional potassic-ultrapotassic volcanic rocks are widely developed in the Lhasa block, Qinghai-Tibet region. In this paper, it was observed for the first time that dilute alkali elements–Li, Rb, and Cs–exhibit supernormal enrichment in the research data of numerous potassic-ultrapotassic volcanic rocks in the Lhasa block. At the same time, there are few studies on the genesis of enrichment. Currently, 568 sets of published volcanic rock data and 8 sets of measured data in the Lhasa block are sorted in detail, and the genesis of dilute alkali element enrichment is explained by means of geochemical research methods. It is believed that the high degree of magmatic fractionation of potassic-ultrapotassic volcanic rocks in the Lhasa block is the main reason for the abnormal enrichment of dilute alkali elements such as Li, Rb, and Cs. The abnormal enrichment area is mainly located in the central and western parts of the Lhasa block with an age range of 25–13 Ma. The discrimination range of Zr/Hf and Nb/Ta with a high degree of differentiation of potassic-ultrapotassic volcanic rocks in the Lhasa block is divided by analogy with the research results of highly fractionated granites, which provides a reference for the study of the supernormal enrichment of dilute alkali elements in potassic-ultrapotassic volcanic rocks in this area.
The Earth's lowermost mantle large low velocity provinces are accompanied by small-scale ultralow velocity zones in localized regions on the core-mantle boundary. Large low velocity provinces are hypothesized to be caused by large-scale compositional heterogeneity (i.e., thermochemical piles). The origin of ultralow velocity zones, however, remains elusive. Here we perform three-dimensional geodynamical calculations to show that the current locations and shapes of ultralow velocity zones are related to their cause. We find that the hottest lowermost mantle regions are commonly located well within the interiors of thermochemical piles. In contrast, accumulations of ultradense compositionally distinct material occur as discontinuous patches along the margins of thermochemical piles and have asymmetrical cross-sectional shape. Furthermore, the lateral morphology of these patches provides insight into mantle flow directions and long-term stability. The global distribution and large variations of morphology of ultralow velocity zones validate a compositionally distinct origin for most ultralow velocity zones.Ultralow velocity zones are detected on the core-mantle boundary, but their origin is enigmatic. Here, the authors find that the global distribution and large variations of morphology of ultralow velocity zones are consistent with most having a compositionally-distinct origin.
SUMMARY Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma. In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.
Abstract The Yellowstone region (western United States) is a commonly cited example of intraplate volcanism whose origin has been a topic of debate for several decades. Recent work has suggested that a deep mantle plume, rooted beneath southern California, is the source of Yellowstone volcanism. Seismic anisotropy, which typically results from deformation, can be used to identify and characterize mantle flow. Here, we show that the proposed plume root location at the base of the mantle is strongly seismically anisotropic. This finding is complemented by geodynamic modeling results showing upwelling flow and high strains in the lowermost mantle beneath the Yellowstone region. Our results support the idea that the Yellowstone volcanism is caused by a plume rooted in the deepest mantle beneath southern California, connecting dynamics in the deepest mantle with phenomena at Earth's surface.
Abstract We investigate broadband SPdKS waveforms from earthquakes occurring beneath Myanmar. These paths sample the core–mantle boundary beneath northwestern China. Waveform modeling shows that two ∼250 × 250 km wide ultra-low velocity zones (ULVZs) with a thickness of roughly 10 km exist in the region. The ULVZ models fitting these data have large S-wave velocity drops of 55% but relatively small 14% P-wave velocity reductions. This is almost a 4:1 S- to P-wave velocity ratio and is suggestive of a partial melt origin. These ULVZs exist in a region of the Circum-Pacific with a long history of subduction and far from large low-velocity province (LLVP) boundaries where ULVZs are more commonly observed. It is possible that these ULVZs are generated by partial melting of mid-ocean ridge basalt.
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