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Abstract: Earth''s lower mantle is dominated by a pair of antipodal large low shear velocity provinces (LLSVPs) that reach >1000km up from the core–mantle boundary (CMB). These are separated by a ring of faster-than-average velocities thought to be related to subduction of oceanic lithosphere. How robustly does global tomography constrain velocity structure in the lower mantle, and are there other robust large scale features that have not been identified? We use cluster analysis to identify structures and seismic characteristics common to a set of recent global tomographic models which have been derived using different data sets, parameterizations, and theory behind approximations used in inversion. We detect a pronounced asymmetry in the velocity gradient with depth between seismically fast and slow regions in the lowermost 500km of the mantle, suggesting the presence of compositional heterogeneity. We find that, in all models, there is a clear separation of lower mantle structure into one fast and two slow regions, and that the boundary of the regions is remarkably similar across models even on length scales as small as <1000km. This inter-model similarity indicates that long wavelength features are not a consequence of lack of fine-scale resolution, but that they truly dominate the structure in the lowermost mantle. There is a single exception to this separation: an isolated slow anomaly ∼900km across (at the CMB) and extending ∼500km upward from the core–mantle boundary, which we call the “Perm Anomaly”. Though it is far smaller than an LLSVP, waveform analysis confirms that this anomaly is robustly constrained and bounded by rapid lateral velocity gradients like those found around LLSVPs, suggesting that the nature and process of formation of both types of structures may be related. [Copyright &y& Elsevier]

Abstract: Global mantle tomography can be improved through better use of data and application of more accurate wave propagation methods. However, few techniques have been developed for objective validation and exploration of the resulting tomographic models. We show that cluster analysis can be used to validate and explore the salient features across such models. We present a cluster analysis of a global upper mantle radially anisotropic model SEMum developed using full waveform tomography and the Spectral Element Method. Applied to SEMum down to 350 km depth, the cluster analysis reveals that absolute shear wave velocity (Vs) depth profiles naturally group into families that correspond with known surface tectonics. This allows us to construct a global tectonic regionalization based solely on tomography, without the help of any a priori information. We find that the profiles of stable platforms and shields consistently exhibit a mid-lithospheric low velocity zone (LVZ) between 80 and 130km depth, while the asthenosphere is found at depths greater than 250km in both regions. This global intra-continental-lithosphere low velocity zone agrees with recent receiver function studies and regional tomographic studies. Furthermore, we identify an anomalous oceanic region characterized by slow shear wave speeds at depths below 150km. Hotspots are found preferentially in the vicinity of this anomalous region. In the Pacific Ocean, where plate velocities are largest, these regions have elongated shapes that align with absolute plate motion, suggesting a relationship between the location of hotspots and small-scale convection in the oceanic upper mantle. [Copyright &y& Elsevier]

Abstract: Close examination of the long wavelength shear velocity signal in the lowermost mantle in the wavenumber domain ties several geophysical observations together and leads to fundamental inferences. When mantle shear velocity model S362ANI at a depth of 2800km is expanded in spherical harmonics up to degree 18, more than one half of the seismic model''s total power is contained in a single spherical harmonic coefficient: the “recumbent” Y 20 spherical harmonic; a Y 20 with its axis of symmetry rotated to the equatorial plane. This degree 2 signal, which continues with decreasing amplitude for more than 1000km above the core–mantle boundary (CMB), is characterized by two antipodal regions of low velocities, separated by a circum-polar torus of higher than average velocities. If the slow regions are associated with net excess mass, then any axis of rotation located in the plane of the circum-polar torus will be close to the maximum moment of inertia axis; this includes, of course, the current axis of rotation. We suggest that the recumbent Y 20 is a very stable feature: once established, it is difficult to erase, and only relatively small departures from this equilibrium configuration are possible. This anomaly correlates strongly with the degree 2 terms of the residual geoid expansion, distribution of the hot spots above the slow regions, high attenuation in the transition zone, and position of subduction zones above the fast band during the last 200Ma. Also, the preferred paths of the virtual geomagnetic pole and true polar wander locations for the last 200Ma lie within the fast band. Since the non-hydrostatic perturbation of the moment of inertia tensor depends only on degree-2 anomalies in the density distribution and deformation of discontinuities, it is natural to infer that rotational dynamics of the Earth have influenced the distribution of heterogeneities in the Earth''s deep interior. We propose that the large-scale heterogeneity at the base of the mantle, which we name Mantle Anchor Structure (MAS) may have formed early in the history of the convecting mantle, remained locked in place with respect to the Earth''s rotation axis ever since, and is currently imposing the planform of flow in the mantle and of plate tectonics at the surface. [Copyright &y& Elsevier]

• Isotopic links between S, W, He, and Pb suggest mixing between several components. • One component is primordial with Δ 33 S = 0. • Primordial sulfur is linked to a dense, undegassed basal reservoir (ULVZ). • Additional components are recycled with variable Δ 33 S. • Recycled sulfur is linked to material with EMII and HIMU characteristics. Understanding present-day mantle heterogeneity is key to understanding the geochemical evolution of our planet. The Samoan islands are the type locality for the Enriched Mantle (II) reservoir that is thought to be produced from the subduction and recycling of marine sediment from upper continental crust. In addition to hosting extreme radiogenic isotope compositions from the EM II reservoir, Samoa also exhibits contributions from other mantle reservoirs in a dilute form including the EM (I) (recycled continental material), HIMU (recycled oceanic crust), and DMM (depleted upper mantle) mantle reservoirs. The plume system feeding the Samoan islands sits above a seismically imaged Large Low Shear Velocity Province (LLSVP) and an Ultra-Low Velocity Zone (ULVZ) that is thought to contribute, in addition to recycled components, the recently discovered early-formed (primordial) components with negative μ 182 W and high 3He/4He. Recent work measuring sulfur isotopes in ocean island basalts has established that recycled oceanic and continental crust host unique S-isotope compositions that can be identified at various hotspot localities. Here we document previously unknown relationships between Δ 33 S and radiogenic tungsten, helium and lead isotopes from 7 Samoan basalts (from the islands of Ofu, Vailulu'u and Malumalu) that suggest mixing between several endmembers. One, a HIMU influence that has slight positive Δ 33 S and positive δ 34 S; another, related to EM II that has near zero Δ 33 S and positive δ 34 S; a third, which is primordial with negative μ 182 W, high 3He/4He, that has Δ 33 S = 0 and negative δ 34 S. From this, we conclude that the indistinguishable Δ 33 S of the primordial endmember from that of the convective mantle indicates that sulfur isotopes were homogenized early in Earth's history. The Vailulu'u sample with HIMU characteristics, carries a small but resolvable Δ 33 S that allows, but does not require mass-independent Archean Δ 33 S to shift the Δ 33 S. The observed correlations involving Δ 33 S support arguments linking Pb, He, and W geochemistry to a deep mantle process and places constraints on questions related to the sources of mantle geochemical heterogeneity. [ABSTRACT FROM AUTHOR]

This study explores the properties of the lithosphere-asthenosphere boundary (LAB) and other shallow mantle discontinuities across the diverse geologic provinces of the northwest United States. Sp phases were used to image three-dimensional discontinuity structure by common conversion point stacking with data from 804 temporary and permanent broadband stations, including 247 from EarthScope's USArray Transportable Array. Substantial variations in mantle discontinuity structure are apparent over a variety of spatial and temporal scales. To the west of the Sevier Thrust Belt, a coherent Sp negative phase coincides with the LAB depth range inferred from tomographic models. To the east, within the stable craton, multiple negative phases typically occur in the high velocity lithospheric layer, although in places a single mid-lithospheric discontinuity is imaged. Sub-cratonic LAB phases are often absent, indicating an LAB velocity gradient that is distributed over >50 km and that is consistent with effects of temperature alone. Where weak and intermittent LAB phases appear, they suggest more vertically localized velocity gradients produced by other factors such as bulk composition, volatile content, or contrasts in grain size or melt. In the tectonically active west, a positive Sp phase at depths consistent with the base of the asthenospheric low velocity zone in tomography models is intermittently observed. Beneath magmatic provinces in the west, the LAB Sp discontinuity deepens by ?10 km from the High Lava Plains, where magmatism has occurred from 0-10.5 Ma, to the northern region of the Columbia River Basalts, which has been magmatically quiet since 15 Ma. Here we suggest a model in which the negative LAB velocity gradient is created by a layer of partial melt ponding beneath a solidus-defined boundary. This model predicts that higher temperatures associated with more recent magmatism would result in a shallowing of the intersection of the geotherm with the solidus. Beneath the Yellowstone Caldera, the absence of an LAB Sp phase suggests that the contrast in seismic velocity between the lithosphere and asthenosphere has been erased by intrusion of partial melt and heat into the lithosphere. [ABSTRACT FROM AUTHOR]