Thermally Dominated Deep Mantle LLSVPs: A Review

Abstract

The two large low shear-waveShear-wave (S wave) velocity gradient velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth’s thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical ‘piles’ comes from seismic constraints, including the following: (i) their long-wavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocityBulk-sound velocity anomalies (and elevated ratios between shear- and compressional-waveCompressional-wave (P wave) velocity anomalies); (v) anti-correlated shear-wave and density anomaliesDensity anomalies ; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an isochemical, well-mixed mantle. In addition, it has been proposed that hotspots and the reconstructed eruption sites of large igneous provinces correlate in location with LLSVP margins. In this paper, we review recent results which indicate that the majority of these constraints do not require thermo-chemical piles: they are equally well (or poorly) explained by thermal heterogeneity alone. Our analyses and conclusions are largely based on comparisonsSeismic tomography models comparisons between imaged seismic structure and synthetic seismic structures from a set of thermal and thermo-chemical mantle convectionConvection models, which are constrained by ~300 Myr of plate motionPlate motion histories. Modelled physical structure (temperature, pressure and composition) is converted into seismic velocities via a thermodynamic approach that accounts for elastic, anelastic and phase contributions and, subsequently, a tomographic resolution filter is applied to account for the dampingSeismic model damping and geographic biasReference seismic model bias inherent to seismic imagingSeismic imaging . Our results indicate that, in terms of large-scale seismic structure and dynamics, these two provinces are predominantly thermal features and, accordingly, that chemical heterogeneity is largely a passiveMargin passive componentChemical heterogeneities passive component of lowermost mantle dynamics.