Seismic Expressions of Thermochemical Mantle Plumes

Over the last decade of geophysical research the concepts of hotspots and plumes have taken a central role in discussions of the interior structure of the Earth and global geodynamic plate and convection models. In this study, I focus on the ability of thermal and/or thermochemical plumes to reproduce global and regional seismic observations at hotspot locations on Earth. In order to make meaningful interpretations of seismic images from global tomographic images I begin with an investigation into the physical meaning of seismic reference models and a full exploration of the temperature and compositional sensitivities of mantle seismic velocities, utilising a fully consistent forward modelling approach with up-to-date mineral physics parameters and associated uncertainties. I determine that, despite three-dimensional complexity of the mantle, averaged seismic structure reflects the average radial physical structure of the mantle except near phase boundaries and within thermal boundary layers. In the second half of the study I produce synthetic plume signatures by converting the thermo-chemical strutures of a range of plausible dynamic whole mantle plumes into seismic velocities-including the effect of seismic resolution in global tomographic models by convolution of the seismic structures with a resolution filter for the global model S40RTS. Quantitative comparison of synthetic signatures with global seismic observations beneath a number of hotspots indicates that more than half of all studied locations are underlain by low-velocity anomalies with widths and magnitudes compatible with thermal plumes. Other locations, e.g. Iceland, require plumes with time-dependent morphologies, modified by chemistry or phase buoyancy forces. I next forward model the predicted transition zone seismic structure for a number for thermal and thermochemical whole mantle plume scenarios, before commenting on suitability of using transition zone thickness beneath hotspots as a proxy for temperature. Lastly, I finish with a discussion of how such an analysis might be extended to other terrestrial planets, such as Mars

Wavefront healing renders deep plumes seismically invisible

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86947/1/j.1365-246X.2011.05173.x.pd

Mapping spherical seismic into physical structure: Biases from 3-D phase-transition and thermal boundary-layer heterogeneity

Earth's mantle is, to a very good first approximation, spherically symmetric, with lateral deviations in seismic velocities and density of only a few per cent. This observation has led to the common assumption that average radial seismic models reflect the mantle's average physical structure. We test this assumption by using a set of dynamically generated mantle structures and comparing seismic velocities for the average radial physical state with laterally averaged seismic velocities. The thermal and thermochemical dynamic circulation models are Earth-like in terms of convective vigour, thermal structure and geographical pattern of heterogeneity. We find that, in general, averaged seismic structure is not distinguishable from the seismic structure of the physical average, within the uncertainty bounds of seismic reference models. An exception is near phase boundaries, where phase-boundary topography broadens the averaged seismic jump relative to the discontinuity at physical reference conditions. In an inversion for 1-D seismic structure, where narrow discontinuities are imposed, these biases may map into a lower jump, and substantially stronger velocity gradients above and below the interface than are actually present. Other small biases in averaged structure occur in thermal boundary layers, including those that form above chemical piles. These biases are caused by large lateral variations in temperature, not compositional variability

Thermochemical interpretation of 1-D seismic data for the lower mantle : The significance of nondiabatic thermal gradients and compositional heterogeneity

International audienceEquation-of-state (EOS) modeling, whereby the seismic properties of a specified thermochemical structure are constructed from mineral physics constraints, and compared with global seismic data, provides a potentially powerful tool for distinguishing between plausible mantle structures. However, previous such studies at lower mantle depths have been hampered by insufficient evaluation of mineral physics uncertainties, overestimation of seismic uncertainties, or biases in the type of seismic and/or mineral physics data used. This has led to a wide, often conflicting, variety of models being proposed for the average lower mantle structure. In this study, we perform a thorough reassessment of mineral physics and seismic data uncertainties. Uncertainties in both the type of EOS, and mineral elastic parameters, used are taken into account. From this analysis, it is evident that the seismic variability due to these uncertainties is predominantly controlled by only a small subset of the mineral parameters. Furthermore, although adiabatic pyrolite cannot be ruled out completely, it is problematic to explain seismic velocities and gradients at all depth intervals with such a structure, especially in the interval 1660­2000 km. We therefore consider a range of alternative thermal and chemical structures, and map out the sensitivity of average seismic velocities and gradients to deviations in temperature and composition. Compositional sensitivity is tested both in terms of plausible end-member compositions (e.g., MORB, chondrite), and via changes in each of the five major mantle oxides, SiO2, MgO, FeO, CaO, and Al2O3. Fe enrichment reduces both P and S velocities significantly, while Si enrichment (and Mg depletion) increases P and S velocities, with a larger increase in P than in S. Using purely thermal deviations from adiabatic pyrolite, it remains difficult to explain simultaneously all seismic observations. A superadiabatic temperature gradient does improve the seismic fit in the lowermost mantle, but should be accompanied by concurrent bulk chemistry changes. Our results suggest that the most plausible way to alter bulk chemistry in the lowermost mantle, simultaneously fitting density, bulk velocity and shear velocity constraints, is an increasing contribution of a hot, basalt-enriched component with depth