Seismic Expressions of Thermochemical Mantle Plumes

Abstract

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