| Summary: | The work in this thesis has been motivated by some of the outstanding questions about Earth structure, in both the mantle and inner core, and has allowed me to draw conclusions not only about physical features but also chemical and temperature characteristics and the limitations of some of our current seismic techniques. The mantle represents the largest single division of the Earth, and seismic data are used to show that it is a region of pervasive heterogeneity, with few globally coherent discontinuities. The mantle transition zone and its bounding discontinuities at 410km and 660km depths are the subject of frequent study, but are still providing us with new insights. Receiver function data provide compelling evidence for complexity at the base of the mantle transition zone. Incompatible results for different seismic phases (SS precursors and P-to-SV conversions) and disagreement with synthetic models, suggest chemical heterogeneity in the middle of the transition zone, which supports models of episodic convective flow across the region. The concept of linking seismological observation of velocity discontinuities with phase transitions in mineral components has permitted direct estimations of chemical and thermal conditions. Synthetic modelling indicates that a linear relation only applies in a restricted, fairly low temperature range however, due to multiple phase transitions near 660km depth. Receiver functions and SS precursors both suggest an ambient potential temperature of ~1250°C, with regional variations up to ±300°C, when compared with a pyrolite mantle model. This temperature is fairly cool and would support a deep plume, rather than shallow, origin for hotspots. The inner core, in contrast, is the most challenging region of the Earth to study, both directly using seismology and indirectly through high pressure and temperature experiments. Many parameters, including shear attenuation and velocity, remain only poorly constrained.
|
|---|
