Utilising mantle seismic structure to constrain mantle circulation

Morgan, Gwynfor 2025. Utilising mantle seismic structure to constrain mantle circulation. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Earth’s mantle’s sub-solidus convection is studied with numerical models. The impact of major phase transformations in the mantle transition zone (MTZ) on flow is already well understood. Phase transformations are associated with ‘discontinuities’ in 1D seismic structure (d410 and d660) whose topography is observed globally using SS-precursors. Recently, advances in computational and experimental mineral physics have improved knowledge of mantle phase relations, revealing a patchwork of stability fields with ‘branching’ and ‘curving’ boundaries in pressure-temperature space. Using parametrisations of physics that relates mineral physics to geodynamics, and both of these to seismological observations, this thesis evaluates the impact more complex phase boundaries have on mantle dynamics and how mantle models might be evaluated with reference to observations of d410 and d660. Others suggest ‘non-linear’ phase boundaries in pressure-temperature space could have distinct dynamic impacts. Chapter 2 investigates this for the ringwoodite-out via akimotoite and garnet-out reactions, ‘branching’ and ‘curving’ phase boundaries respectively. In 3D spherical shell mantle convection and circulation models, impacts are found to be negligible. Chapter 3 introduces methods for predicting topography on d410 and d660 from mantle circulation models and simply filtering for SS-precursors’ sensitivity. A mantle circulation model (MCM) is evaluated and does not satisfy observations, overpredicting MTZ temperature range and underpredicting the recycled chemical heterogeneity. MCMs where viscosity structure, compositional initial condition, and core-mantle boundary temperature are presented in Chapter 4. d410-d660 structure are predicted and the models are assessed against published observations. Viscosity structure dominantly controls the variation of the discontinuity topographies, but no simulation provided a satisfactory fit to d410 and d660. Specifically, the reported positive correlation of the discontinuities was elusive. Various explanations for this correlation are found insufficient. Although synthetic discontinuity structure does not yet seem to be a simple constraint on MCMs, they show promise for understanding Earth observations.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Funders:	NERC GW4+ DTP
Date of First Compliant Deposit:	23 January 2026
Last Modified:	23 Jan 2026 14:23
URI:	https://orca.cardiff.ac.uk/id/eprint/184145

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