Understanding noble gases in the context of mantle dynamics

Recalde, Nicolas 2025. Understanding noble gases in the context of mantle dynamics. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Noble gases are ideal tracers which provide major constraints on the degassing of the Earth’s mantle, and imply preservation of long-term geochemical heterogeneities. These constraints have been challenging to reconcile with the geophysical understanding of the mantle. In this thesis, I aim to better understand the links between thermal and compositional evolution of the mantle using He and Ar data and 3D numerical models of mantle convection. While degassing is linked to the processing history of the mantle, the role of convection in controlling mantle differentiation remains unclear. Models show that the influence of viscosity structure, radiogenic and secular heat on thermal evolution governs processing rates and the type of material that can melt. Efficient convective mixing is responsible of both scattering heterogeneities, which promotes their destruction, and mantle cooling, which tends to favour preservation of primitive undegassed material. Preservation is strongly dependent on the processing history, however current processing can’t be used to distinguish between histories. Atmospheric 40Ar, should nonetheless reconcile with Earth’s processing history. Hence, a second series of simulations investigates the influence of oceanic crust density and timing of continental crust extraction on mantle differentiation and 40Ar degassing. In agreement with previous works, preservation of key heterogeneities is crucial for mantle 40Ar, but is also compromised by heat-producing elements concentration and efficient mixing. To prevent extensive mantle degassing, results suggest early continental crust extraction would be required to better satisfy 40Ar constraints. A last suite of models interrogates the ability of Large Low Shear Velocity Provinces (LLSVPs) to preferentially store materials holding 3He and/or 40Ar. Results suggest mantle plumes are systematically associated with LLSVPs seismic signatures and favour the presence of dense primordial layer in order to explain Oceanic Island Basalts 3He/4He ratios. Models highlight the strong influence of subduction in controlling basal mantle structures’ mobility, shaping plume forests. This thesis further establishes that understanding He and Ar data in the context of mantle dynamics is fundamental to better constrain mantle evolution and key driving processes, i.e. convection, differentiation and degassing.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Funders:	NERC
Date of First Compliant Deposit:	9 April 2026
Last Modified:	10 Apr 2026 12:31
URI:	https://orca.cardiff.ac.uk/id/eprint/186298

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