The dynamics of magmas and mixing in fissure systems

Havard, Tegan (2025) The dynamics of magmas and mixing in fissure systems PhD thesis, University of Liverpool.

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Abstract

The mixing and mingling of magmas are widespread occurrences in igneous systems that have important implications for the petrogenesis of magmatic systems, volcanic eruption triggers, and the severity of their hazards. This thesis examines the dynamics of these mixing and mingling processes within dykes, a near-vertical planar magma conduit that forms an integral part of fissure systems and feeding of volcanic eruptions. The means of understanding the subterranean plumbing system, by necessity, relies on indirect methods. Scaled analogue experiments are simplified representations of processes that allow investigation of processes on time and length scales suitable for the laboratory with results applicable to the natural system. Field studies of exposed, fossilised dyke systems reveal the internal architecture and textures that offer insight into the conditions when the dyke was active. This thesis demonstrates the complexity of magma mingling and mixing dynamics within dykes, how scaled analogue experiments can offer insights into natural systems, and how mingling and mixing textures can offer insights into the formation of dykes. In this thesis, scaled analogue experiments are used to investigate how a dyke geometry and the endmember magma properties (viscosity, density) affect the mixing and mingling process. Fluid pairs differing in viscosity and density are set to interact in a dyke-like geometry. Mixing is enhanced by more similar physical properties between endmembers and low viscosities, and appears to be enhanced by a dyke (slot) geometry relative to a chamber (cuboidal) geometry. The mixing ratio trends of the experimental data are comparable to near-real time geochemical mixing data from the Kīlauea 2018 eruption, suggesting dynamic similarity between the systems. Expanding further, scaled analogue experiments are conducted to explore the dynamics of mingling and mixing during ascent within a growing dyke geometry. Fluid pairs differing in density are successively injected into gelatine, growing as a dyke. Immiscible fluids exhibit mingling behaviour, whilst miscible fluids exhibit mingling and mixing behaviour at low volumetric flux, and mixing at high volumetric flux. The degree of mingling and mixing is higher at high flux. Dyke propagation is primarily controlled by the flux and fluid buoyancy, meaning dyke propagation alone does not offer insights into the internal mingling and mixing fluid dynamics. Two internal flow regimes are resolved using laser-based particle image velocimetry techniques: upwards unidirectional flow only evidenced by the oil, and central jet and recirculation by all other fluids. This reveals fluid buoyancy plays a role in determining the flow regime. The eruptive material can be highly variable and is determined by the magma interaction process, mingling or mixing, along with flux. The potential high variability of eruptive products highlights the need for high-resolution spatial and temporal datasets to better understand the subsurface dynamics. The Streitishvarf composite dyke, Iceland is a natural case study of a well-exposed eroded dyke system that exhibits mingling and mixing textures on a range of spatial scales. For three separate sites, spanning ~15 km horizontal distance and ~700 m vertical distance, field observations, petrographic analyses, and analyses of enclave orientation and morphology are conducted. Three main compositional groups are identified, the geometry and distribution of which is variable across the different sites. Magma mingling is evidenced by enclaves and compositional banding, which may be concurrent processes, and magma mixing is evidenced by the hybrid composition group, petrographic compositions, and crystal textures. These processes likely occurred during the initial magma interaction (proposed basalt dyke intersecting rhyolite reservoir) and during the emplacement of the composite dyke. There are multiple enclave formation events inferred from the variable enclave composition and morphology, whilst the differing orientation of enclaves from sub-parallel to the dyke strike at deeper exposure to sub-perpendicular at shallower exposures indicates differing flow regimes are captured within the dyke. Further work is required to refine the emplacement scenario of the SCD.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Analogue experiment, Composite dyke, Dyke, EDS, Flow dynamics, Gelatine analogue, Magma mingling, Magma mixing, Particle image velocimetry, PIV, SEM, Streitishvarf, Volcanic plumbing system
Divisions:	Faculty of Science & Engineering Faculty of Science & Engineering > School of Environmental Sciences
Depositing User:	Symplectic Admin
Date Deposited:	15 Aug 2025 08:03
Last Modified:	15 Aug 2025 08:04
DOI:	10.17638/03193107
Supervisors:	Kavanagh, Janine Mariani, Elisabetta Jones, Thomas
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3193107
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