title: Porosity and permeability development in evolving fragmental volcanic systems
Creators: name: Weaver, Joshua
Publisher: name: University of Liverpool Repository
Year: 2023

description: The capacity for fluid percolation in volcanic environments may be considered a double-edged sword. On the one hand, high permeabilities offer both a pressure release valve to mitigate the explosive potential of a magma, and access to valuable clean energy in geothermal reservoirs. Conversely, low permeabilities are associated with dangerous, highly pressurised magma which have a propensity to violently erupt. In geothermal systems, impervious rocks may prohibit resource utilisation, or provide the necessary cap rock to seal-in and maintain the desired high temperature fluids. In both active volcanic environments and hydrothermal systems, fluid flow is heavily concentrated through fracture networks and fragmental systems, such that they play a central role in determining the style of volcanism and the potential for geothermal energy production. In this thesis, I investigate how the evolution of several fragmental systems may impact their porosity and permeability development. Specifically, I analyse how dehydration affects juvenile fragmental melts which remain in viscous environments and how altered fragmental deposits respond to thermal stress. Before exploring the evolution of the aggregate fragmental systems, I first assess individual melt fragments which are dehydrating in an open system, such that volatiles may escape the melt into vesicles, via vesiculation, or into the surrounding atmosphere, via diffusion out of the sample. Where volatiles move into vesicles, the isolated bubble growth expands the fragment volume, whereas volatiles released from the fragment do not directly impart a geometry change. I demonstrate that in pyroclasts, vesiculation will strive towards equilibrium with the closed system conditions, and so it is a fragment size independent process; volatiles continually diffuse into vesicles until the water content of the melt drops to the melt solubility limit, such that the closed-system vesicularity of fragments can be assessed using bubble growth models. On the other hand, diffusive outgassing equilibrates the melt with the conditions of the open, surrounding gas, and so the effectiveness of diffusive volatile loss is determined by the surface area of the melt-atmosphere interface. I observe that dehydration caused by diffusive outgassing progressively impacts deeper into the fragment, where it causes exsolved volatiles to resorb and the vesicles to shrink and be lost. Accordingly, a dense and dehydrated rind forms at the sample margin, which thickens with the lengthscale of diffusion. I show that where vesiculation and diffusive outgassing occur in a melt fragment concomitantly, the processes compete to expand and densify the fragment. Because the rate of diffusive outgassing is determined by the melt surface area, the size of pyroclasts controls this competition, such that, as fragment size decreases, the vesicularity moves increasingly away from the closed-system bubble growth models. I find that smaller fragments attain lower vesicularity profiles than larger fragments, and that over time, fragments of all sizes will densify and eventually lose all vesicularity. In fragmental aggregate systems, this evolution in individual melt fragments is likely inversed in the volume of the inter-fragment pore space, leading to implications for the porosity and permeability of the system. I monitor the evolution of vesicularity, connected porosity, and permeability in open fragmental melts with various grain sizes, to assess how the concurrent processes of vesiculation, diffusive outgassing, and sintering interact. I find that as melt vesiculates, the expansion of fragments causes a commensurate loss in the inter-fragment pore space, which causes a reduction in permeability. However, this process is transient whilst the system remains open to the atmosphere, as diffusive outgassing causes fragment contraction, which reverses the porosity and permeability impact of vesiculation. Overprinting these processes, sintering continues to densify the melt and will ultimately close the permeable network. From the complex fragment size controls for these processes, I establish regimes which determine the general evolution of porosity and permeability during sintering of hydrous melts. Finally, I assess the impact of dehydration in hydrothermally altered volcaniclastic reservoir rocks. I explore the thermal stability of hydrous minerals in hyaloclastites and investigate how the dehydration and dissolution of matrix constituents influences the porosity, permeability, and mechanical behaviour of the bulk rock. I find that at relatively low temperatures, which are applicable for geothermal resources, smectite dehydrates, causing the mineral lattices to densify and ultimately, collapse. This dissolution creates pore volume, which is increasingly connected, such that thermal stress increases permeability without necessitating the formation of fractures. The increase in porosity reduces the compressive and tensile strength of the hyaloclastites. I show that rocks containing phyllosilicate minerals may be susceptible to thermal fluctuations, and that this enhances porosity and permeability and reduces strength, which may then facilitate mechanical compaction at lower stresses, with significant implications for geothermal reservoir rocks and magmatic host rocks. Through these studies I highlight that dehydration in fragmental volcanic systems can produce complex porosity and permeability evolution. If these systems are to be well understood, a careful assessment of their compositions (particle size distributions and mineralogies) is required and their thermal, chemical, and physical environmental conditions should be well constrained.