title: Development, impact and longevity of fractures in magmatic, volcanic and geothermal systems
Creators: name: Lamur, AL
Publisher: name: University of Liverpool Repository
Year: 2018

description: The migration of fluids in the Earth’s crust embodies the last stage of the internal heat release of our planet. Either spectacularly expressed at the surface through volcanic activity, or more subtly as internal hydrothermal circulation, this phenomenon involves the upwards motion of fluids and magmas that contribute to more efficient heat transfer. On one hand, volcanic eruptions result from the movement of buoyant magmatic liquids towards the surface. On their way up, these magmas cool down, crystallise and upon decompression, build up an internal pressure that dictate the eruptive style: Effusive when the internal pressure is released as it builds up; explosive when the internal pressure accumulates until it is able to fracture the magma. In nature, the shift from effusive to explosive activity is often periodic, reflecting cycles of pressure accumulation and relaxation in the conduit. On the other hand, hydrothermal circulation results from the infiltration of water, of meteoric or magmatic origin, into the rocks making up the crust. Higher temperatures at depth and/ or due to the presence of a magmatic body, trigger the formation of convective cells in which chilled, denser water percolates downwards then heats up, losing density before moving back upwards. Importantly, the circulation of both hydrothermal fluids and magmatic liquids is controlled by the presence of fractures in the crust and the permeability of the surrounding rocks. In this thesis, I first investigate how fractures affect the localisation of fluids in fractured porous rocks through permeability measurements, both at atmospheric pressure and at shallow confining conditions (&lt;30 MPa; ~1 km depth). I demonstrate that the impact of fractures is greater at lower porosities as the permeability is greatly increased. In more porous rocks, higher pore connectivity means that macro-fractures affect the permeability less significantly, as at least some of the fluid flows through the pre-existing connected porous network. I further demonstrate that, during confinement, most of the mechanical closure occurs at shallow conditions (&lt;5 MPa; ~200m depth) for the porosity range tested. In addition, I show that mechanically shutting a fracture does not seal the permeable pathways, and that the fractured system is unable to retrieve the same permeability as the intact system. I then develop an analytical solution for the permeability of variably porous, fractured systems as a function of depth before numerically solving it. I complement this work with two “case studies” in which tensile fractures form and open or heal, and link this to the system permeability evolution through time and the implications for magmatic, volcanic and geothermal systems. In the first scenario, tensile fractures open in a cooling magmatic body to form columnar joints in a basalt. Because the temperature at which cooling joints form remains elusive, causing a lingering scientific debate, I develop a novel type of mechanical testing and show for the first time that, in basaltic systems, these macro-fractures form purely in the elastic regime. This is further supported by the use of the rock’s thermo-mechanical characteristics (namely thermal expansion and tensile strength) to model the tensile stress build-up upon cooling and, once formed, the evolution of the fracture width between two columns. Applying the analytical solution for fractured systems permeability defined earlier, I further model the permeability evolution of a columnar jointing magmatic body, important for the understanding of fluid migration during drilling close to magmatic chambers. Finally, in magmas, the entrapment of exsolving gases, during ascent, force the accumulation of stress in the liquid, building pore pressure and potentially resulting in magmatic fragmentation. Consequently, the accumulated stresses can then be dissipated at times longer than the relaxation timescale of the melt, allowing fractures to heal and the system to recover strength lose permeability. Using synthetic glasses in a newly designed experimental setup, I show that the time required to start the healing process is proportional to the relaxation timescale of the melt. I further demonstrate that the kinetics of fracture healing involve two distinct stages. The first stage sees the fracture walls viscously deforming to dissipate excessive energy along the fracture plane (wetting regime), while the second stage consists of the diffusive exchange of elements across the fracture interface (diffusive regime). I finally surmise that the cyclic activity of persistently active silicic volcanoes could be explained by dynamic permeability and strength variations of material due to repeating fracture and healing cycles. Overall, I show that the development and longevity of fractures have significant impacts on the localisation of fluid flow, highlighting that fractures significantly contribute to the development of anisotropy in magmatic, volcanic and geothermal environments. A better understanding of the longevity of fractures in these systems is of prime importance in the mitigation of hazards associated to volcanic eruptions, but also in the development of cheaper, more efficient geothermal energy.