The rock physics of cemented volcanic ash in alkaline hydrothermal environments
Keywords: Geophysics.

The interplay between faulting and fluid-mediated cementation processes in the Earth's subsurface results in crustal rock properties that are non-static but instead vary both spatially and temporally. In particular, the reaction between alkali-rich fluids and volcanic ash in hydrothermal systems can create cemented ash seals at geologically rapid rates, with the strength and permeability of the cemented ash determining the performance of the seals. The detection, diagnostics and remediation of subsurface properties requires a rock physics basis for interpreting the corresponding changes in rock physics properties as a function of the new cementing phases. By recreating alkali-fluid-ash cementation reactions in the laboratory under hydrothermal conditions, I investigated how the chemical and hydrothermal conditions control the mineralogy, elastic, mechanical, storage and transport properties. By varying the amount of CaO content available for the cementation reaction, I analyzed how the volume of cement affects the physical properties. I developed a mineralogical model to calculate the relative abundance of volcanic ash, reaction-produced calcium-alumino-silica-hydrate (C-A-S-H) cement, and precipitated portlandite (Ca(OH)2) in the cemented ash samples. The calculated phase separation as a function of CaO content is important for linking chemical and physical modeling. By comparing the calculated mineralogy to the elastic and transport properties, I highlighted that both the C-A-S-H cement and portlandite contribute to decrease the permeability, while only the C-A-S-H cement significantly contributes to increase the elastic moduli. These competing influences result in a change in the rock physics relationship between the permeability and P-wave velocity as a function of alkali content. By changing the hydrothermal temperature during the formation of potassium-rich fluid-ash cements, I analyzed how the type of cement phase that forms affects the physical properties. Chemical structure and grain density measurements indicate that at 25ᵒC geopolymer phases form, at 100ᵒC crystalline zeolites form, and at 300ᵒC crystalline feldspathoids form. By comparing the mineralogy to the mechanical, storage, and transport properties, I highlighted the competing effects of the mineral stability and the mineral densification, with the densification leading to a significant increase in porosity and permeability. The performance of these cementing phases in the subsurface depends on their long-term stability and cohesion with adjacent formations, which cannot be easily evaluated with laboratory samples. To address this issue, I characterized the properties of Roman marine concrete (RMC)—a man-made alkali-fluid-ash cement that has undergone weathering processes and includes rock aggregates of varying type and distribution—to investigate how the cohesion of the cement phases and aggregate rocks affect the mechanical and transport properties. I identify a previously unreported intertwined network of calcium-sulfo-aluminate-hydrate fibers embedded in a matrix of crossbred C-A-S-H and geopolymer phases. The richness in alkalis is particularly important as it enhances the cohesion at the fiber-matrix interface through polymerization. I also show that the use of compositionally similar aggregates that can participate in the cementation reaction limits the formation of a matrix-aggregate interfacial zone and increases the cohesion between the matrix and aggregate phases. Altogether, the increased cohesion enables the RMC samples to creep and exhibit a ductile fracture behavior under triaxial loading and also lowers the permeability. This thesis provides a coherent data set to understand how the reaction conditions, both physical and chemical, control the micro- and macroscopic properties of alkali-fluid cemented alumino-silicates and a framework for modeling the scattered geophysical observations in volcanic hydrothermal systems and cemented faults