The nanoscale control of geofluids in the solid Earth

Fluid-rock interactions are pivotal in numerous geological processes, including heat and mass distribution in the lithosphere, the deep carbon cycle, and environmental geosciences. While fluids navigate through the lithosphere via well-connected pore spaces—if available—and along macroscale cracks and fractures, a growing body of evidence indicates that at smaller scales, interfacial phenomena, and fluid flow through nanoscale pathways also play a significant role in mass transfer within the solid Earth. Fluids exhibit distinct physical and transport properties when confined in nanoscale spaces. Therefore, considering nanoconfinement-induced phenomena is crucial for accurately predicting geofluid interactions with rocks. The present thesis aims to unravel these nanoscale controls on geofluids and fluid-rock interactions in the solid Earth. The thesis begins with imaging nanoporosity in serpentinites. Our multi-scale, multi-dimensional imaging techniques, along with discrete element modelling, reveal that serpentinites function as nanoporous media with pore sizes predominantly less than 100 nm. In the following chapter, I extended the characterisation of nanoporosity in Earth materials by analysing small-angle neutron scattering data and high-resolution electron microscopy images. My results reveal that a diverse range of lithospheric rocks consistently exhibit nanoporosity, with pore sizes predominantly smaller than 100 nanometres in diameter. Using molecular dynamics simulations, I reveal that water shows unique dielectric properties when confined within slit nanochannels of different minerals. My results show that both perpendicular and parallel permittivity components significantly deviate from their bulk water counterparts. The perpendicular permittivity, which affects mineral solubility, decreases drastically under nanoconfinement. Incorporating these nanoconfinement-based permittivities into thermodynamic equilibrium models suggests that this reduction in perpendicular permittivity due to nanoconfinement will lower the solubility of calcite and quartz under high P-T conditions. Geofluids in nature are typically not pure water but a mixture of water, salts, and gases in specific settings. Therefore, it is crucial to quantify how these additional components influence the permittivity of fluids confined within rock nanopores. To assess the effect of salinity, I used molecular dynamics simulations to calculate the permittivity of saline water in calcite slit nanopores under low P-T conditions. NaCl at various concentrations served as the salt, reflecting the composition of geofluids confined within nanoporous geological structures. The results show that the dielectric properties are weakly dependent on salinity for both permittivity components. This indicates that surface-induced phenomena primarily govern the dielectric responses, dominating the effects of salt ions in the solution. This thesis concludes by evaluating how changes in dielectric permittivity due to nanoconfinement influence the geochemistry of water in the solid Earth. I incorporated the newly determined nanoconfinement-based permittivities into existing thermodynamic equilibrium models and explored water self-ionization, ion pairing, pH, and aqueous speciation under nanoconfinement in the deep Earth. The results show that nanoconfinement profoundly affects water behaviour. This thesis highlights the significance of nanoporosity in geological formations and its impact on geofluid behavior. Nanoconfinement affects geochemical processes such as fluid transport, reaction rates, and mineral solubility. To improve predictions of fluid-mediated processes within the Earth, geochemical models should integrate nanopore dynamics and the unique properties of nanoconfined fluids.