Geochemistry of lithospheric aqueous fluids modified by nanoconfinement

Water is a principal component of Earth's fluids, and its interaction with rocks governs lithospheric geochemical and geodynamic processes. Water-rock interactions are crucial in societally relevant resource management, including subsurface extraction and storage of energy, the deep carbon cycle and generating critical metal deposits. The prevailing view is that fluids navigate through the lithosphere without being influenced by the distinct properties that arise from matter confined at the nanoscale. Here we use electron microscopy and neutron scattering data to show that a diverse range of lithospheric rocks, including sandstones, peridotites and serpentinites, consistently show nanoporosity, predominantly with pore sizes < 100 nanometres. Using molecular dynamics simulations, we demonstrate that water's dielectric permittivity-a fundamental property that governs its geochemical behaviour-diverges in nanoconfinement from its bulk counterpart under conditions ranging from ambient to extremes of 700 °C and 5 GPa. Our geochemical simulations suggest that changes in water permittivity due to confinement will decrease mineral solubility, a process that is not currently considered in models of fluid-rock interactions. Given that permittivity is also intimately linked to ion speciation, pore-size-dependent properties should be expected to exert a primary influence on rock reactivity and the geochemical evolution of fluids during fluid-rock interactions.