Aqueous fluids are critical agents in the geochemical evolution of Earth’s interior. Fluid circulation and fluid–rock reactions in the Earth take place at temperatures ranging from ambient to magmatic, at pressures from ambient to extreme, and involve fluids that range from nearly pure H2O through to complex, multicomponent solutions. Consequently, the physical and chemical properties of hydrothermal fluids vary widely as functions of geologic setting; this variation strongly impacts fluid-driven processes. This issue will focus on the nature of geologic fluids at hydrothermal conditions and how such fluids affect geologic processes in some major settings.

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