Thematic Articles

Hot fluids are nearly ubiquitous in volcanic environments in the Earth’s crust. Magma at depth heats groundwater which then ascends towards the Earth’s surface through faults, fractures, and otherwise permeable rocks. Fluids in geothermal systems offer direct insight into the many complex chemical and physical processes that occur in these extreme environments. They are also analogues of many ore-forming systems. Scientists have advanced our understanding of fluids in geothermal systems by studying wells sunk ~2–3 km deep into many geothermal fields. Today, we are targeting deeper and hotter reservoirs, at or near the contact of magmatic bodies, which provide unique opportunities to study, and potentially utilize, supercritical fluid resources in the near future.

Magmatic-hydrothermal fluids play a key role in a variety of geological processes, including volcanic eruptions and the formation of ore deposits whose metal content is derived from magmas and transported to the site of ore deposition by means of hydrothermal fluids. Here, we explain the causes and consequences of fluid saturation in magmas, the corresponding fluid-phase equilibria, and the behavior of metals and ligands during the transition from magma to an exsolved hydrothermal fluid. Much of what we know about magmatic-hydrothermal systems stems from the study of fluid inclusions, which are minute droplets of fluids trapped within minerals during mineral growth.

Fluids are essential to the physical and chemical processes in subduction zones. Two types of subduction-zone fluids can be distinguished. First, shallow fluids, which are relatively dilute and water rich and that have properties that vary between subduction zones depending on the local thermal regime. Second, deep fluids, which possess higher proportions of dissolved silicate, salts and non-polar gases relative to water content, and have properties that are broadly similar in most subduction systems, regardless of the local thermal structure. We review key physical and chemical properties of fluids in two key subduction-zone contexts—along the slab top and beneath the volcanic front—to illustrate the distinct properties of shallow and deep subduction-zone fluids.

Metamorphic reactions within the Earth’s crust produce fluids of variable composition that play a major role in the evolution of continents. Metamorphic fluids facilitate reactions that alter crustal rheology, reduce melting temperature, cycle elements between geological reservoirs and form ore deposits. These fluids are relatively inaccessible, other than by study of fluid inclusions, so most studies rely on a combination of indirect evidence and predictive thermodynamic models to determine the characteristics and roles of the fluids. In this article, the origins, compositions, controlling phase equilibria, and roles of metamorphic fluids are reviewed, followed by a discussion of selected areas of current and future research.

Metamorphic reactions within the Earth’s crust produce fluids of variable composition that play a major role in the evolution of continents. Metamorphic fluids facilitate reactions that alter crustal rheology, reduce melting temperature, cycle elements between geological reservoirs and form ore deposits. These fluids are relatively inaccessible, other than by study of fluid inclusions, so most studies rely on a combination of indirect evidence and predictive thermodynamic models to determine the characteristics and roles of the fluids. In this article, the origins, compositions, controlling phase equilibria, and roles of metamorphic fluids are reviewed, followed by a discussion of selected areas of current and future research.

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.