Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014 Hydrothermal Vent Fluids (Seafloor) Andrea Koschinsky* Earth and Environmental Sciences, School of Engineering & Science, Jacobs University, Bremen, Germany Synonyms Black-smoker fluids; Hydrothermal solutions; Seafloor hot springs Definition Hydrothermal vent fluid (seafloor): a hot (up to >400 C) aqueous solution discharging at the seafloor that typically originates from the reaction of seawater with oceanic crust under high- temperature high-pressure conditions, leading to enrichments in dissolved components such as metals and gases. Hydrothermal vent fluids may also originate from discharges of magma-derived waters from beneath submarine volcanoes, which typically mix with seawater or other vent fluids prior to venting. Introduction Hydrothermal fluids on the seafloor typically form by circulation of seawater in fractured oceanic crust at volcanically active sites in the ocean (Fig. 1) (Von Damm, 1995; German and Seyfried, 2014). These places include mid-ocean ridges, back-arc spreading centers, and hot-spot or arc-related submarine volcanoes. Circulating seawater is heated by a heat source such as a magma chamber or associated hot rock and, during heating and chemical reaction with the surrounding rock, undergoes a suite of chemical modifications. These include acidification, leaching of major and trace components (including trace metals such as copper, zinc, and gold) from the rock, precipitation of specific mineral phases, and redox reactions including the formation of reduced gases such as methane, hydrogen sulfide, and hydrogen. In the reaction zone in the subsurface, the fluids can reach temperatures of several hundred degrees Celsius, which makes them very buoyant. Furthermore, if fluid temperatures reach the boiling point under the respective pressure conditions during their circulation and ascent back to the seafloor, the fluids can boil and phase-separate into a vapor phase and a residual brine phase. This process is a major cause of salinity variations in vent fluids. After ascent the hot, low pH, and reduced fluids discharge at the seafloor through larger channels or finer cracks, reacting with the cold seawater and forming hot (up to >400 C) focused black-smoker fluids and, in most cases, sulfide edifices or “chimneys.” Mixing with seawater and the associated cooling and chemical changes induce the precipitation of sulfide minerals either forming sulfide structures or “black smoke” particles. When the fluid is cooled or diluted deeper in the subsurface prior to discharging at the seafloor, diffuse and translucent fluid emissions at lower temperatures seep through cracks in the seafloor. The metals discharged and minerals formed by the vent fluids are important for the formation of potentially valuable ore deposits at the seafloor and contribute to the *Email: [email protected] Page 1 of 8 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014 Fig. 1 Simplified sketch showing the evolution of hydrothermal fluids in a geologically active site; cold seawater entrains the fractured crust, heats up and reacts with the rock in the reaction zone, and then flows up again as hot hydrothermal fluid to the seafloor. If it is not significantly cooled and remains largely undiluted, it forms hot black smokers or white smokers (with whitish minerals forming at slightly lower temperatures) during mixing with cold ambient seawater. If mixing and cooling already takes place in the subsurface, part of the mineral load will precipitate here, and the fluids will be emitted as mostly transparent, cooler diffuse fluids. Green ovals show sites where hydrothermal habitats can be found chemical composition of the ocean. The reduced gases in the fluids also nurture rich specially adapted hydrothermal ecosystems that are based on chemosynthetic primary production (i.e., by gaining energy and chemical components directly from the fluids) rather than photosynthesis (see Fig. 1). Occurrence of Hydrothermal Vent Fluids The majority of hydrothermal circulation producing vent fluids occurs along the mid-ocean ridges, which span about 60,000 km through the global oceans (see, e.g., http://vents-data.interridge.org/ ventfields and Fig. 2). The first low-temperature (up to 17 C) hydrothermal vents had been discovered at the Galapagos Spreading Center in 1977 (Corliss et al., 1979; Edmond et al., 1979), while the first black smokers with temperatures around 380 C had been found at 21N on the East Pacific Rise (EPR) in 1979 (Spiess et al., 1980). While hydrothermal fluid emanations seem to be more abundant on fast-spreading ridges such as the EPR, dozens of active hydrothermal vent sites have also been discovered on slow-spreading ridges such as the Mid-Atlantic Ridge (MAR) or Central Indian Ridge (see compilation by Edmonds (2010)). Even a few off-axis systems with fluids of a very distinct composition, such as the Lost City field on the MAR (Kelley et al., 2001), have been discovered. While hydrothermal activity and fluid composition seem to be highly variable on fast-spreading ridges on timescales of months to years, due to frequent volcanic activities (e.g., Baker et al., 1998), vent fluid composition has been shown to be rather stable over many years in systems on the MAR (e.g., Edmonds, 2010; Schmidt et al., 2011). Hydrothermal fluid venting is also found associated with hot-spot-related intraplate volcanism, such as at some Pacific islands like Hawaii, as well as at arc volcanoes and back-arc spreading centers such as the Manus Basin and North Fiji Basin in the Western Pacific. Page 2 of 8 # DOI 10.1007/978-94-007-6644-0_17-1 Encyclopedia of Marine Geosciences Springer Science+Business Media Dordrecht 2014 Fig. 2 Global distribution of seafloor hydrothermal systems and related mineral deposits, with about 300 sites of high-temperature hydrothermal venting (Beaulieu, 2010) Page 3 of 8 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014 Composition of Hydrothermal Vent Fluids Hydrothermal fluids from different locations or even within discrete vent fields can span a wide range of chemical composition, with individual parameters often varying over several orders of magnitude (see, e.g., German and Seyfried, 2014). The fluids are usually acidic, with pH values down to 2, major ions (such as Na, Ca, Cl, etc.) either increased or decreased relative to seawater, and the majority of elements including Si and most trace metals significantly enriched relative to seawater. The reducing nature of the hydrothermal fluids is reflected by the absence of oxygen and the presence of reduced gases such as hydrogen, methane, and hydrogen sulfide. Parameters determining the chemical composition of hydrothermal vent fluids include pressure (both the pressure of the sub-seafloor reaction zone and the emanation site at the seafloor), temperature during fluid evolution and emanation, the mineralogical and chemical composition of the host rock, and the reaction time. In back-arc hydrothermal systems, magmatic fluids (i.e., waters exsolved from water- rich magmas) also can affect hydrothermal fluids if they are added at depth within the reaction zone (Gamo et al., 2006; Reeves et al., 2011). These magmatic fluids may also discharge directly toward the seafloor forming unique acidic hydrothermal vents. Apart from in situ studies and direct sampling and analysis, laboratory experimental studies and thermodynamic calculations (e.g., Bischoff and Rosenbauer, 1985) have also helped to understand factors controlling the composition of hydrothermal fluids. High temperatures, as well as acidity, are necessary to leach large amounts of metals such as Fe and Cu from the rock and keep them in solution. Fluids emanating with very high temperatures and very rich in metals (up to millimolar amounts of Fe and Mn and micromolar amounts of Cu and other metals) can be found in deep (3,000 m water depth) hydrothermal systems, where the pressure- dependent boiling point allows the fluids to reach temperatures of 400 C (e.g., Koschinsky et al., 2008). Fluids reaching the boiling point phase-separate into a vapor phase rich in gases and a residual brine phase being rich in major ions and trace metals. Emanation of low-chlorinity vapor and high-chlorinity brine phases can be observed both spatially and temporally segregated, with the denser brine phase often emanating subsequent to the vapor phase (Butterfield et al., 1997; von Damm et al., 1997). The difference in composition for fluids from different types of hydrothermal systems is also related to the different host rocks that react with the entraining seawater. At fast-spreading centers, the fluids are characterized by reaction with basalt, resulting in high concentrations of S, Si, and many metals. At slow-spreading ridges such as the MAR, fluids often carry a pronounced ultramafic signature from reactions with mantle rocks, such as very high hydrogen and methane concentrations due to serpentinization reactions (e.g., Kelley et al., 2001; Charlou et al., 2002; Kelley et al., 2005; Schmidt et al., 2011). The few available fluid data from back-arc basins and island arcs indicate typically very low pH values and a strong enrichment of trace metals including As, Au, Hg, Pb, Sn, and Sb originating potentially from magmatic sources as well as leaching of the rocks (e.g., andesites) (e.g., Hannington et al., 2005; Yang and Scott, 2006; De Ronde et al., 2011; Reeves et al., 2011). Role of Hydrothermal Vent Fluids for the Formation of Ore Deposits When the hot metal-rich hydrothermal fluids mix with ambient seawater, either in the sub-seafloor or when they emanate at the seafloor, they precipitate large amounts of its metal and sulfur load as minerals, forming massive sulfide deposits, mineral chimneys, and black smoke in the rising plume Page 4 of 8 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014 (e.g., Hannington et al., 2005).