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Encyclopedia of Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014

Hydrothermal Vent Fluids (Seafloor)

Andrea Koschinsky* 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 with under high- temperature high-pressure conditions, leading to enrichments in dissolved components such as and . Hydrothermal vent fluids may also originate from discharges of -derived from beneath 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 (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 , , and ) from the rock, of specific mineral phases, and redox reactions including the formation of reduced gases such as , 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 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 phase. This process is a major cause of 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” . 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 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 can be found chemical composition of the ocean. The reduced gases in the fluids also nurture rich specially adapted hydrothermal that are based on chemosynthetic (i.e., by gaining and chemical components directly from the fluids) rather than (see Fig. 1).

Occurrence of Hydrothermal Vent Fluids

The majority of producing vent fluids occurs along the mid-ocean ridges, which span about 60,000 km through the global (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 , 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 Basin in the Western Pacific.

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Fig. 2 Global distribution of seafloor hydrothermal systems and related mineral deposits, with about 300 sites of high-temperature hydrothermal venting (Beaulieu, 2010) ae3o 8 of 3 Page 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 (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 of the hydrothermal fluids is reflected by the absence of 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 . In back-arc hydrothermal systems, magmatic fluids (i.e., waters exsolved from - rich ) 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 , 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 -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 load as minerals, forming massive sulfide deposits, mineral chimneys, and black smoke in the rising plume

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(e.g., Hannington et al., 2005). The steep temperature and geochemical gradients in the mixing zones to a sequence of precipitation of sulfide minerals and others, such as , , silicates, and finally, in more oxidized zones, oxyhydroxides of Fe and Mn. These deposits are an important sink of metals mobilized from the oceanic crust by the hydrothermal fluids and thus limit the amount of material that is transported into the oceanic . They are very rich in many valuable metals such as Au, Ag, Cu, and Zn (e.g., de Ronde et al., 2011), depending on the composition of the ore-forming fluid. Hence, they may represent important metal resources for our future needs in modern technologies, although estimated overall quantities of deposits are small in scale relative to terrestrial metal resources (Hannington et al., 2011; Hein et al., 2013).

Role of Hydrothermal Vent Fluids for Supporting Deep- Vent Communities and the Origin of

Hydrothermal vent fluids do not only feed hydrothermal ore deposits with precious metals but also nurture rich hydrothermal ecosystems with their chemical energy and material released in the fluids mixing with seawater (e.g., van Dover, 2000). The members of these ecosystems are both adapted to extreme conditions in the fluids, such as low pH, high temperatures, and high metal contents, and are able to sustain bioproductivity without any light but based on the material and energy available in the vent fluids (e.g., Tunnicliffe, 1992). During , gain energy from redox reactions in the mixing zone to build up and feed a rich of grazers, symbiotic , , tube worms, and other . While the type and composition of the fluid has an impact on the evolution of the , at the same time the organisms change the fluid composition in the mixing zone. They mediate redox reactions, take up chemical components, and excrete others that may influence the (bio)chemical reactivity and fluxes of these metals. Hydrothermal fluids are also discussed to be potential sites where life on Earth may have evolved. While in early Earth history the surface of the Earth was a rather hostile place, the seafloor was a more protected place with hydrothermal fluids providing energy and chemical components suitable to sustain microbial life (Martin et al., 2008). However, also in the search for , the potential existence of hydrothermal fluids plays a major role. Astrobiological research focuses on geothermally active bodies in our in which water exists or may have existed, such as and Venus, or the icy of the giant in the outer solar system, such as and (e.g., Vance et al., 2007). Hence, knowledge on the functions of hydrothermal fluids for life on Earth is an important prerequisite for the search for life elsewhere in our solar system.

Role of Hydrothermal Vent Fluids for Heat and Energy Transfer from the Oceanic Crust into the Ocean

Hydrothermal fluids are a crucial medium for the exchange of heat and between the oceanic crust and the ocean (Kadko et al., 1995; Baker, 2007). The globally widespread occurrence of hydrothermal fluid emanations on the seafloor, which has been operating for most of Earth’s history, makes hydrothermal input one of the main, but poorly quantified, sources of components such as trace metals into the global oceans. The magnitude of hydrothermal input seems to be on the order of riverine input for some elements, such as Mn (Edmond et al., 1979; Elderfield and Schulz, 1996). Among the many problems that exist for a more precise assessment of hydrothermal element fluxes to the ocean are the unknown total number and size of hydrothermal vent fields and their chemical

Page 5 of 8 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_17-1 # Springer Science+Business Media Dordrecht 2014 variability, the role of poorly quantified or undiscovered off-axis flow, and an incomplete under- standing of the transformation processes of fluid components during mixing of fluid and ambient seawater and within the rising and dispersing hydrothermal plume. Formation of mineral deposits, oxidation of Fe and Mn in the plume, and scavenging of trace metals on Mn and Fe oxides in the hydrothermal plumes (German et al., 1997) significantly reduce the hydrothermal metal fluxes of many elements, for example. However, modeling and chemical studies have shown that for Fe, the hydrothermal metal flux must be significantly higher than previously assumed, probably contributing to the biogeochemical cycling of Fe in the oceans (Tagliabue et al., 2010). While the exact mechanisms of metal and transformation from the vent sites into the ocean are still unclear, several studies hint at a strong role of colloidal sulfides or hydroxides and organic metal complexes (Sander and Koschinsky, 2011).

Summary and Conclusions

Hydrothermal fluid emanations at the seafloor are a widespread phenomenon in volcanic, magmatic, or tectonically active areas in the oceans, such as mid-ocean ridges and submarine volcanoes. Their wide range in emanation temperatures (up to >400 C), chemical composition (up to millimolar concentrations in reduced gases and metals), and temporal variability reflect the specific nature (such as water depth and host rock) of the site location. Hydrothermal fluids are important on a global scale for the formation of metal ore deposits on the seafloor, for nurturing chemosynthetic ecosystems specifically adapted to extreme conditions, and for metal fluxes into the water column and the chemical budget of the ocean. However, the exact nature and quantity of the related processes is still not fully understood. It can be expected that each expedition discovering new hydrothermal vent systems will broaden our view on the diversity of hydrothermal vent fluids and help understand their relevance for the exchange of heat and material between the oceanic crust and the ocean.

Cross-References

▶ Hydrothermal Plumes ▶ Hydrothermalism ▶ Marine Mineral Resources ▶ Oceanic Spreading Center ▶ Serpentinization ▶ Spreading Axis ▶ Volcanogenic Massive Sulfides

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