KeywordsHydrothermal Fluid Hydrothermal System Massive Sulfide Sulfide Deposit Black Smoker
Hydrothermalism refers to the convective circulation of seawater through oceanic lithosphere with possible contributions of volatiles and metals from underlying magma chambers.
The circulation of seawater through oceanic lithosphere is the principal mechanism for the transfer of heat and chemicals between the ocean and underlying crust and has a direct effect on the composition and heat budget of oceanic lithosphere and the composition of seawater (e.g., German and Seyfried, 2014). Along submarine tectonic plate boundaries (see “Mid-Ocean Ridge Magmatism and Volcanism,” “Magmatism at Convergent Plate Boundaries,” “Intraoceanic Subduction Zones,” “Hydrothermal Plumes”) and hot-spot chains, the heat associated with crystallization in shallow magma reservoirs and cooling of hot rock drives the circulation of seawater through the permeable crust. As the fluid circulates, it is heated, reacts chemically with the surrounding rock, and ultimately discharges back at the seafloor (see “Hydrothermal Vent Fluids”). To date, over 500 hydrothermal discharge sites have been discovered along submarine tectonic boundaries, at an average spacing of about one major discharge site per 100 km of ridge axis (http://www.interridge.org/IRvents). Cooling of the crust by hydrothermal circulation accounts for about 30 % of the heat lost from newly formed (<1 million year old) oceanic lithosphere (Wolery and Sleep, 1976; German and Seyfried, 2014).
The massive sulfide deposits that form on the seafloor are composed primarily of sulfide minerals, as well as sulfates (anhydrite and barite) and amorphous silica. The deposits can contain economically significant concentrations of Cu, Zn, Pb, Au, and Ag, and massive sulfide deposits that have been discovered in ancient oceanic crust now obducted on land (referred to as volcanogenic massive sulfide deposits) have been mined for their precious and base metal content and continue to be an important source for these metals. With the increasing global demand for metal resources and the decreasing rate of discovery of large ore deposits on land to satisfy this demand, modern hydrothermal sulfide deposits on the seafloor are increasingly being considered as a viable future source for metals (see “Marine Mineral Resources”). Although, at the time of writing, no modern seafloor massive sulfide deposits have been exploited, several companies and national governments are actively exploring the seafloor for massive sulfide deposits that are sufficiently large and exhibit sufficiently high metal concentrations to be exploited.
The following sections provide an overview of the physical and chemical characteristics of typical submarine hydrothermal systems. Many aspects of hydrothermal systems (e.g., black and white smokers, hydrothermal fluids, hydrothermal plumes, volcanogenic massive sulfides, chemosynthetic life) are covered in more detail in their own separate entries, and the reader is directed to those entries for further details.
The discovery of active hydrothermal venting on the seafloor, and especially their link to chemosynthetic life, was among the most significant scientific advances of the twentieth century. The link between seafloor volcanism and hydrothermalism in the oceans was first articulated as an outgrowth of plate tectonic theory in the 1960s, about 20 years before the discovery of black smokers, although the notion of massive sulfide deposits forming at submarine volcanoes existed among economic geologists long before then. By the late 1970s, many of the details of ore formation in the submarine environment had already been established from the study of fossil hydrothermal sulfide deposits on land. A significant metal enrichment had also been identified in sediments flanking the East Pacific Rise mid-ocean ridge, which was attributed to, as yet undiscovered, hydrothermal activity along the nearby ridge (Boström, 1966). Thus, the discovery of seafloor hydrothermal vents and the associated massive sulfide deposits came about not only because of technological advances that allowed deep diving to the mid-ocean ridges but also because of existing ideas about the likelihood that some form of hydrothermal activity should exist on the seafloor. However, those who participated on the discovery cruises did not foresee the spectacular “black smoker” chimneys that formed at the vents or the unusual biological communities that were found thriving in such extreme environments.
With the exception of well-defined discharge sites, the parameters that describe the geometry of hydrothermal systems (e.g., depth, fluid volumes, and recharge sites) are poorly defined. Insights can come from direct observations of the seafloor, seismic tomography, or analysis of bathymetric data. However, these observations are limited to only the surface expressions of hydrothermal systems. For example, although seafloor discharge sites are often observed to occur on or in close proximity to exposed faults, the relationship between hydrothermal upflow, discharge, and the permeable fault planes remains unclear (Tivey and Johnson, 2002). Much of our understanding of the subsurface geometry of hydrothermal systems has come from studies of ancient hydrothermal systems preserved in ophiolites (Hannington et al., 1995). For example, Richards et al. (1989) showed that hydrothermal upflow zones occur as discrete pipes beneath vent sites. Further evidence for a narrow, focused upflow zone has come from studies of thermally induced crustal demagnification (or “burn holes”) beneath hydrothermal discharge sites (Tivey and Johnson, 2002) and numerical fluid flow simulations (Coumou et al., 2008).
Hydrothermal recharge zones are especially enigmatic, since recharge is effectively invisible on the seafloor, especially compared to the dynamic activity at high-temperature discharge sites. Numerical models and heat flow measurements indicate that hydrothermal recharge can occur at scales ranging from tens of meters to several kilometers from the discharge area (Fig. 1) (Coumou et al., 2008; Johnson et al., 2010; Hasenclever et al., 2014). Recharge that occurs near the discharge site can include a component of recirculated, hot, hydrothermal fluid that did not vent at the surface, which results in a significant increase in the temperature of the circulating fluids in proximity to the discharge site. Recent numerical modeling indicates that about two thirds of the recharge occurs on-axis by warm (up to 300 °C) recharge close to the discharge sites, while one third occurs as colder and broader recharge up to several kilometers away from the axis (Hasenclever et al., 2014).
The following is a brief description of the evolution of hydrothermal fluid as it circulates through the oceanic lithosphere and the fluid-rock interactions that occur. The reader is directed to the entry on hydrothermal vent fluids for a more detailed description of the physical and chemical properties and evolution of hydrothermal fluids.
Seawater undergoes important chemical changes when heated. These changes were first established experimentally by Bischoff and Seyfried (1978) and Bischoff and Rosenbauer (1988) and are in agreement with the first measurements of hydrothermal fluids (Edmond et al., 1979). At ~130 °C, anhydrite, which has retrograde solubility, will begin to precipitate from dissolved Ca2+ and SO4 2− in the seawater, removing almost all of the Ca2+ and about one third of the sulfate. Beginning at about 25 °C, magnesium begins to precipitate from seawater in a brucite-like molecule [Mg(OH)2] as part of the smectite structure. When seawater is heated slowly in the recharge zone, Ca2+ is released at the same rate that Mg2+ is taken up (Elderfield et al., 1999) and the pH remains near neutral. At temperatures in excess of 150–250 °C, calcium is then lost from the fluid, when Ca silicates (prehnite, clinozoisite, tremolite) become stable. If seawater is heated rapidly (i.e., in chimney walls), protons may be released in exchange for Mg when magnesium hydroxide sulfate hydrate (MHSH) may precipitate and the pH drops. Considerable acidity is also generated when protons are released into solution in exchange for Ca2+. The released Ca2+ also combines with the remaining sulfate to form more anhydrite. Leftover sulfate is thermochemically reduced to hydrogen sulfide by reaction with ferrous iron in the host rock. As a result, SO4 2− concentrations decrease to nearly zero by the time fluids reach their maximum temperature.
Sequential water-rock reactions along the hydrothermal fluid flow path are responsible for the exchange of different elements between the fluid and substrate at progressively higher temperatures. At temperatures of about 40–60 °C, basaltic glass in the substrate is altered to clay minerals, zeolites, and Fe oxyhydroxides. Alkali elements such as K, Rb, and Cs are precipitated out of seawater and fixed in the rock at temperatures up to about 100 °C. Initially, this exchange is balanced by the leaching of Fe and Si from volcanic glass. At 150 °C, the alkali elements that were originally fixed in the rocks at low temperatures are leached, and Ca, Mg, and Na are added to the rock to maintain charge balance. Magnesium is fixed first as Mg-rich smectite (<200 °C) and then as chlorite (>200 °C). The H+ released during Ca fixation is partly consumed by the hydrolysis of the igneous minerals, but contributes to the overall acidity of the fluids and promotes metal leaching from the rock. At about 250 °C, Na+ is fixed as albite, partly replacing the remaining plagioclase. In the highest-temperature parts of the hydrothermal system (>350 °C), all Mg is lost from the hydrothermal fluid, and Ca fixation causes the formation of secondary plagioclase and epidote and the release of even more H+, which is then buffered in a system that has become rock-dominated (e.g., Seyfried, 1987). At the same time, reaction of water with Fe-bearing minerals in the rock (e.g., olivine, pyroxene, Fe sulfides) liberates metals and H2 and results in major reduction of the fluids. Thus, for a system hosted in basaltic/gabbroic oceanic crust, the highest-temperature fluids are in equilibrium with an assemblage of plagioclase-epidote-quartz-magnetite-anhydrite-pyrite, referred to as the PEQMAP buffer (Pester et al., 2012). The major sources of Fe, Mn, Zn, and Cu in hydrothermal fluids are immiscible sulfides and ferromagnesian minerals in basalt. Feldspar is a major source of Pb and Ba especially in more felsic volcanic rocks associated with hydrothermal systems related to volcanic arcs and in back-arc regions.
High-Temperature Reaction Zones in Different Geological Environments
The sum of the reactions occurring along the path of hydrothermal circulation at basaltic mid-ocean ridge segments results in a fluid that is slightly acidic, reduced, silica- and alkali-rich, and Mg-poor relative to seawater. Metals and reduced sulfur reach maximum concentrations in the high-temperature “reaction zone,” at temperatures in excess of ~400 °C and pressures of 400–500 bars (~2 km depth). Here, fluids in equilibrium with the PEQMAP buffer assemblage have a narrow range of aH2S and aH2, and elements such as Si and Fe become so enriched that they are major constituents of the hydrothermal fluids. Under these conditions, SiO2 concentrations quickly rise to near quartz saturation. Because quartz solubility is pressure dependent, silica concentrations measured in the highest-temperature fluids at the seafloor may provide an indication of the depth of the reaction zone, which commonly reaches the top of the crystallizing subvolcanic intrusions.
In subduction-related environments, the more felsic composition of the melts results in higher concentrations of alkali elements and other trace elements such as Ba and Pb (Hannington et al., 2005). Higher volatile contents of the fluids (e.g., CO2 and SO2) are also common, relative to mid-ocean ridge fluids, and likely reflect direct contributions from volcanic degassing.
Hydrothermal venting is the most recognizable feature of hydrothermal systems. High-temperature, focused venting produces black and white smokers and volcanogenic massive sulfide deposits that are often associated with diverse chemosynthetic ecosystems.
The hot hydrothermal fluid that formed at depth has a low density relative to cold seawater and thus rises buoyantly to the seafloor in a matter of minutes (at meters per second) along focused zones of high permeability (e.g., deep normal faults). Whereas convecting seawater has a residence time in the crust of several years from the point of recharge to the reaction zone, the transit from the base of the hydrothermal system to the seafloor is rapid (Kadko and Moore, 1988; Kadko and Butterfield, 1998). Observations of the rock record indicate that the rate of ascent is so fast that the fluid does not reequilibrate with the surrounding rock during ascent, although there may be some reaction (e.g., silicification of the wall rock and precipitation of metals). Minor amounts of sulfide are precipitated, but the major subseafloor deposition of sulfide occurs only in well-developed “stockwork zones” or feeder systems immediately beneath the seafloor vents where entrainment of seawater (which reduces the fluid temperature) and possibly boiling contribute to the precipitation of sulfide minerals (see below). Significant mixing and precipitation of sulfides below the seafloor result in the venting of clear fluids and a distinct lack of black or white “smoke.”
Measured concentrations of most elements in fluids that are discharged at the seafloor are somewhat lower than in the reaction zone because of mineral precipitation and dilution caused by mixing with seawater. However, the concentrations in the unmixed fluids can be calculated by assuming that Mg and SO4 were quantitatively removed from hydrothermal seawater at depth and any Mg or SO4 in the sample fluids must therefore be a result of mixing with seawater at or below the seafloor. This is the basis for the “end-member” fluid model (Edmond et al., 1979; Von Damm et al., 1985). In this model, the concentrations of major elements in samples collected at different temperatures or degrees of mixing can be extrapolated back to their original values along mixing lines between seawater and an undiluted, zero-Mg or zero-SO4 fluid. Under some conditions, the uptake of Mg by altered rocks can be reversed, and SO4 may be added to the fluids from other sources, so the zero-Mg, zero-SO4 end-member model cannot be universally applied.
Venting of hydrothermal fluids results in the precipitation and accumulation of sulfide-rich chimneys and mounds at the vent site. Chimneys that form at ~350 °C “black smoker” vents on mid-ocean ridges are composed of an assemblage of Cu, Fe, and Zn sulfide minerals including pyrite (±marcasite), pyrrhotite, chalcopyrite, Fe-rich sphalerite, and minor isocubanite and bornite, together with anhydrite. Lower-temperature vents (so-called white smokers) are dominated by pyrite, marcasite, sphalerite, and minor galena and sulphosalts, together with abundant amorphous silica, barite, and minor anhydrite; pyrrhotite and chalcopyrite are rare or absent. Mineral assemblages from extinct chimneys can be readily ascribed to a high- or low-temperature origin by analogy with the mineralogy of active vents. Mineralogical zoning in the chimneys reflects strong gradients in fluid temperatures (e.g., Tivey, 2007). High-temperature fluid conduits in black smokers commonly are lined by chalcopyrite together with pyrrhotite, isocubanite, and locally bornite; the outer portions of chimneys are commonly made up of the minerals deposited at lower temperature, such as sphalerite, pyrite, and marcasite. However, detailed examination reveals complex intergrowths, replacements, and recrystallization of the minerals resulting from a dynamic and locally chaotic environment of sulfide precipitation within the chimneys.
Subseafloor Stockwork Mineralization
The feeder zones of most hydrothermal vent sites contain vertically extensive networks of sulfide veins and disseminated sulfides, referred to as the “stockwork zone,” and intensely altered rocks (i.e., the “alteration pipe”). Stockwork mineralization can contain as much as 30–40 % of the total metal transported by hydrothermal fluids. The main alteration minerals are quartz, chlorite, illite, and kaolinite. These are produced by reaction of the hydrothermal fluids with the host rocks and by entrainment of local seawater into the upflow zone. Pyrite, which occurs as a replacement of Fe-bearing igneous phases, is commonly associated with the intense clay alteration in the central portion of the stockwork. Intense silicification adjacent to sulfide veins typically destroys any primary igneous textures in the volcanic rocks and converts them to a dominantly quartz-chlorite assemblage. Alteration temperatures in the cores of the stockwork were likely close to the exit temperatures of black smoker vents at the surface, as indicated by fluid inclusion homogenization temperatures in quartz and anhydrite in the veins (Petersen et al., 2000). In most cases, the alteration also causes the rocks surrounding the discharge zone to become insulated, sealing the system against the ingress of seawater and allowing higher-temperature fluids to occupy the alteration pipe (Lowell et al., 1993). However, chloritized pillow rims are also common beyond the main stockwork zone, indicating fluid flow between pillows even where sulfide veins are not present. Much of this chlorite alteration is likely caused by entrainment of seawater into the hydrothermal upflow rather than precipitation from the high-temperature fluids (Janecky and Shanks, 1988). Abundant anhydrite is also found within some feeder zones, precipitated from the progressive heating of seawater that is drawn into the mound beneath black smoker vents. Lenses of massive sulfide are also found within permeable zones in the volcanic rocks below the seafloor, such as interflow breccias, hyaloclastite, or sediments.
The Role of Phase Separation
At lower pressures and temperatures, fluids rising to the seafloor may intersect the two-phase curve for seawater at temperatures below the critical point and will begin to boil. At the depth ranges typical for the majority of mid-ocean ridges (>2,500 m), most fluids at the point of discharge are well below the two-phase curve for seawater. However, at water depths of less than about 1,500 m, for example, at locations where hot-spot and mid-ocean ridges interact (Iceland, Azores, Ascension, Axial Seamount), the near-vertical adiabats of typical black smoker fluids at 350 °C will intersect the two-phase boundary more than 100 m below the seafloor (Fig. 4). These fluids will cool dramatically as they rise along the boiling curve. At these low pressures, the density difference between the vapor and liquid is much greater than in the reaction zone, resulting in the rapid separation of the vapor phase.
As a result, there are important physical limitations to the depth at which high-temperature massive sulfide deposits can form if the pressure at the seafloor is insufficient to prevent boiling (Ohmoto, 1996). In particular, it is unlikely that a Cu-rich massive sulfide deposit could develop at water depths of less than 500 m (i.e., a boiling temperature of <265 °C), unless the composition of the hydrothermal fluids is very different from that of typical black smokers. Metals such as Cu, which are most effectively transported at high temperatures, will be deposited in a subseafloor boiling zone; metals that are soluble at lower temperatures (e.g., Au, Ag, As, Sb, Hg, Tl, Pb, and Zn) will be transported to the seafloor above the boiling zone and deposited in the seafloor precipitates (Hannington et al., 1999). The extent of boiling is therefore a major control on the bulk composition of the hydrothermal deposits forming at the seafloor and could lead to abundant subseafloor vein mineralization over a large vertical depth range coincident with the boiling zone.
On mid-ocean ridges spreading at between 50 and 150 mm/year, the occurrence and distribution of hydrothermalism is roughly proportional to the spreading rate (Baker and German, 2004). The frequency of hydrothermal vents along the magmatically robust fast-spreading ridges is high, relative to the more magma-starved slow-spreading ridges (Beaulieu et al., 2013). However, associated hydrothermal sulfide deposits on the fast-spreading ridges tend to be small, and the fewer deposits on slow-spreading ridges are commonly larger, reflecting longer-lived hydrothermal deposition. The incidence of large hydrothermal plumes is unexpectedly high at ultraslow-spreading ridges. Although most high-temperature hydrothermal systems occur in the axial zones of the ridges, a number of important hydrothermal systems also have developed at ridge-axis volcanoes and at the intersections of ridges with major transform faults or hot spots (mantle plumes). Widespread but typically low-temperature hydrothermal activity also has been found on off-axis volcanoes. Curiously, although intraplate hotspot volcanoes account for a large proportion of seafloor volcanism (about 12 % of the Earth’s magmatic budget (Schmidt and Schmincke, 2000), almost no high-temperature hydrothermal activity has been reported that is associated with these volcanoes, except in proximity to ridges. This apparent contradiction may be due to the great depth of the heat source and the narrow magma conduits beneath hotspot volcanoes (Hannington et al., 2005). The lack of documented hydrothermalism in other parts of the oceans (e.g., the polar regions and the Southern Ocean, Fig. 3) mainly reflects the difficulties of marine research at these latitudes. Discoveries of hydrothermal plumes and massive sulfide deposits in the high Arctic (e.g., Gakkel Ridge) and in Antarctica (e.g., Bransfield Strait, East Scotia Ridge) confirm that seafloor hydrothermal activity in remote parts of the oceans is little different from that observed elsewhere. Along slow- and ultraslow-spreading ridges (<20 mm/year full spreading rate), at high latitudes and elsewhere, hydrothermal activity appears to be demonstrably more abundant than predicted based on the relation between magma budget and hydrothermal activity established for ridges spreading at faster rates.
A variety of geophysical measurements indicate that about 3 to 6 × 1013 kg/year of seawater must be circulated through the axial zones of the world’s mid-ocean ridges and heated to a temperature of at least 350 °C to remove the heat from newly formed crust (Schultz and Elderfield, 1997; Alt, 2003; Mottl, 2003). Other estimates, based on geochemical mass balances, have ranged as high as 1.5 × 1014 kg/year (Baker et al., 1996) and as low as 7 × 1012 kg/year (Nielsen et al., 2006). These calculations assume that the component of lower-temperature diffuse flow (responsible for up to 90 % of the total heat discharge) represents end-member fluid in which the elements have been conservatively diluted with seawater. The combined flux of low- and high-temperature fluids is >1014 kg/year, enough to cycle the entire mass of the world’s oceans (1.37 × 1021 kg) through the axis of the mid-ocean ridges every 5–10 Ma (Elderfield and Schultz, 1996). If the flux of heat from the flanks of the ridges is included in this calculation (crust older than 1 Ma but <65 Ma), the cycling time for the worlds’ oceans through the ridges is less than 1 Ma.
Based on an assumption that fast-ridge EPR-style venting was globally representative, Mottl (2003) and Sinha and Evans (2004) estimated that the heat transport associated with high-temperature convection at ridge axes is 1.8 + 0.3 × 1012 W. As noted above, only about 10 % of this heat is discharged at black smoker temperatures because of mixing and conductive cooling of fluids on their way to the seafloor (Ginster et al., 1994). Conversely, German et al. (2010) argue that closer to 50 % of the total on-axis heat flow on slow-spreading ridges may be in large black smoker systems. Assuming a heat flux of 2–5 MW for a single black smoker vent (Converse et al., 1984; Bemis et al., 1993; Ginster et al., 1994), between 50,000 and 100,000 black smokers would be required to account for the high-temperature flux (10 % of 1.8 + 0.3 × 1012 W) or a density of at least one black smoker for every 1 km of ridge axis. Because their distribution is far from uniform, it is more likely that this heat is removed by fewer larger-scale hydrothermal systems. For example, some large vent fields contain as many as 100 black smoker vents with a heat output equivalent to 200–500 MW (Becker and von Herzen, 1996; Kelley et al., 2002). Only 1,000 high-temperature vent fields of this magnitude would be required to account for the high-temperature flux, equivalent to one large hydrothermal system every 100 km along the ridges. Fewer than 100 vent fields of this size are presently known, but only about 20 % of the mid-ocean ridge system has been explored in detail so far. The annual flux of metals and sulfur from these vents is estimated to be on the order of 106 tonnes per year, assuming a discharge rate of 200–500 kg/s and total combined metal and sulfur concentrations of about 250 mg/kg. This may be only a small fraction of the metal and sulfur originally mobilized by high-temperature fluids at depth, if a large proportion is retained in the crust as a result of cooling, mixing, and water-rock interaction in the subseafloor (Alt, 2003).
Independent estimates based on the actual distribution of known vent sites suggest that the spacing of sulfide occurrences is quite regular. Hannington and Monecke (2009) examined the spacing of large vent fields on 32 ridge segments and calculated an average spacing between sulfide deposits of 98 km. The spacing is greater on the slow-spreading ridges (167 km) than on the fast-spreading ridges (46 km), but the individual sulfide occurrences, and therefore the cumulative heat output, on the slow-spreading ridges are larger on average (Bach and Frueh-Green, 2010; Hannington et al., 2011). The spacing of known mineral deposits is similar to the global incidence of venting detected remotely as hydrothermal plumes (Baker and German, 2004; Baker, 2007).
At some slow-spreading ridges, tectonic processes locally expose gabbroic and ultramafic rocks. Near large faults, vertical tectonics and especially low-angle detachment faulting expose large sections of the underlying oceanic lithosphere, referred to as oceanic core complexes. At several locations on the Mid-Atlantic Ridge, hydrothermal systems have been located on top of oceanic core complexes (e.g., Rainbow, Logatchev, Ashadze, Nibelungen, Semyenov, Irinovskoe, Von Damm fields). Although it has been suggested that some hydrothermal circulation at these sites is driven by the heat of serpentinization of the exposed ultramafic rocks, high-temperature black smoker activity in these settings is considered to reflect a deep magmatic heat source (Kelley et al., 2002; Mevel, 2003; Melchert et al., 2008). Gabbro has been shown to intrude the lithospheric mantle prior to uplift and exposure (Cannat, 1996), which may drive high-temperature hydrothermal circulation in the ultramafic rocks, and the hydrothermal fluids are interpreted as products of high-temperature reactions with both gabbroic subvolcanic intrusions and peridotite.
The Lost City vent field, located at the intersection of the Mid-Atlantic Ridge and the Atlantis Fracture Zone, is an example of a rare hydrothermal system that may be driven primarily by the heat of serpentinization. Here, peridotite exposed at the seafloor is reacting directly with cold seawater, releasing significant quantities of a Mg-poor, CH4- and H2-rich fluid but no metals (Kelley et al., 2001). The fluids are buffered by equilibrium with diopside, brucite, and serpentine produced by the hydration of peridotite, which results in pH values of ~10–11. As the warm, up to 91 °C, high-pH (thus OH-rich) fluids mix with Mg-rich seawater, brucite is formed directly on the seafloor and in subseafloor fractures (Kelley et al., 2005). The formation of serpentine and other Mg-silicates also buffers Si to very low concentrations, and only traces of silica are deposited at the seafloor. The high pH of the vent fluids causes the conversion of HCO3 − in the seawater to carbonate ions and triggers carbonate precipitation. Towering carbonate (aragonite) chimneys, 10–30 m in height (up to 60 m), and travertine-like deposits form (Kelley et al., 2001; Palandri and Reed, 2004). Polymetallic sulfides are absent because of a lack of metals and H2S in the vent fluids, and the low temperatures prevent anhydrite saturation. The serpentinization of olivine is exothermic and generates enough heat to raise the subseafloor temperatures to as much as 150 °C. However, theoretical calculations based on major cation concentrations in the fluids and heat balance suggest that exothermic reactions cannot be the only source of heat (Allen and Seyfried, 2004).
Hydrothermal systems in subduction-related environments are broadly similar to those at the mid-ocean ridges, and features typical of mid-ocean ridge hydrothermal systems (e.g., black smokers, hydrothermal plumes) are also observed in arc and back-arc settings. In contrast to the mid-ocean ridges, however, convergent margins are characterized by a range of different crustal thicknesses, heat flow regimes, and magma compositions. The compositions of the volcanic rocks vary from mid-ocean ridge basalt (MORB) to more felsic lavas (andesite, dacite, and rhyolite) with geochemical affinities to island arcs. This variability leads to major differences in the composition of the hydrothermal fluids and the mineralogy and bulk composition of the sulfide deposits. For instance, many of the arc-related systems are directly affected by influx of magmatic SO2, which disproportionates to native sulfur and sulfuric acid. These seafloor solfataras are characterized by large accumulations of sulfur and fields of white smokers venting fluids with pH <1.5. About one third of the known high-temperature seafloor hydrothermal systems occur at submarine volcanic arcs or in back-arc basins. Although there are no estimates of the total hydrothermal flux, distributions along axis, as determined from overlying hydrothermal plume incidence, appears directly proportional to the lengths of the arc and back-arc segments, just as seen along mid-ocean ridges (Beaulieu et al., 2013) hence, accounting for about 20 % of the mid-ocean ridge flux. The spacing of hydrothermal systems along the volcanic fronts of island arcs remain poorly known, as these volcanoes are still being discovered and explored. On the Mariana and Kermadec arcs, which are the most completely surveyed, hydrothermal plumes have been found above one third of the submarine volcanoes, which occur at intervals of ~70 km along the lengths of the arcs (de Ronde et al., 2003). However, the incidence and intensity of hydrothermal activity associated with these volcanoes are highly variable and not yet well characterized. Some plumes detected within or above the summit calderas of arc volcanoes are related to high-temperature hydrothermal venting and black smoker activity, but many of the plume signals appear to be due to passive degassing or diffuse venting rather than high-temperature hydrothermal vents (Embley et al., 2006).
The major controls on vent fluid compositions in arc and back-arc settings are the same as those at mid-ocean ridges, although there is abundant evidence that magmas directly supply a number of components, including CO2 and metals, to the hydrothermal fluids and boiling affects many of the hydrothermal systems at the volcanic fronts of the arcs. Nearly all of the volcanoes at the fronts of the arcs have summit calderas at water depths of less than 1,600 m. This has a major impact on the temperatures of the hydrothermal fluids due to boiling, and there is abundant visual evidence of phase separation at many of the vents (Stoffers et al., 1999) (see section on phase separation for further details). Higher-temperature hydrothermal vents are found mainly in the back-arc rifts or on volcanoes with large and deep calderas, which are below the boiling depth of the hydrothermal fluids. The Lau Basin was among the first of the Western Pacific arc- and back-arc systems to be recognized as a young ocean basin formed by the splitting of an island arc (Karig, 1970). And, one of the first known occurrences of seafloor hydrothermal mineralization anywhere in the oceans was recovered by accident from the northern part of the Lau Basin in 1975, long before the discovery of black smoker deposits on the East Pacific Rise (Bertine and Keene, 1975). Black smoker activity has since been found on all of the spreading segments of the Lau Basin and in all of the other back-arc spreading centers of the Western Pacific region.
Massive sulfide deposits in volcanic arc/back-arc environments also may contain abundant native gold, Ag-rich galena (PbS), Pb-As-Sb sulphosalts (including tennantite and tetrahedrite), stibnite (Sb2S3), orpiment (As2S3), realgar (AsS), and locally cinnabar (HgS). These differences reflect the composition of the underlying volcanic rocks, which commonly included more fractionated suites of lava (andesite, dacite, rhyolite) and possibly direct contributions of metals from magmatic volatiles.
About 5 % of the world’s active spreading centers are covered by sediment from nearby continental margins (Hannington et al., 2005). These include sediment-covered ridges such as Middle Valley and Escanaba Trough on the Juan de Fuca and Gorda Ridges and rift grabens that have become major depocenters for sediment (e.g., Guaymas Basin in the Gulf of California). Subseafloor intrusions (e.g., sill-sediment complexes) are common and are indicated by high heat flow in the sedimented valleys, where high-temperature hydrothermal venting is locally focused around the margins of buried sills (Einsele, 1985; Zierenberg et al., 1993). The highest-temperature fluids originate in the volcanic basement where the upper few hundred meters of fractured basalt is several orders of magnitude more permeable than the overlying sediments and can accommodate large-scale fluid flow.
Although the hydrothermal fluids originate in the basement, extensive interaction and buffering by sediments occur en route to the seafloor. The fluids are generally depleted in metals and enriched in the alkalies, boron, ammonia, and organic-derived hydrocarbons compared to seawater-basalt systems. They have a higher pH and are more reduced than fluids that reacted only with basalt. Sill intrusions into the sediments produce organic matter derived hydrocarbons, mainly methane, as well as CO2 and H2. Lower venting temperatures compared to sediment-free mid-ocean ridges result from subseafloor mixing and interaction with the sediments; together with the higher pH, this accounts for the low metal concentrations at the seafloor (German and Von Damm, 2004). The high pH and alkalinity of the vent fluids, combined with high SiO2 concentrations, results in the precipitation of abundant Mg-silicates (e.g., talc and stevensite) and carbonate during mixing with seawater, and chimneys composed only of anhydrite, barite, amorphous silica, smectite, or carbonate are common. Calcite, dolomite, and manganiferous carbonates are also important constituents of the hydrothermal precipitates. Products of the thermal degradation of organic matter, including liquid petroleum and solid bitumens, are commonly preserved in the hydrothermal precipitates (Peter et al., 1991), and higher hydrocarbons (e.g., C1 through C6) are also present in the vent fluids (Simoneit, 1988). The high concentrations of ammonia in the vent fluids, which are several orders of magnitude higher than in typical mid-ocean ridge black smokers (e.g., 10–15 mmol/kg at Guaymas Basin compared to <0.01 mmol/kg NH4 at 21°N EPR), originate from the breakdown of N-bearing organic compounds in the sediments, and this produces a strong pH buffer via the reaction NH3 + H+ = NH4 +.
The massive sulfide deposits that form at sediment-covered mid-ocean ridges (e.g., Escanaba Trough, Southern Gorda Ridge, and Guaymas Basin, Gulf of California) are locally more complex than the black smoker chimneys at sediment-starved mid-ocean ridges. Hydrothermal fluids that interact with continent-derived turbiditic sediments leach Pb, Ba, and other elements from feldspar and other detrital components in the sediments, and the sulfide deposits include such minerals as arsenopyrite (FeAsS), tetrahedrite (Cu12Sb4S13), loellingite (FeAs2), stannite (Cu2FeSnS4), jordanite (Pb14As7S24), native bismuth, and other Pb-As-Sb sulphosalts (Koski et al., 1988; Zierenberg et al., 1993). Due to the strongly reduced nature of hydrothermal fluids that have reacted with organic material in the sediment, pyrrhotite is also a common constituent of these deposits.
Many of the characteristics of hydrothermal systems within sedimented ridge environments are also observed in sedimented back-arc basins, such as in the Okinawa Trough (Ishibashi et al., 2015). The basin sediments are dominated by terrigenous material, commonly with a high organic matter content. This can lead to concentrations of hydrocarbons, including gas hydrates, in the sediment-filled basins (Glasby and Notsu, 2003). Abundant carbonate minerals, similar to those found on sediment-covered mid-ocean ridges, also occur in sedimented back-arc environments (Nakashima et al., 1995).
A characteristic feature of high-temperature hydrothermal vents is hydrothermal plumes that form above the active vent-sources. The plumes, consisting of a mixture of hydrothermal fluid diluted by seawater, rise several hundred meters above the seafloor until they attain neutral buoyancy, at which point the plume spreads laterally along the direction of the local currents (see “Hydrothermal Plumes”). Plumes can readily be detected for several kilometers away from their source, from measurements of physical and chemical parameters in the water column (e.g., positive temperature, 3He, CH4 and turbidity anomalies, and negative Eh anomalies). The detection of plumes in the water column has been the primary method for detecting active hydrothermal venting on the seafloor. While it was long believed that most transition metals from hydrothermal vents are deposited near the vent sites, hydrothermal plumes have recently been recognized as a potentially important source of dissolved iron throughout the oceans (Tagliabue et al., 2010).
The minerals that precipitate out of hydrothermal fluids at a vent site and are entrained into the buoyant ascending plume (i.e., the “smoke”) are commonly dispersed over a wide area by near-bottom and mid-depth currents and do not contribute to the growth of sulfide deposits, except in rare cases. At typical black smoker vents, at least 90 % of the metals and sulfur discharged at the seafloor is lost to the hydrothermal plume. Sulfide particles larger than about 50 50 μm mu;m in size settle out of the plume within a kilometer of the vent, but finer particles, typically less than 20 20 μm mu;m in size, are dispersed (Baker et al., 1985; Mottl and McConachy, 1990; Feely et al., 1994). The finer particles account for most of the particle load of the plumes, but a large proportion of these may oxidize and dissolve before settling, releasing metals back into seawater (Feely et al., 1987; Metz and Trefry, 1993). However, up to 10 % of total Fe within a plume may be in the form of nanoparticles (<200 nM size fraction) that are resilient to both oxidation and particle settling and can therefore be dispersed significant distances off-axis (Yucel et al., 2011).
The metalliferous sediments in the Atlantis II Deep of the Red Sea are an exceptional example of metal deposition from hydrothermal fluids vented into the water column. In the Atlantis II Deep, metal-rich fluids are injected into the base of a highly stratified brine pool by hydrothermal vents located within the deep (Zierenberg and Shanks, 1983; Scholten et al., 2000). These metal-bearing brines have salinities, which are many times greater than vent fluids on the mid-ocean ridges, most likely as a consequence of seawater circulation through Miocene evaporites on the flanks of the rift. The brines sink, rather than rise as buoyant hydrothermal plumes, settling into several deep anoxic basins and depositing the metals as thick horizons of oxides and sulfides on the basin floor.
The circulation of seawater through oceanic lithosphere and the associated fluid-rock thermal and chemical exchange that occurs in the subsurface has a major effect on the composition of both seawater and oceanic lithosphere. The wide range of different mineralogical and geochemical characteristics of hydrothermal systems is a result of a combination of source rock composition, pressure- and temperature-dependent solubility controls, and possible direct contributions of metals and gases from subvolcanic magmas. These characteristics provide clues to the geology and chemical processes that are occurring in the underlying crust. The heating and subsequent venting of circulated seawater is the major mechanism for cooling hot, newly formed oceanic crust. The resulting chemical exchange includes the leaching and transport of base and precious metals from the underlying rock to the seafloor. The mixing of hot, metal-rich hydrothermal fluids with cold seawater at sites of focused black or white smoker discharge on the seafloor result in the formation of volcanogenic massive sulfide deposits that often host chemosynthetic-based ecosystems. The massive sulfide deposits that formed in ancient oceans and are now exposed on land are an important source for Cu, Zn, Pb, Au, and Ag and are mined for their metal contents. In the future, hydrothermal sulfide deposits on the modern seafloor may similarly be exploited for their rich metal content.
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