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Home , Anhydrite, Hydrothermal circulation, Hydrothermal vent, Ocean, Seafloor spreading, Seawater

This article has been published in published been This has article or collective redistirbution other or collective or means this reposting, of machine, is by photocopy article only anypermitted of with portion the approval The O of S p e c i a l i ss u e F e at u r e

Generation of B y M a r g a r e t K i ngs to n T i v e y Seafloor Hydrothermal , Volume 1, a quarterly 20, Number The O journal of Vent Fluids and Associated

Mineral Deposits ceanography Society. C

In the nearly 30 years since the discovery of hydrothermal ery of high-temperature fluids and -rich deposits based venting along open- spreading centers, much has been on studies of fossil deposits uplifted and exposed on (see learned about the generation of vent fluids and associated de- review by Skinner, 1983), the expectation was that these might opyright 2007 by The O posits. The hot, reducing, metal-rich, - and - be the exception, not the rule. poor hydrothermal fluids that exit “black smoker” and “white Thirty years later, we now know the role these systems play smoker” chimneys are formed through interactions of in transferring mass and from the and with . These interactions (1) modify the compo- to the . has proven to be an ceanography Society. A ceanography Society. Send all correspondence to:ceanography Society. Send [email protected] Th e O or sition of oceanic crust, (2) affect , (3) form important sink for Mg and a source for other elements such metal-rich deposits (possible analogs to ore deposits present on as Fe, Mn, Li, Rb, and Cs; thus it affects ocean chemistry (Von land), and (4) provide energy sources for biological communi- Damm et al., 1985). Analogs for ore deposits have been dis- ll rights reserved. P reserved. ll rights ties in the deep . covered, as have unusual biological communities (see Fisher et The discovery of seafloor vents was a result of a number al., this issue). Compilations of global data demonstrate that, of factors. It came about in part through hypothesis-driven in general, the heat flux from venting along sections of mid- inquiry, which predicted that hydrothermal activity at mid- ocean ridges is roughly proportional to spreading rate (though ermission is granted to in teaching copy this and research. for use article R ocean ridges is a logical outgrowth of plate-tectonic theory. at ultraslow-spreading ridges, estimates of heat flux based on Measurements of heat flow near ridge crests showed scattered plume incidence fall off of this trend, and are significantly values, with many significantly lower than values predicted for greater than predicted [Baker et al., 1995, 2004]). Investiga- cooling of newly emplaced oceanic crust by conduction alone, tions of individual vent fields along fast-, medium-, and slow- consistent with of heat near ridge crests via convec- spreading ridges, however, have produced the less-intuitive Box 1931, R Box ceanography Society, PO tion of fluid (e.g., Talwani et al., 1971). Technological advances observation that the largest individual vent deposits tend to be that allowed deep diving in occupied submersibles also played found on slower-spreading ridges (Hannington et al., 1995). a role, allowing views of the seafloor at the scale needed to Through comparisons of systems in different tectonic settings, observe and sample active vents. But there were also aspects of in substrates of different compositions, and at different depths exploration and serendipity involved. While those on the 1977 in the ocean (Figure 1), coupled with data from laboratory and vent-discovery cruise had predicted the presence of warm flu- theoretical experiments, significant progress has been made ockville, MD 20849-1931, U ockville, ids, they had not foreseen the unusual biological communities in understanding the factors that control vent-fluid and de- epublication, systemmatic reproduction, that were found to thrive in these environments (Corliss et al., posit composition (e.g., see recent reviews by German and Von 1979). And while some had anticipated the eventual discov- Damm [2004] and Hannington et al. [2005]). S A .

50 Oceanography Vol. 20, No. 1 By comparing the fluids and deposits formed in distinct geologic and tectonic settings, it is possible to examine the role that specific factors play in determining fluid composition ... and deposit size, shape, and composition ...

Figure 1. Known sites of hydrothermal venting along mid-ocean ridges, in back-arc basins, rifted arcs, and at submerged island-arc volcanoes (red), and areas of activity as indicated by mid- chemical anomalies (yellow). EPR= . TAG= Trans Atlantic Geotraverse, MEF = Main Endeavour , and GR-14 = Sea Cliff hydrothermal field on the northern Gorda Ridge. Figure after Baker et al., 1995; German and Von Damm, 2004; Hannington et al., 2005; Koschinsky et al., 2006

Oceanography March 2007 51 Generation of Seafloor crust (seawater). The composition of hot These factors affect the depth and scale Hydrothermal Fluids fluids that exit at vent fields reflects a of fluid circulation, the temperature and Through Water-Rock number of factors: the initial fluid com- pressure at which water-rock reactions Interaction position (seawater); the composition of take place, and whether the fluid under- Circulation of fluids within the oceanic the rock that reacts with the fluid as it goes phase separation. Most mid-ocean crust at spreading centers occurs because circulates and the structure of that rock ridge vent fields are hosted within , of the presence of a heat source ( (e.g., the distribution of fractures and and chemical reactions occur as fluids or newly solidified hot rock), a perme- fissures, the depth to the brittle/ductile circulate, first at low temperatures in the able medium (faulted and fissured igne- transition); and the depth, size, and down-flowing limb or “recharge” zone, ous crust), and a fluid that saturates the shape of the heat source (Figure 2a). then at much higher temperatures in the

Figure 2. (a) Schematic drawing of a hydrothermal sys- (a) components/processes involved tem within oceanic crust showing the different com- ponents and processes that can affect the composition in generating reduced fluid of the fluid that vents at the seafloor (e.g., initial fluid composition, substrate composition, permeability initial fluid Mixing of two fluids -chemical rxns - structure of the substrate, and geometry and of (e.g., seawater) mineral /dissolution heat source, all of which contribute to the temperatures and pressures at which reactions occur). (b) Elaboration of the processes that contribute to formation of mid- Modified fluid ocean ridge vent fluids. As seawater penetrates down into the crust, basaltic glass, olivine, and plagioclase are altered to ferric micas, smectite, and Fe-oxyhydroxides at low temperatures (40°C–60°C). As the modified fluid penetrates deeper into the crust and is heated to higher porous media composition (e.g., basalt, temperatures, precipitation of smectite and chlorite peridotite, andesite, rhyolite, dacite ± results in removal of Mg from the fluid in exchange sediment) and structure 2+ + + 2+ = for Ca , H , and Na . Ca and SO4 are lost from the

fluid as (CaSO4) precipitates at temperatures (temperature-pressure of fluid-rock greater than 150°C. Deeper in the system, anorthite is interaction; geometry of heat source) altered to albite, a process called albitization, with Na and Si being added to the crust in exchange for Ca, which is released from the rock into the fluid. The sum (b) Generic ridge vent system of these reactions results in a fluid that is slightly , anoxic, alkali-rich, and Mg-poor relative to seawater. Diffuse, This fluid then leaches S and from the rock. low-temp Focused, high-temp flow flow through chimneys Volatiles from magma (He, CO2, CH4, H2) may be added, further modifying the fluid composition. Further fluid Seawater modification can occur from separation of the fluid into a low-, vapor-rich phase and a phase low-t alt. ≥ 350° vent fluid if the temperatures and pressures exceed those of the Mg smectite/chlorite boiling curve. Lastly, as the hot, buoyant fluids rise rap- H+, ca2+, Na+ 2+ = water -rock rx n (?) idly to the seafloor, there may be some equilibration, ca +So4 anhydrite with minor amounts of precipitation and/or dissolu- (e.g., Si, cu , H 2 ) tion of phases as the fluid rises. becomes MorB phase separation/segregation “reaction zone” saturated, but does not precipitate due to kinetic barri- albitization } or “root zone” ers. Close to the seafloor, the fluid may exit directly into S, cu, Fe , Mn, the ocean, or be modified in the subsurface if seawater 3 Zn, etc. He, co2, cH4, H2 is entrained in the vicinity of the vents. ~1200°c Heat source = magma or hot rock

52 Oceanography Vol. 20, No. 1 deepest portions of the circulation sys- fractures and scarps that expose deeper (Figure 3a), with alkali metals (K, Rb, tem (the “root” or “reaction” zone), and parts of the crust, from drill cores, and Cs), B, and H2O removed from seawater lastly as the hot, buoyant fluid rises rap- from (slices of oceanic crust to the altered and Si, S, and, in idly through the “up-flow” zone to exit at that have been thrust onto land by plate- some cases, Mg, lost from the minerals the seafloor (Alt, 1995) (Figure 2b). tectonic processes). At temperatures up Our understanding of processes oc- to about 40°C to 60°C, reactions of sea- Margaret Kingston Tivey (mktivey@ curring in the down-flowing limb relies water with basalt result in the alteration whoi.edu) is Associate Scientist, Depart- largely on data and observations of al- of basaltic glass, olivine, and plagioclase ment of Chemistry and Geochemis- teration mineral assemblages within oce- by oxidation to ferric micas and smectite, try, Woods Hole Oceanographic Institution, anic crust recovered by submersible from Mg-rich smectite, and Fe oxyhydroxides Woods Hole, MA, USA.

Figure 3. (a) Altered basalt recovered on Ocean (a) (c) Drilling Program (ODP) Leg 51 from Hole 417a, composed of ferric smectites and Fe-oxyhydrox- ides (red-brown) that replace plagioclase, olivine, and basaltic glass; and veins of (white). (b) The two-phase boundary, critical point, and surfaces for seawater as a function of tem- perature and pressure, or depth beneath the sea- floor, assuming hydrostatic pressure (after Bischoff and Rosenbauer, 1985). The red parallelogram indicates temperatures and pressures of vent- ing observed at the seafloor in different locations along the ’s spreading centers. (c) A piece of stockwork, or chloritized basalt , recovered from 116 m beneath the TAG active hydrothermal mound on ODP Leg 158. The sample is composed 1 cm of highly altered chloritized basalt (gray-green), 1 cm sulfide veins (), and quartz .Photos (b) courtesy of S. Humphris (Woods Hole Oceanographic Institution and Ocean Drilling Program)

Oceanography March 2007 53 to the fluid (see review by Alt, 1995). into the fluid from basalt. That sig- ~ 400 to 500 bars). If the fluid tempera- As seawater penetrates deeper and is nificant Ca is released from the rock is ture and pressure exceed those of the heated to temperatures above ~ 150°C, demonstrated by the presence of Ca in boiling curve for seawater (Figure 3b), Mg is removed from the fluid through hydrothermal fluids venting at the sea- the fluid will separate into a low-salin- precipitation of clays, such as Mg-rich floor. Other reactions that affect fluid ity, vapor-rich phase and a brine phase. smectite and chlorite at temperatures composition in the down-flowing limb During this partitioning, volatiles (e.g.,

less than and greater than 200°C, respec- include reaction of water with ferrous H2S) partition preferentially into the tively (Alt, 1995). Mg removal can be Fe-bearing minerals (e.g., olivine, pyrox- vapor-rich phase (Von Damm, 1995). represented by Reaction 1, showing chlo- ene, pyrrhotite), which results in reduc- Most vent fluids exhibit a com-

rite formation. ing conditions (high H2 concentrations). position either significantly greater or The reactions are important, both in Reduction of seawater sulfate occurs, less than that of seawater, which is con- affecting the seawater Mg budget and which results in the observed slightly sistent with phase separation being the also in making the recharge seawater elevated δ34S values of hydrothermal rule rather than the exception (e.g., Von more acidic. However, H+ formed by Mg sulfide in altered sheeted dikes and in Damm, 1995) (Table 1). The large differ- fixation is also consumed by silicate hy- fluids venting at the seafloor≥ ( 1‰ vs. ences in chloride also affect metal con- drolysis reactions. For example, results of 0‰ for basaltic sulfide) (Alt, 1995). tents as most metal are carried in experimental reaction of seawater with exchange reactions also occur, including the fluid at high temperatures as chloride

basalt demonstrate that uptake of Mg by albitization, which affects the concentra- complexes (e.g., FeCl2(aq)) (Helgeson et the rock is roughly balanced by release of tions of Ca and Na in the resulting vent al., 1981). Evidence of phase separation Ca (Mottl, 1983). fluid through alteration of anorthite to is also found in the rock record, where Significant Ca is also lost from sea- albite (Alt, 1995) (Reaction 2). small amounts of fluids trapped in min- water as a result of anhydrite forma- The sum of reactions occurring in erals as fluid inclusions exhibit

tion. Anhydrite (CaSO4) has retrograde the recharge zone (Figure 2b) results in both greater and less than seawater (e.g., and precipitates from seawater a fluid that is slightly acidic, anoxic, and Delaney et al., 1987; Vanko 1988; Kelley at temperatures in excess of ~ 150°C alkali-rich and Mg-poor relative to the et al., 1993). (Bischoff and Seyfried, 1978). Its precipi- starting seawater. This fluid then leaches A final process that can affect vent tation removes all of the Ca from sea- S and metals (e.g., Cu, Fe, Mn, Zn) from fluid compositions is the addition of water and about one-third of seawater the rock into solution in the deep reac- magmatic volatiles to the circulating 3 sulfate. Additional anhydrite precipita- tion (e.g., Alt 1995) or root zone (e.g., fluid, such as He, CO2, CH4, and H2 tion can occur following release of Ca Butterfield et al., 2003) (~ 425°C at (Alt, 1995). In some back-arc and arc

REACTION 1 2+ + 2+ + 4(NaSi)0.5(CaAl)0.5AlSi2O8 + 15Mg + 24H2O ⇒ 3Mg5Al2Si3O10(OH)8 + SiO2 + 2Na + 2Ca + 24H Albite-Anorthite in Basalt Chlorite

REACTION 2 + 2+ CaAl2Si2O8 + 2Na + 4SiO2 (aq) ⇒ 2NaAlSi3O8 + Ca Anorthite Albite

54 Oceanography Vol. 20, No. 1 systems where magma is more siliceous tribute metals to the hydrothermal sys- rapid rate to the seafloor. Observations and richer in H2O, very-low-pH fluids tem (e.g., Cu, Zn, Fe, As, Au) (Ishibashi of the rock record, which integrate the are observed, consistent with addition and Urabe, 1995; Yang and Scott, 1996; effects of water-rock interaction over of magmatic SO2 that disproportionates Hannington et al., 2005). long periods, combined with re- to form sulfuric acid (e.g., Gamo et al., The evolved fluid in the root or reac- sults of thermodynamic calculations that 1997). Some of these back-arc- and arc- tion zone is very buoyant relative to cold consider the measured compositions of related magmatic fluids may also con- seawater (Figure 3b) and thus rises at a fluids sampled at vents, indicate that the

Table 1. Compositions of fluids venting from different settings.

Mid-Ocean Sediment- Ridge Back-Arc Rainbow Lost City Hosted Seawater T (°C) ≤ 405 278–334 365 ≤ 91 100–315 2 pH (25°C) 2.8–4.5 < 1–5.0 2.8 10–11 5.1–5.9 8 Cl, mmol/kg 30.5–1245 255–790 750 548 412–668 545 Na, mmol/kg 10.6–983 210–590 553 479–485 315–560 464 Ca, mmol/kg 4.02–109 6.5–89 67 < 30 160–257 10.2 K, mmol/kg -1.17–58.7 10.5–79 20 - 13.5–49.2 10.1 Ba, µmol/kg 1.64–18.6 5.9–100 > 67 - > 12 0.14

H2S, mmol/kg 0–19.5 1.3–13.1 1 < 0.064 1.10–5.98 -

H2, mmol/kg 0.0005–38 0.035–0.5 13 < 1–15 - -

CO2, mmol/kg 3.56–39.9 14.4–200 na bdl - 2.36

CH4, mmol/kg 0.007–2.58 .005–.06 0.13–2.2 1–2 - -

NH3, mmol/kg < 0.65 - - - 5.6–15.6 - Fe, µmol/kg 7–18700 13–2500 24000 - 0–180 - Mn, µmol/kg 59–3300 12–7100 2250 - 10–236 - Cu, µmol/kg 0–150 .003–34 140 - < 0.02–1.1 - Zn, µmol/kg 0–780 7.6–3000 160 - 0.1–40.0 - Pb, µmol/kg 0.183–0.1630 0.036–3.900 0.148 - < 0.02–0.652 - Co, µmol/kg 0.02–1.43 - 13 - < 0.005 - Cd, µmol/kg 0–0.910 - 0.130 - < 0.01–0.046 - Ni, µmol/kg - - 3 - - -

SO4 , mmol/kg 0 0 0 1–4 0 28 Mg, mmol/kg 0 0 0 < 1 0 53

Data from Von Damm et al., 1985; Welhan and Craig, 1983; German and Von Damm, 2003; Trefry et al., 1994; Ishibashi and Urabe, 1995; Jeffrey S. Seewald, Woods Hole Oceanographic Institution, pers. comm., 2006; Douville et al., 2002; Kelley et al., 2001, 2005; Proskurowski et al., 2006.

Oceanography March 2007 55 fluid does not attain equilibrium with (Hajash and Chandler, 1981). Fluids and types of organic , the surrounding rock during ascent, sampled at vent fields hosted in andesite carbonate, and clay), the structure of though there may be some equilibration show elevated trace metals (e.g., Zn, Cd, the sediment-rich oceanic crust (i.e.,

(e.g., of Si, H2(aq), Cu) (Von Damm, Pb, As) (Fouquet et al., 1993a) (Table 1), the presence of faults or sills that can 1995; Ding and Seyfried, 1994). Quartz but interpretation of these observations affect fluid flow paths and temperatures becomes saturated as the fluid rises (due is complicated by observations of low- of reaction), and amounts of unreacted to decreasing pressure), but does not pH fluids, where the low pH may reflect sediment. Sediment composition largely precipitate due to kinetic barriers at the input of magmatic volatiles (e.g., mag- reflects the source of the sediment, for low pH of the fluids (and is not observed matic SO2) (Gamo et al., 1997; Douville example, whether it is dominantly from within actively forming chimneys), and et al., 1999). turbidites as at Middle Valley (Juan there may be minor amounts of pre- Theoretical and laboratory experi- de Fuca Ridge, Northeast Pacific) or cipitation and/or dissolution of sulfide ments of peridotite-seawater reaction marine-derived as in the Guaymas Basin phases as the fluid rises. Over long time at low water-rock ratios indicate that (East Pacific Rise, ). periods, results of minor amounts of serpentine and talc may replace smectite In either case, the presence of - reaction may be left in the rock record, as an alteration phase left in the oce- ate and organic matter buffers the pH such as epidote + quartz ± chlorite as- anic crust and that high-temperature of the vent fluid (German and Von semblages observed in ophiolites (Alt, fluids should exhibit lower Ca, Si, Mn, Damm, 2004). While fluids from vents 1995), or the well-developed stockwork and Fe; slightly higher pH; and much in the Guaymas Basin, Middle Valley, beneath vent fields, for example, at the higher CH4 and H2 relative to fluids from and Escanaba Trough (Gorda Ridge), Trans-Atlantic Geotraverse (TAG) ac- basalt-seawater reaction at similar tem- show a wide range in composition, all tive hydrothermal mound (Figure 3c), peratures (Hajash and Chandler, 1981; are similar in exhibiting a higher pH though in the latter case significant near- Wetzel and Shock, 2000), though con- (5.1 to 5.9 at 25°C) and lower metal con- surface entrainment of seawater has centrations vary with differing propor- tents than fluids formed in unsediment- likely enhanced the amount of reaction. tions of olivine and pyroxene (Allen and ed settings (German and Von Damm, While detailed experimental, - Seyfried, 2003). Fluids from the perido- 2004) (Table 1). retical, and field studies have been car- tite-hosted Rainbow hydrothermal field Another controlling factor for vent- ried out that consider basalt-hosted (Mid-Atlantic Ridge), however, exhibit fluid composition is the source of heat hydrothermal activity, far fewer studies low pH and very high Fe relative to most that drives hydrothermal and have been done considering alternative mid-ocean ridge vent fluids (Douville affects the temperatures and pressures at substrates, for example, andesite, rhyo- et al., 2002) (Table 1). Laboratory and which reactions occur between fluid and lite, dacite (present in back-arc basins, theoretical experiments that consider the substrate. The Lost City hydrothermal rifted arcs, and submerged island-arc slow rate of hydrolysis of olivine provide system, located 15 km from the axis of volcanoes), peridotite (present along an explanation, indicating that dissolu- the Mid-Atlantic Ridge on the Atlantis some portions of slow-spreading ridges), tion of pyroxene can to a silica-rich Massif (~ 30°N), composed of mantle or sediment. Laboratory experiments fluid, tremolite formation, acid genera- rocks (peridotite and ) and of andesite-seawater interaction at a tion, and high Fe in the fluid relative to gabbro, is an excellent example (Kelley low water-rock ratio (< 5) suggest that reaction of seawater with basalt (Allen et al., 2001). It has been proposed that alteration assemblages within the oce- and Seyfried, 2003). generation of Lost City fluids, and their anic crust should be similar to those Vent-fluid compositions can also be associated mineral deposits and micro- occurring when basalt and seawater affected greatly by reaction with sedi- biologic communities, does not require interact, though resultant fluids were ments. Factors affecting fluids in sedi- heat from magma or cooling of recently enriched in Ca, Mn, Si, and Fe relative to ment-hosted systems include the compo- solidified rock. Instead, the isotopic fluids from basalt-seawater experiments sition of the sediment (e.g., abundances compositions of fluids venting at this

56 Oceanography Vol. 20, No. 1 field are consistent with formation from ability structure of the oceanic crust and/ tops and sides of shimmering spires. exothermic (heat-producing) serpenti- or deposits at the seawater/oceanic crust Fluids venting from the hotter “black nization reactions (a result of seawater- boundary and in the upper few hun- smoker” chimneys have not mixed with mantle peridotite interaction) at tem- dred meters. These factors are important any entrained seawater during their peratures of 110°C to 150°C, producing because they determine, in large part, ascent, as demonstrated by an absence of

Mg-poor, CH4- and H2- rich, very high the styles of mixing between the vent Mg in the sampled fluids (Von Damm et pH (10–11) fluids that vent at the sea- fluid and seawater. Some of these dif- al., 1985). Study of the first black smok- floor at temperatures up to 91°C (Kelley ferences can be illustrated by compar- ers sampled at 21°N on the East Pacific et al., 2001, 2005; Proskurowski et al., ing three distinctly different types of Rise led to a model of chimney forma- 2006). However, theoretical calculations deposits located along mid-ocean ridges tion that is still accepted today (Haymon, to reproduce the chemical composition and hosted in basalt: those found on 1983; Goldfarb et al., 1983): When the of the fluids, specifically the near-sea- the fast-spreading East Pacific Rise, the hot, slightly acidic, metal-, sulfide-, and water values of Cl and K/Cl and Na/Cl intermediate-spreading Endeavour Seg- Ca-rich vent fluid exits at meters-per- ratios, coupled with heat balance models, ment of the , and the second velocities into cold (2°C), slightly suggest that exothermic reactions likely TAG active hydrothermal mound on alkaline, metal-poor, sulfate- and Ca- are not a significant source of heat to the the slow-spreading Mid-Atlantic Ridge rich seawater, anhydrite (CaSO4) and system; it is more likely that the source (Figure 4). The effect that more drastic fine-grained Fe, Zn, and Cu-Fe of heat for the Lost City hydrothermal differences in vent-fluid compositions precipitate. A ring of anhydrite deposited system is deep penetration of fluids and can have on the formation and composi- around the vent opening provides a bar- access to heat from hot rock or magma tion of the deposits can be illustrated by rier to direct mixing of vent fluid with (Allen and Seyfried, 2004). comparing the structure and composi- seawater and a substrate on which other Further examination and further dis- tion of basalt-hosted mid-ocean ridge minerals can precipitate. covery of non-basalt-hosted hydrother- deposits to deposits hosted in other sub- (CuFeS2) is deposited against the inner mal systems is needed to resolve the roles strates and geologic settings: those found wall, and hydrothermal fluid and sea- that different processes, such as water- in back-arc, rifted-arc, and submerged water “mix” via and rock reaction at different temperatures island-arc settings, and those found off- through the newly emplaced wall. These and with different substrates, magmatic axis and hosted in peridotite. processes result in sulfide and sulfate volatile input, and subsurface precipita- minerals becoming saturated and pre- tion, play in determining the composi- East-Pacific-Rise-Type cipitating within pore of the wall, tions of fluids that exit the seafloor. Vent Deposits which gradually becomes less permeable Along the East Pacific Rise, active vent (Figure 5a). As long as the chimney con- Formation of Seafloor fields consist of combinations of small duit remains open, the majority of vent Mineral Deposits (< 10-m-diameter), low-lying mounds fluid exits at the top into seawater and Just as there are several key factors that with individual 1–2-m-diameter struc- rises in a large plume where abundant control the composition of vent fluids, tures that stand < 15 m high, formed metals precipitate. there are key factors that affect the for- by the coalescence of smaller chimneys The style of mixing between vent fluid mation and composition of the deposits (Haymon and Kastner, 1981; Goldfarb and seawater is vastly different in chim- that form within the crust and at the et al., 1983) (Figure 4a). The indi- neys that vent lower-temperature, white seafloor from interaction of these fluids vidual chimneys either vent hot fluid to clear fluids (< 300°C to 330°C); much with seawater. They include the compo- (~ 330°C to 405°C) directly into cold more of the metals in the fluid remain sition and temperature of the fluid that seawater, forming plumes of black pre- within the deposit as the fluid perco- rises from depth toward the seafloor cipitates (black “smoke”) above the vent lates less vigorously through the porous (including its density) and the perme- opening, or cooler fluids through the spires (Figure 5b) (Haymon and Kastner,

Oceanography March 2007 57 (b) MeF Vent Structure Diffuser (a) epr Vent Site Flange Black smoker 10 m

5–10 m impermeable silicified pipe

10 m 5–10 m

(c) taG active Hydrothermal Mound 100 m Black smoker complex

Seawater anhydrite-rich zone entrainment

crust Zn-rich Seawater white smokers entrainment Sulfide talus

Zn mobilization

Massive and pyrite pyrite and silica zone

Demagnetized zone Silicified and pyritized stockwork

58 Oceanography Vol. 20, No. 1 Figure 4. Schematic drawings showing the 1981; Koski et al., 1994). In contrast to different styles of mixing between vent different size and morphology, and differ- black smoker chimneys, these spires fluid and seawater that occur within ent processes affecting, vent structures from often lack anhydrite, consistent with a black-smoker chimneys vs. more porous different tectonic and geologic settings. (a) An East Pacific Rise (EPR) vent site show- lack of entrained seawater sulfate. Flow is spires vs. within the subsurface provide ing tall spires topped by black smoker chim- through narrow, anastomosing conduits a range of different thermal and chemi- neys. Total accumulation of mass at each vent site is low likely due to hot fluids passing that seal with time, resulting in flow cal environments and potential through the structure into the plume above, being diverted horizontally (Fouquet et for the unusual found at vents (see coupled with eruption frequency that can al., 1993b; Koski et al., 1994; Tivey et al., Fisher et al., this issue). bury deposits (based on photographs and data from Ferrini et al., in press). (b) A steep- 1995). Differences between these - sided structure from the Main Endeavour rich chimneys and the -rich black Large, Steep-Sided “Endeavour” Field (MEF) of the Juan de Fuca Ridge (after smoker chimneys provide information Type Structures—Flanges and Hannington et al., 1995, and Sarrazin et al., 1997) that forms from deposition of minerals about the very different environmen- Fluids with “Higher” pH from a fluid that has a higher pH at tempera- tal (thermal, chemical) conditions that At the Main Endeavour Field (MEF) on tures less than 300°C due to the presence likely exist in their interiors and at their the Juan de Fuca Ridge, vent structures of in the fluid T( ivey et al., 1999). The entire MEF includes ~ 15 structures and exteriors where micro-, macro-, and and styles of venting differ greatly from covers an area of ~ 400 x 200 m (Delaney mega-fauna may reside. those on the East Pacific Rise, reflect- et al., 1992). It has also been proposed that the steep-sided Endeavour structures are At East Pacific Rise fields, lower- ing differences in the composition of underlain by pipelike stockworks, with in- temperature, diffuse flow is also observed the vent fluids rising from depth and tense silicification of the alteration pipes seal- exiting from cracks and crevices in the in styles of mixing, and greater longev- ing the stockwork, preventing entrainment of seawater (Hannington et al., 1995). The pres- basaltic seafloor (Haymon and Kastner, ity of venting. Steep-sided structures ence of higher-pH fluids provides an explana- 1981). A recent comparison of high- and rise nearly vertically from the seafloor tion for the silicification T( ivey et al., 1999). low-temperature fluids at the East Pacific to heights greater than 10 or 20 meters (c) The Trans Atlantic Geotraverse (TAG) active hydrothermal mound is forming from Rise at 9°N showed that concentrations (Delaney et al., 1992) (Figure 4b). Struc- vigorous venting through the black smoker, of many elements are consistent with tures host multiple small smokers and combined with significant entrainment of cooler fluid forming from mixing of large overhanging flanges that trap pools seawater into and beneath the mound. This process triggers: deposition of pyrite, chalco- high-temperature fluid with seawater; of hot fluid (Figure 5c). While small pyrite and anhydrite; generation of a more however, the concentrations of some flanges have been observed at some East acidic fluid; and remobilization of Zn and oth- er trace metals, which are then deposited at chemical were not conservative Pacific Rise vent fields, they attain large the outer edges and on the upper surface of with mixing, providing evidence for pos- sizes and trap significant pools of fluid the mound (after Humphris and Tivey, 2000). sible biological consumption of H S and at the MEF because of the presence of The very large size compared to structures 2 from the EPR and MEF result from a combina- H2 (and CO2) and production of CH4 silica, which stabilizes the flanges and tion of efficient mineral deposition because (Von Damm and Lilley, 2004). Similar prevents them from breaking (Delaney of seawater entrainment, and recurrence conclusions were reached in a study of et al., 1992). The large structures that of hydrothermal activity at this same loca- tion over a period of 20,000 to 50,000 years. low-temperature fluids exiting basalt at form dominantly by flange growth, dif- Examples of portions of structures outlined Axial on the Juan de Fuca Ridge fuse flow through sealed spires and other by green boxes are shown in Figure 5. (Butterfield et al., 2004). portions of structures, and incorpora- Overall, the deposits at most East tion of flanges into edifices are also sta- Pacific Rise vent fields are small because bilized by deposition of late-stage silica much of the fluid and precipitate is car- (Tivey et al., 1999). ried upward into plumes above the vent The prevalence of amorphous silica fields, and spreading rates are high and at the MEF results from conductive eruptions frequent so that deposits do cooling of vent fluids that have high not have time to attain a large size. The concentrations of ammonia; as tempera-

Oceanography March 2007 59 (a) black smoker Figure 5. (a) Photograph of a black smoker chimney from the southern East Pacific Rise, taken from the submersible Alvin on Dive 3296 (courtesy of Woods Hole Oceanographic Institu- tion (WHOI); M. Lilley and K. Von Damm chief scientists) and a schematic drawing showing a cross section of a black smoker chimney, and likely directions of fluid flow through the con- duit, with slower advection and diffusion occur- ring across the walls. (b) Photograph of a 284°C diffusely venting spire from the Vienna Woods vent field in the Manus Basin taken onJason Dive 207 (courtesy of WHOI; M. Tivey chief scien- tist), and a schematic drawing of the cross sec- tion across an East Pacific Rise diffusely venting spire that is composed of an inner, very porous

zone of pyrrhotite (Fe1-xS), wurtzite (Zn,Fe)S,

and cubanite (CuFe2S3); a less-porous mid-

layer of wurtzite, pyrite (FeS2) and chalcopyrite

(CuFeS2); and an outer layer of marcasite (FeS2) (after Kormas et al., 2006). (c) Photograph of a flange from the Tui Malila vent field, , (b) diffuser taken on Jason Dive 134 (courtesy of WHOI; M. Tivey chief scientist), and a schematic draw- ing showing a cross section of a flange with a trapped pool of high-temperature fluid. Fluids percolate up through the porous flange layers, precipitating minerals as they traverse the steep temperature gradient, or “waterfall” over the lip of the flange. (d) Photograph of diffuse warm fluid exiting the top of a “crust” sample on the upper tier of the TAG mound, taken from Alvin (courtesy of G. Thompson, WHOI). Textures of re- covered crust samples indicate that much hot- ter fluid is pooled beneath these crusts within the mound and that the hot fluids percolate (c) flange upward through cracks. (e) Photograph of low- pH fluids (pH < 1– 2) venting from the sides of the flank of the North Su vent field in the east- ern Manus Basin, taken on Jason Dive 221 (cour- tesy of J. Seewald, WHOI). It has been proposed that the very low pH results from input of mag- matic volatiles (e.g., Gamo et al., 1997).

(d) crust (e) low pH fluids

60 Oceanography Vol. 20, No. 1 ture decreases, ammonia-ammonium gram exposed a sequence of pyrite, anhy- desite at the 400 m x 100 m Vai Lili field equilibrium buffers pH and allows more drite, silica, and chloritized basalt brec- on the Valu Fa Ridge in the southern Lau efficient deposition of sulfide minerals cias and stockwork beneath the mound Basin are rich in barite (BaSO4), sphaler- and silica from fluids that have a higher (Humphris et al., 1995). All fluids exiting ite ((Zn,Fe)S)), tennantite (Cu12As4S13), pH than conductively cooled ammo- the mound, including lower-temperature and (PbS) relative to mid-ocean nia-poor fluids present at most other diffuse fluids, higher-temperature white ridge deposits (Fouquet et al., 1993a). At mid-ocean ridge vent fields (Tivey et al., smoker fluids, and diffuse fluids exit- the Brothers volcano in the Kermadec 1999). The presence of significant am- ing sulfide-rich crusts on the upper tiers island arc, located at a water depth of monia in the vent fluids is attributed to of the mound (Figure 5d), are related 1600 m, active black smokers and mas- reaction of fluids with buried organic- through mixing, subsurface deposition, sive sulfide deposits are present on the rich sediments (Lilley et al. 1993). As at and remobilization (Edmond et al., 1995; caldera wall, as are -rich fuma- East Pacific Rise fields, the variable styles James and Elderfield, 1996). This deposit roles and very-low-pH vent fluids (de of mixing within the structures—above has been noted as an excellent analog of Ronde et al., 2005). At the PACMANUS flange pools, from chimneys, from fluids a Cyprus-type massive sulfide deposit vent area on Pual Ridge in the eastern percolating through sides of structures (Hannington et al., 1998). Manus Basin, sulfide deposits are en- or through cracks in the substrate— riched in Au, Ag, Pb, As, Sb, and Ba rela- affect the deposition of minerals and cre- Deposits in Back-Arc Basins, tive to mid-ocean ridge deposits (Scott ate a range of environments and habitats Submerged Island-Arc Volcanoes, and Binns, 1995; Moss and Scott, 2001; for . and Rifted Arcs Binns et al., 2002). At the DESMOS cal- Vent deposits found at intra-oceanic, dera, farther east in the Manus Basin, The TAG Active Hydrothermal back-arc basin spreading centers (e.g., advanced argillic alteration of the lavas is Mound—Effects of Seawater Lau and North Basins and Mariana observed (i.e., alteration of igneous rocks

Entrainment Trough), in marginal back-arc basins to (KAl3(SO4)2(OH)6), alumi- The largest single vent deposit discov- (e.g., Okinawa Trough), at submerged num-rich clay, and quartz ± pyrite), pro- ered to date along open-ocean spreading island-arc volcanoes (e.g., Izu-Bonin, posed to result from alteration by acid- centers is the TAG active hydrothermal Mariana, and Tonga-Kermadec arcs), sulfate fluids; vent fluids with extremely mound at 26°N on the Mid-Atlantic and in areas with more complex tectonic low pH (as low as 0.87) are also present Ridge, where black-smoker fluids are ex- histories (e.g., the Manus Basin where (Gamo et al., 1997; Seewald et al., 2006). tremely well focused and exit vigorously arc and back-arc rifting are The differences and similarities from a central black-smoker complex to occurring in old arc crust) display both observed in the composition of deposits form a large, buoyant black plume (Rona similarities and differences when com- present in back-arc basins, rifted arcs, et al., 1986) (Figure 4c). The large size pared with deposits found on mid-ocean and submerged island-arc volcanoes can results in part from significant seawater ridges (Figure 1). Some deposits are as be attributed to a number of factors, entrainment into the mound, which trig- large as those found along mid-ocean including the composition of the sub- gers precipitation of anhydrite, chalco- ridges. They can be enriched relative strate (basalt, andesite, rhyolite, dacite), pyrite, and pyrite within the mound, and to mid-ocean ridge deposits in Zn, Pb, the contribution of magmatic volatiles remobilization of metals (Edmond et al., As, Sb, Ag, Au, and Ba (see reviews by to the hydrothermal system, and the 1995; Tivey et al., 1995). The large size Ishibashi and Urabe, 1995; Hannington depth and structure of the substrate. also reflects the age of the deposit and its et al., 2005). For example, deposits host- However, because many of these factors formation from repeated episodes of hy- ed in basalt in the northern Lau Basin co-vary with one another, it is difficult drothermal activity over the last 20,000 to are not enriched in trace metals relative to determine which is most responsible 50,000 years (Lalou et al., 1995). Recovery to mid-ocean ridge deposits (Bortnikov for observed differences in deposits. For of rock core by the Ocean Drilling Pro- et al., 1993), while those hosted in an- example, deposits hosted in basalt in

Oceanography March 2007 61 back-arc basins (e.g., in the northern Lau and island-arc vent sites. Anomalously this boiling can enhance metal enrich- Basin and in the Manus Basin) are most low-sulfur isotope values of sulfides (δ34S ments (Hannington et al., 2005). similar in composition and structure to as low as -7.3‰ to -13.9‰) at Broth- Volcanism in shallower water can also mid-ocean ridge deposits; these depos- ers volcano in the Kermadec arc, the result in pyroclastic rock (e.g., Fiske et its, however, are also present in water DESMOS caldera in the Manus back-arc al., 2001) that is very porous, with much depths most similar to those observed at basin, the Hine Hina vent field in the greater permeability than lavas erupted mid-ocean ridges (e.g., Bortnikov et al., Lau Basin, and Conical near in deeper water. If permeability is en- 1993; Hannington et al., 2005). In con- Papua New Guinea are consistent with hanced, then seawater entrainment may trast, deposits hosted in andesite, rhyo- input of magmatic SO2 to these systems occur, with subsurface deposition of sul- lite, and dacite (e.g., those on the Valu (see review by Hannington et al., 2005). fide minerals and anhydrite, generation Fa Ridge in the Lau Basin, on Brothers It has also been proposed that magmatic of a more acidic fluid, and subsequent volcano in the Kermadec island arc, and fluids at some sites may carry metals in metal remobilization that could lead to on the Pual Ridge in the eastern Manus addition to SO2, so that some of the met- further metal enrichment. It has been Basin) exhibit metal enrichments relative al enrichment could be from direct input proposed that the more siliceous mag- to mid-ocean ridge deposits. The more of magmatic fluids to the hydrother- mas in back-arc and arc environments felsic (or siliceous) substrate composi- mal systems. Evidence for this includes result in substrates with greater perme- tions (andesite, rhyolite, dacite) reflect the presence of high concentrations of ability, also allowing greater amounts of effects of the addition of H2O and other Cu, Zn, and Fe sulfides and seawater entrainment (Butterfield et al., volatiles from subducted sediments and in CO2-rich bubbles in both melt 2003). Further detailed study of the flu- hydrated oceanic crust, and partial melt- inclusions and matrix glass of volcanic ids and deposits in these settings should ing in the mantle wedge (see review by rocks recovered from the eastern Manus allow better constraints to be placed on Hannington et al., 2005). These deposits, back-arc basin (Yang and Scott, 1996). the roles that substrate composition, however, are also located in shallower In addition, high concentrations of Au substrate structure, and magmatic vola- water depths, and low-pH fluids at these and As in back-arc and arc-related mas- tile contribution play in determining sites suggest the addition of magmatic sive sulfide deposits have been proposed vent fluid and vent deposit composition. volatiles (e.g., SO2) (see reviews by Ishi- to reflect magmatic input, given the ex- bashi and Urabe, 1995 and Hannington treme enrichments in deposits relative -Rich “Lost City” et al., 2005). So, while the observed metal to host rocks, and that such enrichments Type Deposits enrichments in deposits (e.g., of Zn, Pb, are unlikely to occur solely from leaching For each setting described above, dif- As, Sb, Ag, Au, and Ba) are attributed to from host rocks (Ishibashi and Urabe, ferences in the size, morphology, and vent fluids being enriched in these ele- 1995; Hannington et al., 2005). composition of vent structures have ments from reaction of seawater with Two additional factors, the shallow been attributed to some combination rocks that are richer in silica and water water depth of many arc and back-arc of differences in vent fluid composi- (e.g., Fouquet et al., 1993a; Scott and related systems, and differences in the tion (e.g., pH, presence or absence of Binns, 1995), the enrichments are also structure of felsic vs. basaltic oceanic ammonia at the MEF), style of mixing thought to result from input of mag- crust, may also modify fluid composi- between vent fluid and seawater (e.g., matic volatiles into these systems, such as tions and affect metal enrichments. rapid venting of vent fluid into the ocean

SO2, which results in a more acidic hy- At volcanic arcs, vents are at the East Pacific Rise vs. mixing of vent drothermal fluid that can mobilize more located on conical volcanoes, sometimes fluid with entrained seawater beneath metals (Figure 5e) (e.g., Douville et al., within a summit caldera, and often in the seafloor as at TAG), and longevity of 1999; Gamo et al., 1997). water depths that are < 1000 m. At these venting (related to spreading rate, erup- There is evidence for the presence of shallow depths, boiling of fluids may tion frequency). At the Lost City vent magmatic SO2 at a number of back-arc occur as they ascend from depth, and field, the fluid composition is extremely

62 Oceanography Vol. 20, No. 1 different from mid-ocean ridge or back- into the subsurface at TAG, and from impact that hydrothermal systems have arc basin vent fluids, with an alkaline mixing of warm, very-high-pH, reduced on processes will be made through pH (~ 10–11) and low metal and sulfide (H2-rich) vent fluid with seawater at Lost taking advantage of the range in tectonic content (Kelley et al. 2005; Proskurowski City. More importantly, there is evidence and geologic settings of venting. Many et al., 2006) (Table 1). The fluids exit- that both systems have been active on of the world’s spreading centers have yet ing the structures are also significantly and off for tens of thousands of years to be explored (Figure 1), particularly in cooler than at many seafloor vent sites, (30,000 years based on 14C dating of the the polar on ultraslow-spreading with maximum observed temperatures Lost City deposits (Fruh-Green et al., ridges. As always in oceanography, prog- of 91°C (Kelley et al., 2005). The depos- 2003) and 20,000 to 50,000 years at TAG ress will continue to be made through a its present are tall spires with flanges. (Lalou et al., 1995), with the longevity a combination of hypothesis-driven explo- Active portions consist of porous and result of slow rates of spreading and low ration and serendipity. friable networks of aragonite (CaCO3) eruption frequency. enclosed in brucite (Mg(OH)2); arago- Acknowledgements nite becomes saturated and precipitates Summary Support for M.K.T. was provided as high-pH, Ca-rich fluids mixes with Despite the wide range in spreading by National Science Foundation - Ca- and (HCO3 )- rich rates, depths, substrate compositions, grants OCE-0241796 and OCE- seawater, while brucite forms as warm, and geometries or even types of heat 0327448. Illustrations were done high-pH (thus hydroxide (OH-)-rich) sources (e.g., serpentinization reactions by Margaret Sulanowska with help vent fluid mixes with Mg-rich seawater vs. presence of magma) along mid-ocean from Lauren Ledwell. Reviews by (Kelley et al., 2005, Kelley, 2005). Older, ridge and back-arc basin spreading cen- Sven Petersen and David Vanko are inactive portions of the structures are ters, and at submerged arc volcanoes and gratefully acknowledged. calcite-rich, lithified, and brucite-poor, a rifted arcs, there are strong similarities result of aragonite converting to calcite among all seafloor vent fields in terms of References over time, and brucite dissolving as it is the processes of heat and mass transfer Allen, D.E., and W.E. Seyfried Jr. 2003. Composition- al controls on vent fluids from ultramafic-hosted exposed to cold seawater that percolates that result in venting of hydrothermal hydrothermal systems at mid-ocean ridges: An into older parts of the structure (Kelley fluids to the oceans, formation of min- experimental study at 400°C, 500 bars. Geochi- et al., 2001, 2005). eral deposits, and creation of chemical mica et Cosmochimica Acta 67:1,531–1,542. Allen, D.E., and W.E. Seyfried Jr. 2004. Serpentiniza- The Lost City structures differ signifi- and thermal environments conducive tion and heat generation: Constraints from Lost cantly from other seafloor vent deposits to biological activity in the . City and Rainbow hydrothermal systems. Geochi- in composition. They lack metal-sulfide By comparing the fluids and deposits mica et Cosmochimica Acta 68:1,347–1,354. Alt, J. 1995. Subseafloor processes in mid-ocean ridge minerals because of a lack of metals formed in distinct geologic and tectonic hydrothermal systems. Pp. 85–114 in Seafloor Hy- and sulfide in the vent, and they lack settings, it is possible to examine the drothermal Systems: Physical, Chemical, Biological, and Geological Interactions. S.E. Humphris, R.A. anhydrite (CaSO ) because mixing tem- role that specific factors play in deter- 4 Zierenberg, L.S. Mullineaux, and R.E. Thomson, peratures are low (< 91°C), preventing mining fluid composition (e.g., sub- eds, AGU Monograph Series, No. 91, American anhydrite-saturation, and pH is high, strate composition, input of magmatic Geophysical Union, Washington, DC. Baker, E., C.R. German, and H. Elderfield. 1995. resulting in saturation of aragonite volatiles, permeability structure of the Hydrothermal plumes over spreading-center

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