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Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ 3 He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ∼3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable δ 3 He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land.

The Monowai volcanic center is located at the midpoint along the ~2,530-km-long Tonga-Kermadec arc system. The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ~35 km2; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ~100 m of the sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature venting (

Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW Caldera and Cone sites, whose geology, permeability, vent fluid compositions, mineralogy, and ore-forming conditions are in stark contrast to each other. The NW Caldera site strikes for ∼600 m in a SW–NE direction with chimneys occurring over a ∼145-m depth interval, between ∼1,690 and 1,545 m. At least 100 dead and active sulfide chimney spires occur in this field and are typically 2–3 m in height, with some reaching 6–7 m. Their ages (at time of sampling) fall broadly into three groups

A comprehensive survey of hydrothermal plumes and their geochemistry revealed 16 hydrothermally active volcanoes along 1,200 km of the Mariana arc from 13.5° N to 22.5° N. Of these 16, one volcano, NW Rota-1, is discharging sulfuric acid-rich fluids producing some of the largest chemical anomalies ever observed in non-buoyant hydrothermal plumes. Two types of venting were observed, one of focused flow rich in Al, S, Si, CO2, Fe, Mn, and 3He, and a second of diffuse flow rich in Fe, Mn, CO2, and 3He but without Al, S, and Si. The plume waters from the acid-rich focused flow showed decreases in pH up to 0.73 pH units compared to ambient seawater and an estimated 80 percent of this change was brought about by volcanic and/or hydrothermal SO2 (and possibly HCl) with the remaining 20 percent by CO2. Conservative estimates suggest that the pH of the venting fluids was

Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity‐temperature‐depth‐optical backscatter device (CTDO) and are associated with elevated light scattering and flare‐shaped backscatter signals revealed by the ship's echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water ...

Earth’s fastest present seafloor spreading occurs along the East Pacific Rise near 31°–32° S. Two of the major hydrothermal plume areas discovered during a 1998 multidisciplinary geophysical/hydrothermal investigation of these mid-ocean ridge axes were explored during a 1999 Alvin expedition. Both occur in recently eruptive areas where shallow collapse structures mark the neovolcanic axis. The 31° S vent area occurs in a broad linear zone of collapses and fractures coalescing into an axial summit trough. The 32° S vent area has been volcanically repaved by a more recent eruption, with non-linear collapses that have not yet coalesced. Both sites occur in highly inflated areas, near local inflation peaks, which is the best segment-scale predictor of hydrothermal activity at these superfast spreading rates (150 mm/yr).

Brothers volcano, which is part of the active Kermadec arc, northeast of New Zealand, forms an elongate edifice 13 km long by 8 km across that strikes northwest-southeast. The volcano has a caldera with a basal diameter of ~3 km and a floor at 1,850 m below sea level, surrounded by 290- to 530-m-high walls. A volcanic cone of dacite rises 350 m from the caldera floor and partially coalesces with the southern caldera wall. Three hydrothermal sites have been located: on the northwest caldera wall, on the southeast caldera wall, and on the dacite cone. Multiple hydrothermal plumes rise ~750 m through the water column upward from the caldera floor, originating from the northwest caldera walls and atop the cone, itself host to three separate vent fields (summit, upper flank, northeast flank). In 1999, the cone site had plumes with relatively high concentrations of gas with a ΔpH of −0.27 relative to seawater (proxy for CO2 + S gases), dissolved H2S up to 4,250 nM, high concentrations of particulate Cu (up to 3.4 nM), total dissolvable Fe (up to 4,720 nM), total dissolvable Mn (up to 260 nM) and Fe/Mn values of 4.4 to 18.2. By 2002, plumes from the summit vent field had much lower particulate Cu (0.3 nM), total dissolvable Fe (175 nM), and Fe/Mn values of 0.8 but similar ΔpH (−0.22) and higher H2S (7,000 nM). The 1999 plume results are consistent with a magmatic fluid component with the concentration of Fe suggesting direct exsolution of a liquid brine, whereas the much lower concentrations of metals but higher overall gas contents in the 2002 plumes likely reflect subsea-floor phase separation. Plumes above the northwest caldera site are chemically distinct, and their compositions have not changed over the same 3-year interval. They have less CO2 (ΔpH of −0.09), no detectable H2S, total dissolved Fe of 955 nM, total dissolved Mn of 150 nM, and Fe/Mn of 6.4. An overall increase in 3He/4He values in the plumes from R/RA = 6.1 in 1999 to 7.2 in 2002 is further consistent with a magmatic pulse perturbing the system. The northwest caldera site is host to at least two large areas (~600 m by at least 50 m) of chimneys and sub-cropping massive sulfide. One deposit is partially buried by sediment near the caldera rim at ~1,450 m, whereas the other crops out along narrow, fault-bounded ledges between ~1,600 and 1,650 m. Camera tows imaged active 1- to 2-m-high black smoker chimneys in the deeper zone together with numerous 1- to 5-m-high inactive spires, abundant sulfide talus, partially buried massive sulfides, and hydrothermally altered volcanic rocks. 210Pb/226Ra dating of one chimney gives an age of 27 ± 6 years; 226Ra/Ba dating of other mineralization indicates ages up to 1,200 years. Formation temperatures derived from Δ34Ssulfate-sulfide mineral pairs are 245° to 295° for the northwest caldera site, 225° to 260°C for the southeast caldera and ~260° to 305°C for the cone. Fluid inclusion gas data suggest subsea-floor phase separation occurred at the northwest caldera site. Alteration minerals identified include silicates, silica polymorphs, sulfates, sulfides, Fe and Mn oxide and/or oxyhydroxides, and native sulfur, which are consistent with precipitation at a range of temperatures from fluids of different compositions. An advanced argillic assemblage of illite + amorphous silica + natroalunite + pyrite + native S at the cone site, the occurrence of chalcocite + covellite + bornite + iss + chalcopyrite + pyrite in sulfide samples from the southeast caldera site, and veins of enargite in a rhyodacitic sample from the northwest caldera site are indicative of high-sulfidation conditions similar to those of subaerial magmatic-hydrothermal systems. The northwest caldera vent site is a long-lived hydrothermal system that is today dominated by evolved sea-water but has had episodic injections of magmatic fluid. The southeast caldera site represents the main upflow of a relatively well established magmatic-hydrothermal system on the sea floor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each of these different vent sites represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.

The eruption of Axial Volcano in January 1998 produced extensive plumes in the overlying water column with large anomalies in Fe, Mn, pH, light attenuation, and temperature. A strong correlation between total iron and light attenuation (dc) suggests a low abundance of particulate sulfur (PS) in the plumes. Because total carbon dioxide (ΣCO2) samples were not collected, high-precision pH measurements were used to estimate maximum CO2 anomalies (ΔCO2) which, when compared to the other physical and chemical data, suggest that the fluids being vented 3 weeks after eruption were formed by the interaction between an intruded dike and a mixture of interstitial seawater and mature hydrothermal fluids.

Hydrothermal event plumes are unique water-column features observed over mid-ocean ridges, presumably generated by the sudden release of large volumes of hot, buoyant fluid. Although the specifics of event plume generation are unknown, event plumes have been attributed to the rapid emptying of a hydrothermal reservoir or to rapid heat extraction from a recently emplaced dike or seafloor lava flows. The chemical and thermal signatures of event plumes as compared to the underlying steady-state plumes offer important clues to the generation of event plumes. Event plumes have low 3He/heat ratios of ∼0.4 × 10−17 mol J−1, similar to vent fluids from mature hydrothermal systems. In contrast, the steady-state plumes found beneath the event plumes have elevated and variable 3He/heat ratios of 2 to 5 × 10−17 mol J−1. Fluids collected directly over fresh lava flows have even higher 3He/heat ratios of 2 to 8 × 10−17 mol J−1, up to 30 times the event plume values. These disparate 3He/heat ratios place strong constraints on models of event plume generation, especially models which rely on heat extraction from seafloor eruptions. Published by Elsevier Science B.V.

The superfast-spreading portion of the East Pacific Rise at 17.5°S is a magmatically robust section of mid-ocean ridge. Plumes characterized by high volatile/metal ratios, which has been hypothesized to be indicative of recent magmatism, were found between 17°22′S and 17°35′S in November–December 1993. Dives with the French submersible Nautile in December 1993 observed young sheet flows and widespread venting centered on the segment's shoalest point at 17°26′S. Eight submersible dives made one year later with the Japanese submersible Shinkai 6500 in September–November 1994 found glassy, unsedimented lavas, a range of diffuse and high-temperature vents, and several types of biologic communities on the ridge crest beneath the volatile-rich 1993 plumes. The diffuse vents range from small areas in the youngest lavas characterized by very low-temperature flow (

In June 1993, a seafloor dike intrusion along the CoAxial segment of the Juan de Fuca Ridge was acoustically detected. A near-immediate field response and repeated plume mapping and sampling surveys during eight cruises over the next 3 years have provided a unique opportunity to estimate heat and mass fluxes from both event and chronic discharge during the life cycle of a newly created hydrothermal system. The intrusion triggered the release of at least three event plumes followed by chronic discharge focused at two sites: Flow, the site of a lava eruption at the distal end of the intrusion; and Floe, 30 km back along the trend of the dike. We have combined measurements of plume temperature anomalies, plume areal extent, and year-long averages of current flow at both sites to estimate the chronic hydrothermal heat flux Hu. Initial values of Hu at both sites were of order 104 MW, declining over time t as Hu = at−k, with k ≈ 1. Significant plumes were no longer detectable at Flow by June 1995, or at Floe by June 1996. Elemental fluxes from the CoAxial system have been derived from Hu and measurements of the ratios Mn/heat, Fe/heat, and particulate S/heat in chronic plumes. While Mn and Fe fluxes mirrored the power curve decline of heat, the combined regional particulate S (PS) flux experienced a second pronounced maximum some months after the eruption owing to a sharp increase in the S/heat ratio at Floc. Integrated inventories from chronic discharge were ∼4×1017 J for heat, ∼3×108 mol for Mn, ∼2×108 mol for Fe, and ∼1×108 mol for PS. Realistic uncertainties for all species are roughly a factor of 2. The three event plumes accounted for

The episodic magmatic intrusions that accompany seafloor spreading can profoundly affect the discharge of hydrothermal heat and fluid. A rapid field response to the acoustic detection of seismic activity on the CoAxial segment of the Juan de Fuca Ridge, June/July 1993, provided the opportunity to conduct a unique series of observations of hydrothermal event plumes. Between July 3 and 26 we detected at least three event plumes ranging in plume volume from 1.3 to 4 × 1010 m³, and in excess heat from 2.2 to 12.4 × 1015 J. Two were discovered directly above a new lava extrusion. One of these was first sampled within days of its release, as evidenced by its asymmetric shape, complex structure, and a steadily increasing light attenuation anomaly apparently generated by ongoing precipitation of hydrothermal Fe. We hypothesize that event plumes were produced intermittently in response to successive volcano-tectonic events that produced pulses in seismic activity between June 28 and July 14.

Magmatic injection during June/July 1993 on the CoAxial segment of the Juan de Fuca Ridge resulted in an axial lava eruption, an overlying chronic-style hydrothermal plume, and at least three megaplume-like event plumes. A comprehensive view of Mn and Fe liberated as hydrothermal discharge during this volcano-hydrothermal event was obtained as part of a rapid field response that included continuous in situ measurements of Mn and Fe. Distribution maps for Mn, Fe and temperature anomaly within the chronic and event plumes reveal significant thermochemical diversity that we ascribe to variable extraction efficiencies for heat and metals by source fluids, and to plume age differences. The Mn/heat data suggest that diffuse fluids could be the primary source of the chronic plume at northern CoAxial segment but make only a minor contribution to event plumes. Measurements of ephemeral Fe(II) were used to estimate that the event plumes were ∼2, ∼4, and >6–8 days old when first sampled, corresponding to formation times during peak seismic activity. The inventories of Mn and Fe within the three CoAxial event plumes were similar to values previously reported for megaplumes.

The 1993 volcanic/hydrothermal event on the CoAxial segment, northern Juan de Fuca Ridge, generated a complex set of event plumes and steady-state type plumes with a pattern of 3 He/heat ratios very similar to that observed during the 1986 megaplume event on the southern JdFR. The event plumes residing -600 m above the seafloor had normal He/heat ratios averaging 0.34 x 10 -12 cm 3 STP cal -1 , while the deeper, steady-state plumes had elevated 3 He/heat values ranging from 0.6 to 4.5 x 10 -12 cm 3 STP cal -1 . These results generally confirm that elevated 3 He/heat ratios in some of the water-column plumes are a reliable indicator of a recent magmatic/tectonic event. However, in contrast to the 1986 JdFR event, there was no evidence for hydrothermal activity on the CoAxial segment prior to 1993, thereby requiring revision of our models for event plume generation.

[1] The Northwest Lau Backarc Basin, consisting of the Northwest Lau Spreading Center (NWLSC) and the Rochambeau Rifts (RR), is unique in having elevated 3He/4He ratios (up to 28 Ra) in the erupted lavas, clearly indicating a hot spot or ocean island basalt (OIB)-type signature. This OIB-type helium signature does not appear in any other part of the Lau Basin. Water column plume surveys conducted in 2008 and 2010 identified several sites of active hydrothermal discharge along the NWLSC-RR and showed that the incidence of hydrothermal activity is high, consistent with the high spreading rate of ∼100 mm/year. Hydrocasts into the Central Caldera and Southern Caldera of the NWLSC detected elevated3He/4He (δ3He = 55% and 100%, respectively), trace metals (TMn, TFe), and suspended particles, indicating localized hydrothermal venting at these two sites. Hydrocasts along the northern rift zone of the NWLSC also had excess δ3He, TMn, and suspended particles suggesting additional sites of hydrothermal activity. The RR are dominated by Lobster Caldera, a large volcano with four radiating rift zones. Hydrocasts into Lobster Caldera in 2008 detected high δ3He (up to 239%) and suspended particle and TMn signals, indicating active venting within the caldera. A repeat survey of Lobster in 2010 confirmed the site was still active two years later. Plumes at Lobster Caldera and Central Caldera have end-member3He/4He ratios of 19 Ra and 11 Ra, respectively, confirming that hot spot-type helium is also present in the hydrothermal fluids.

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor-of-2 variation in CO2 (and H2S) is larger than expected for loss of a very high-chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.

In situ measurements of dissolved manganese and total dissolved iron were conducted in hydrothermal plumes over the Juan de Fuca Ridge using a submersible chemical analyzer (Scanner). The Scanner was deployed as part of a CTD/transmissometer/rosette instrument package on both tow-yos and vertical casts during the VENTS Leg I cruise in 1989. Dissolved manganese and total dissolved iron concentrations, along with temperature and light attenuation anomalies, were determined over the ridge crest every 5 s. Discrete samples for laboratory analyses of dissolved iron II, total dissolved iron II+III and manganese were also collected. Metal to heat ratios (Me:Q) measured in situ were extremely variable in one steady state plume, while an event plume had constant Me:Q. Uniform values of Mn:Q in the event plume demonstrate that Mn behaves conservatively in the near-field plume. Variability in the Mn: Q ratios in a steady state plume indicated the presence of at least two hydrothermal sources with distinct Me:Q values. A simple mixing model shows that the contribution of Mn from high Me:Q sources, with a composition characteristic of black smoker vents, varies between 1% and 99% within the core of the steady state plume with an average value of 55%. On average, over 50% of the excess heat within the plume originates from low Me:Q ratio sources, with a composition characteristic of low-temperature, diffuse flow vent fluids. Less than 4% of the volume of hydrothermal fluids in the plume originates from black smokers. The Fe II concentrations were used to provide an estimate of plume age on a transect across the ridge axis. Plume ages were about 2.5 days on axis and >12 days off axis. These plume ages were modeled to provide estimates of plume transport and horizontal diffusion and show excellent agreement with ages determined using 222Rn.

Concentrations of trace metals in Fe- and Cl-rich hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) have been determined and corrected for residual precipitates formed in the sampler. Precipitate corrections for Ag, Cd, Cu, Mo, Pb, Sb, and Zn were variable, contributing an average of 20% to total solution concentrations whereas corrections for Co averaged less than 5% and essentially no corrections were required for Fe and Mn. Values for Cu, Co, and Mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320°C. In addition, and unlike most other metals studied, all vent fluids from the SJFR were almost completely depleted in Mo relative to seawater values of about 110 nmol kg−1. In contrast to the Cu-Co-Mo group, concentrations of Ag, Cd, Sb, and Pb correlated well with those for Zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332°C range encountered in this study and more by the combined chemical processes that control Zn levels along the transport pathway from the deep reaction zone to the vent orifice.

Dissolved and total 234Th (t1/2 = 24.1 days) profiles were measured over the hydrothermal field located within the North Cleft segment of the Juan de Fuca Ridge. Above the hydrothermal effluent plume, which rose approximately 200 m from the seafloor, ≥90% of the 234Th was dissolved, and the total 234Th activity was in equilibrium with its parent uranium. This is typical of deep water for which the residence time of dissolved 234Th with respect to particle uptake is greater than 200 days. Within the effluent plume, however, as little as 30% of the 234Th was in the dissolved state and the residence time of dissolved 234Th with respect to particle uptake was then of the order of only a few weeks. The flux (JTh) of dissolved 234Th to the particle phase was about 300 disintegrations per minute (dpm) cm−2 yr−1. The total 234Th was still approximately in equilibrium with U, indicating that the residence time of the 234Th-bearing particles with respect to sinking was greater than 100 days. Particulate P, V, and As, while insignificant above the plume, attained concentrations as high as 60, 0.65, and 0.45 nmol/L within the plume. The particulate [P]/[234Th] ratio and the 234Th flux (JTh) are used to calculate a removal flux of P resulting from hydrothermal scavenging of ∼50 μg cm−2 yr−1 or approximately 5.0× 106 g P yr−1 per kilometer of active ridge crest. Similarly, 1.0×105 g km−1 of V and As are removed per year from the water column by hydrothermal scavenging over this region. Extrapolation to a global scale suggests that ridge crest scavenging removes ≤10% of these elements from the ocean.

A unique data set for hydrothermal Mn and Fe was collected at Cleft segment on the Juan de Fuca Ridge between 1983 and 1991. The data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting. Manganese/heat and iron/heat ratios for plumes from the north end of the Cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for Mn of 0.36±0.17 mol s−1 and for Fe of 0.61±0.34 mol s−1. Over 6 years of plume measurements at North Cleft segment, observed episodic hydrothermal discharge accounted for −15% of the total vented Mn and −35% of vented Fe. The chronic fluxes for Mn and Fe at a second venting center located at the south end of the Cleft segment were estimated to be approximately equal to the fluxes at North Cleft segment. Chronic plumes at North Cleft segment are mixtures of focused and diffuse discharge that contribute heat, Mn, and Fe in variable proportions. Similar examination of South Cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in Mn and Fe but contributing substantially to the overall heat. Temporal and spatial variations in the concentrations of Mn and Fe and in Mn/heat and Fe/heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes. Manganese/heat and iron/heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes. The accurate assessment of segment-scale hydrothermal fluxes of Mn and Fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge.

In 1990 and 1991, particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the North Cleft segment of the Juan de Fuca Ridge were sampled to study their changing composition and fluxes away from the vent field. In the rising buoyant plume, >75% of the P, V, Cr, and As scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent. Cu and Zn are most enriched in buoyant plume particles collected from the first few meters above the vent. However, the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense Cu- and Zn-rich sulfide phases. Using the plume data, coupled with the results of our analysis of sediment trap samples, we estimated that more than 99% and 99.9%, respectively, of the total hydrothermal Fe and Mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean.

Hydrothermal vent fluids from the North Cleft segment of the Juan de Fuca Ridge between 44°54′ and 45°00′N were sampled in 1988, 1990, 1991, and 1992. In 1988, chloride-depleted (relative to seawater) diffuse fluids with low dissolved metal concentrations (relative to other mid-ocean ridge (MOR) fluids) were sampled over a distance of ∼10 km along axis. In 1990 and 1991, both high-temperature (>200°C) and diffuse vents were consistently metal- and chloride-enriched. The end-member compositions of high-temperature vents and nearby diffuse vents were very similar. There were small but significant correlated changes in diffuse and high-temperature composition from 1990 to 1991. The large-scale but temporary venting of low-chlorinity fluids over the entire North Cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride-enriched fluids. It is possible that a volcanic event along the North Cleft segment prior to 1987, for which there is firm geologic evidence, initially caused a boiling event which resulted in the preferential venting of vapor-enriched fluids through 1988, followed by a transition to brine-enriched fluids by 1990. High iron, low sodium, and low Sr/Ca ratios in the high-chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event. The absence of chloride-depleted fluids from 1990 onward, and the systematics of lithium, boron, and manganese with chloride in the high-temperature fluids from North Cleft suggest that the evolution toward lower chlorinity at Monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater.

We conducted a survey of water-column hydrothermal plumes along a 350-km long section of the East Pacific Rise (EPR) axis stretching from 8°45′N to 11°50′N, including the region at 9°45′–54′N where 1991 ALVIN dives found evidence for recent seafloor volcanic eruptions. Our survey included measurements of temperature anomaly, light attenuation (suspended particles), methane, hydrogen, iron, manganese, helium, and aluminum. We detected strong light attenuation plumes in two main regions: south of the Clipperton Transform Fault (CTF) from 9°27′N to 9°57′N and north of the CTF from 11°05′N to 11°35′N. However, the plumes at these two regions had very different physical and chemical characteristics. South of the CTF, where the ridge is thought to be magmatically robust, the plumes had very high ratios of ³He/heat, methane/Mn and S/Fe. Plumes north of the CTF had much lower ratios of ³He/heat, methane/Mn, and S/Fe. These striking differences in volatile/heat and volatile/metal ratios suggest that the plumes in the vicinity of 9°50′N were derived from a young, evolving hydrothermal system, while the activity at ∼11°10′N is from an older, stable system. This survey demonstrated that water-column plume measurements can be used not only to accurately locate regions of hydrothermal activity, but also to determine the first-order chemical and physical characteristics of the hydrothermal fluids exiting from the seafloor.

Dissolved silica (Si) and inorganic germanium (Ge) concentrations were measured in hydrothermal fluids from black smoker vents on the East Pacific Rise (21°N EPR) and the Southern Juan de Fuca Ridge (45°N SJdFR: North and South Cleft Sites, Axial Volcano). These typically display end-member concentrations ranging from 16 to 23 mM (Si) and 150 to 280 nM (Ge), and end-member Ge/Si ratios clustering between 8 and 14 × 10−6, more than 10-fold greater than the ratio entering the ocean via rivers (0.54 × 10−6) and being recycled in seawater (0.7 × 10−6). ‘Excess’ concentrations of dissolved Si and Ge above oceanic background are observed in mid-water hydrothermal plumes over mid-ocean ridge (MOR) spreading centers on the Southern EPR (SEPR) (10°–20°S) and the SJdFR. The largest Si and Ge concentration anomalies occur over the North Cleft Segment of the SJdFR. These are a factor of three greater than anomalies over the SEPR (10°–20°S). Excess Ge correlates with excess3He in plumes at a Ge/3He molar ratio of about 1 × 104, approximately the same ratio as in black smokers. These observations, combined with low abundances of Ge in FeMn-rich metalliferous sediments, suggest that Ge (and Si) behave conservatively in mid-ocean ridge hydrothermal plumes. A simple ocean Si and Ge balance, constrained by the global river silica flux and Ge/Si ratios in hydrothermal vents, rivers and biogenic silica, suggests that the global hydrothermal silica flux is about 1–4 × 1011 mole yr−1, much lower than that estimated from3He. Either (1) 70–80% of the Ge flux to the ocean is removed in as-yet undiscovered sinks (not opal), or (2) only 10% of the mantle to ocean3He and heat fluxes is associated with MOR hydrothermal convection through the 350°C isotherm (90% is off-ridge), or (3) the oceanic Ge/Si,3He/ (and87Sr86Sr) balances today are far from steady-state.

In 1988 and 1989, particles from on- and off-axis neutrally buoyant plumes over the Juan de Fuca Ridge were sampled to study their changing composition with distance from the ridge crest. Plume depth and particle composition in off-axis hydrothermal plumes were used to identify plume sources and trajectories. Iron and phosphorus concentrations and, in particular, P/Fe molar ratios were found to be sensitive indicators of hydrothermal phases in the suspended matter. Phosphorus, vanadium, and arsenic are scavenged from solution where dissolved iron from hydrothermal vents oxidizes in seawater and forms submicrometer-sized particles of Fe oxyhydroxide. The P/Fe molar ratio (∼0.23) is essentially the same for vent fields along Cleft and Endeavour segments, indicating that the scavenging processes are the same for both regions. High P/Fe ratios are observed over the South and North Cleft Segment vent fields and to the west along the Vance Seamount Chain. The seamounts evidently function as a barrier, bathymetrically steering the hydrothermal plumes to the west away from the ridge crest. Since particles from the Endeavour vent field are greatly enriched in copper relative to hydrothermal plumes from other vent fields along the Juan de Fuca Ridge, copper concentrations in the plumes can be used to distinguish plume sources. Based on water column depth and elemental composition, hydrothermal plume particles from specific vent fields on the Juan de Fuca Ridge may be tracked as much as several tens of kilometers from their source using particle compositions.

[1] Fifty submarine volcanoes of the Mariana Arc, covering 1200 km from 13.5°N to 23.2°N, were surveyed for hydrothermal activity. Of these 50 volcanoes, eight showed limited evidence of hydrothermal activity, while another 10 volcanoes displayed intense, chemically rich, hydrothermal plumes that allow detailed chemical characterization and insight into the hydrothermal activity forming the fluids that generate the plumes. The most active volcanoes exhibit a wide range of CO2 to pH relationships, from the venting of acid-rich fluids, to CO2-rich fluids, to fluids rich in alkalinity and CO2. These pH-CO2-alkalinity relationships are partially responsible for the wide range of Fe:Mn (3 to 32) observed at the different volcanoes. This chemical heterogeneity is further manifest in a wide range of CO2:3He (3 × 109 to 55 × 109), indicating that for all but one, >80% of the CO2 venting from these volcanoes has a slab source. The helium, by contrast, has an upper mantle isotopic signature. The range of chemical conditions suggests that these volcanoes occupy various states of eruptive evolution ranging from ongoing magmatic activity to highly evolved hydrothermal systems.

Submersible dives on 22 active submarine volcanoes on the Mariana and Tonga-Kermadec arcs have discovered systems on six of these volcanoes that, in addition to discharging hot vent fluid, are also venting a separate CO2-rich phase either in the form of gas bubbles or liquid CO2 droplets. One of the most impressive is the Champagne vent site on NW Eifuku in the northern Mariana Arc, which is discharging cold droplets of liquid CO2 at an estimated rate of 23 mol CO2/s, about 0.1% of the global mid-ocean ridge (MOR) carbon flux. Three other Mariana Arc submarine volcanoes (NW Rota-1, Nikko, and Daikoku), and two volcanoes on the Tonga-Kermadec Arc (Giggenbach and Volcano-1) also have vent fields discharging CO2-rich gas bubbles. The vent fluids at these volcanoes have very high CO2 concentrations and elevated C/3He and δ 13C (CO2) ratios compared to MOR systems, indicating a contribution to the carbon flux from subducted marine carbonates and organic material. Analysis of the CO2 concentrations shows that most of the fluids are undersaturated with CO2. This deviation from equilibrium would not be expected for pressure release degassing of an ascending fluid saturated with CO2. Mechanisms to produce a separate CO2-rich gas phase at the seafloor require direct injection of magmatic CO2-rich gas. The ascending CO2-rich gas could then partially dissolve into seawater circulating within the volcano edifice without reaching equilibrium. Alternatively, an ascending high-temperature, CO2-rich aqueous fluid could boil to produce a CO2-rich gas phase and a CO2-depleted liquid. These findings indicate that carbon fluxes from submarine arcs may be higher than previously estimated, and that experiments to estimate carbon fluxes at submarine arc volcanoes are merited. Hydrothermal sites such as these with a separate gas phase are valuable natural laboratories for studying the effects of high CO2 concentrations on marine ecosystems.

[1] Submarine volcanic arcs extend for fully a third of the length of the mid-ocean ridge system, but systematic investigation of hydrothermal venting on those arcs is sparse. Here we report on the distribution of hydrothermal activity along the intraoceanic Mariana arc. We identify 76 volcanic edifices along 1370 km of arc, grouped into 60 “volcanic centers,” of which at least 26 (20 submarine) are hydrothermally or volcanically active. The overall volcanic center density is 4.4/100 km of arc, and that of active centers is 1.9/100 km. An equal length of the Tonga-Kermadec arc holds fewer centers, 2.9/100 km, but a similar density of active centers, 1.8/100 km. The lower percentage of active centers on the Mariana arc results from its having both fewer caldera volcanoes, since on both arcs calderas are twice as likely as cones to be active, and a lower percentage of active calderas and cones. Active centers are found from 80 to 230 km above the subducting Pacific slab, and a quarter lie behind the arc front. The frequency distribution of center spacing along the arc front peaks between 20 and 30 km and shows the asymmetric, long-tail shape typical for many other arcs, especially those with volcano populations >∼20. There is no evidence for a regular spacing of volcanic centers. Using the new Mariana data and recent data from the Tonga-Kermadec arc, we estimate that all intraoceanic arcs combined may contribute hydrothermal emissions equal to ∼10% of that from the global mid-ocean ridge.

The distributions of dissolved, particulate and sedimentary phosphorus were measured in the region of the Juan de Fuca Ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans. Significant negative dissolved phosphate anomalies, ranging from 0 to 60 nmol/l, were observed in the water column at depths between 1900 and 2300 db. The largest anomalies (

[1] Quantifying hydrothermal venting at the boundaries of tectonic plates is an outstanding geoscience problem. Considerable progress has been made by detailed surveys along mid-ocean ridges (MORs), but until recently little was known about fluid venting along volcanic arcs. We present the first systematic survey for hydrothermal venting along the 425-km-long south Tonga arc and new chemistry data for particle and thermal plumes previously reported along an adjacent 88-km-long section of the back-arc Valu Fa Ridge (VFR). Eleven hydrothermal plumes, recognized by their anomalous light backscattering, Eh, temperature, pH, dissolved He-3, CH4, and total dissolvable Fe and Mn, were identified arising from seven volcanic centers along the arc. Five plumes on the VFR were characterized chemically. Vent field density for the south Tonga arc was 2.6 sites/100 km of arc front, comparable to that found by surveys of the Kermadec arc (1.9 to 3.8 sites/100 km) and to MORs in the eastern Pacific (average value for 2280 km of surveyed ridgecrest: 3.2 sites/100 km). A "vent gap'' occurs along a 190 km section of the arc closest to the VFR, and a site density twice the average for MORs on the eastern edge of the Pacific plate was found on this part of the VFR (6.6 sites/100 km). We suggest magmas ascending under the adjacent south Tonga arc have been captured by the VFR. While chemical enrichments of plumes on the south Tonga arc were, in general, slightly less than those on the Kermadec arc, several instances of excessive anomalies in pH suggest a similar presence of fluids enriched in magmatic volatiles (CO2-SO2-H2S). Locally, venting on the VFR has contributed to accumulations of 3 He, Fe, and Mn within the southern Lau basin. On a broader scale, our results provide considerable support for the notion that venting from intraoceanic arcs on the convergent margin of the Pacific plate adds significantly to the total hydrothermal input into the Pacific Ocean.

[1] The 2,500-km Kermadec-Tonga arc is the longest submarine arc on the planet. Here, we report on the second of a series of cruises designed to investigate large-scale controls on active hydrothermal venting on this arc. The 2002 NZAPLUME II cruise surveyed 12 submarine volcanic centers along ∼580 km of the middle Kermadec arc (MKA), extending a 1999 cruise that surveyed 260 km of the southern Kermadec arc (SKA). Average spacing between volcanic centers increases northward from 30 km on backarc crust along the SKA, to 45 km on backarc crust along the southern MKA, to 58 km where the MKA joins the Kermadec Ridge. Volcanic cones dominate in the backarc, and calderas dominate the Kermadec Ridge. The incidence of venting is higher along the MKA (83%, 10 of 12 volcanic centers) than the SKA (67%, 8 of 12), but the relative intensity of venting, as given by plume thickness, areal extent, and concentration of dissolved gases and ionic species, is generally weaker in the MKA. This pattern may reflect subduction of the ∼17-km-thick oceanic Hikurangi Plateau beneath the SKA. Subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab, initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the SKA. Conversely, volcanic centers in the southern MKA are starved of magma replenishment and so their hydrothermal systems are waning. Farther north, where the MKA centers merge with the Kermadec Ridge, fewer but larger magma bodies accumulate in the thicker (older) crust, ensuring more widely separated, caldera-dominated volcanic centers.

[1] Decadal time series observations of hydrothermal fluid emissions from Lō'ihi volcano were initiated in 1992 using a combination of submersible and shipboard sampling strategies. Magmatic-tectonic processes associated with a spectacular seismic event in July–August 1996 led to the collapse of Pele's Vents (31°C) near the volcano summit into a new pit crater (Pele's Pit) inundated by high-temperature (Tmax = 198°C) focused and lower-temperature ( 30- to 200-fold ∼1.5 months postcollapse and, during the following year, decreased a further twofold to threefold at Pele's Pit and ∼30-fold at East Pit. While a steady concentration of ∼400 nmol/L TDFe prevailed throughout the remaining years of this study at Pele's Pit, a gradual and threefold decrease in the concentration of DMn to about 15 nmol/L was observed. High-temperature fluids (128–198°C 1997–1999, ∼90°C 2001) venting simultaneously from different orifices within Pele's Pit had distinguishable Fe/Mn ratios that can be attributed to different subseafloor origins. Fe/Mn ratios characteristic of fluids moderated by high-temperature water-rock reaction had low values in 1997–1998 (1.6 ± 0.7), increasing to about 7 in 1999. Fluids moderated by magmatic degassing of CO2 had much higher Fe/Mn ratios, increasing from 24 ± 15 in 1997–1998 to 50 in 1999 and 63–87 in 2001. Fe/Mn values of dispersing plumes at Lō'ihi reflect an admixture of these sources and a relative Fe abundance that is consistently high compared to mid-ocean ridge systems. The pulsed injection of Mn and Fe into the surrounding ocean associated with the 1996 tectonic-magmatic event at Lō'ihi was massive. Our decadal observations confirm that Mn and Fe are useful markers of the magnitude and evolution of the effects of magmatic perturbation on hydrothermal systems influenced by chronic magmatic degassing.

[1] The spatial density of hydrothermal venting is strongly correlated with spreading rate on mid-ocean ridges (with the interesting exception of hot spot–affected ridges), evidently because spreading rate is a reliable proxy for the magma budget. This correlation remains untested on spreading ridges in back-arc basins, where the magma budget may be complicated by subduction-induced variations of the melt supply. To address this uncertainty, we conducted hydrothermal plume surveys along slow-spreading (40–60 mm/yr) and arc-proximal (10–60 km distant) sections of the southern Mariana Trough and the Valu Fa Ridge (Lau Basin). On both sections we found multiple plumes overlying ∼15–20% of the total length of each section, a coverage comparable to mid-ocean ridges spreading at similar rates. These conditions contrast with earlier reported results from the two nearest-arc segments of a faster spreading (60–70 mm/yr) back-arc ridge, the East Scotia Ridge, which approaches no closer than 100 km to its arc. There, hydrothermal venting is relatively scarce (∼5% plume coverage) and the ridge characteristics are distinctly slow-spreading: small central volcanic highs bookended by deep median valleys, and axial melt lenses restricted to the volcanic highs. Two factors may contribute to an unexpectedly low hydrothermal budget on these East Scotia Ridge segments: they may lie too far from the adjacent arc to benefit from near-arc sources of melt supply, and subduction-aided migration of mantle from the Bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening, analogous to the Reykjanes Ridge. Thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid-ocean ridges: a well-defined trend of spreading rate versus hydrothermal activity on most ridge sections, plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity.

[1] Samples were collected from hydrothermal plumes along the East Pacific Rise (EPR) from 28° to 32°S during the Ridge Axis Plume and Neotectonic Unified Investigation (RAPANUI) cruise (5 March to 12 April 1998). Forty-five vertical casts and tow-yos were conducted: 3 off axis and 42 over the axes of two overlapping propagating ridges, the West and East ridges. These ridges are composed of several nontransform offset ridge segments. Spreading rates range from 149 mm/yr in the segment interiors to 0 mm/yr at the propagating rifts. These maximum spreading rates are considered the fastest on the mid-ocean ridge system and affect the structure of the ridge. Segment-averaged methane plume maxima ranged from 1.7 to 7.2 nM. Mean methane concentrations on the West Ridge were nearly double those of the East Ridge. Westwardly advecting hydrothermal methane persisted to our most distal station, nearly 480 km west of the East Pacific Rise. Background concentrations were less than 1 nM. The highest methane concentration measured was 50 nM in a buoyant plume. Methane did not covary with manganese or any other hydrothermal tracer in plumes over portions of a segment that exhibited recent magmatic activity, possibly as a result of the hydrothermal system's recovery from a phase separation event. In contrast, methane/manganese ratios on the other segments ranged from 0.077 to 0.091. Methane δ13C values in plume maxima ranged from −27 to −33‰ versus Peedee belemnite; background values were around −40‰. These data are compared with hydrothermal plume methane data from slower spreading ridges and illustrate similarities in hydrothermal processes and sources between these systems.

[1] We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

[1] A multidisciplinary expedition to the southern East Pacific Rise (27°30′–32°20′S) provided an opportunity to compare the efficiency and effectiveness of two methods for mapping hydrothermal plumes: the standard conductivity-temperature-depth-optical (CTDO) tow-yo method and a towed fixed array of hydrographic and optical sensors (Miniature Autonomous Plume Recorders (MAPRs)). Six second-order segments were mapped twice: once with CTDO tow-yos, and then again with a fixed array of MAPRs attached to the cable of a deep-towed side-scan sonar. We found a high degree of overall agreement between the two methods in both the distribution and optical intensity of hydrothermal plumes. Between-survey differences increased as time between surveys increased from 6 days, presumably because of advection of the plumes by local currents. Plume locations changed by as much as ∼10 km, implying a confidence limit in predicting vent site location using segment-scale hydrothermal plume surveys. Towed MAPR arrays proved an efficient and effective method for acquiring coregistered geological and hydrothermal plume data.

[1] Observations of the decay of heat and particle discharge following the reinvigoration of a hydrothermal field by a dike intrusion and eruption can provide insights about subsurface hydrothermal circulation. In January 1998, a lava eruption at the summit of Axial Volcano, Juan de Fuca Ridge, created numerous new vents along a 5-km length of the caldera boundary. Five cruises over the next 3.5 years found the inventory of hydrothermal heat and particles (determined by optical backscattering) in the overlying plume decaying as time t−0.9. These results are consistent with the 1993 CoAxial eruption, where the heat flux decayed as ∼t−1.2 and ∼t−0.7 at the Flow and Floc sites, respectively. All three eruption sites decayed significantly faster than t−0.5, the rate attributable to cooling only by simple conduction through the impermeable layer separating magma from hydrothermal circulation. We present a more realistic model of hydrothermal cooling that adds conductive heat loss by ascending fluid to the surrounding rock walls, increasing the theoretical loss rate to t−0.75. We suggest that variability in the observed decay rates are not sampling variations but reflect real differences in the local heat supply and cooling processes. The decay rate was slowest, and matched our model result, where no seafloor eruption occurred and a long-lasting heat source was likely present (Floc). The rate was fastest where a shallow source erupted a thick lava pavement (Flow). The Axial eruption, where a substantial magma chamber emplaced only a thin sheet flow, may be intermediate between these extremes.

[1] During March/April 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of Earth's fasting spreading mid-ocean ridge, 27.5°–32.3°S on the East Pacific Rise. We compared the distribution and chemistry of hydrothermal plumes to geological indicators of long-term (spreading rate) and moderate-term (ridge inflation) variations in magmatic budget. In this large-offset, propagating rift setting, these geological indices span virtually the entire range found along fast spreading ridges worldwide. Hydrothermal plumes overlaid ∼60% of the length of superfast (>130 km/Myr) spreading axis surveyed and defined at least 14 separate vent fields. We observed no plumes over the slower spreading propagating segments. Finer-scale variations in the magmatic budget also correlated with hydrothermal activity, as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation. Along the entire ridge crest, the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume. Plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years. Plume samples with high volatile/metal ratios, generally indicating recent seafloor volcanism, were scarce. Along-axis trends in both volatile (3He; CH4; ΔpH, a proxy for CO2; and particulate S) and nonvolatile (Fe, Mn) species showed a first-order agreement with the trend of ridge inflation. Nevertheless, a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments.

A two-stage scavenging model is used to describe the transport of hydrothermal Mn to the sediments adjacent to ridge crests. Dissolved Mn is hypothesized to be scavenged by slowly settling metal-depositing capsuled bacteria which, in turn, are incorporated into rapidly settling macroaggregates. Upon reaching the seafloor, the Mn is subject to resuspension in particulate form and to remobilization within the sediment column and release back into the water column as dissolved Mn. Measured Mn distributions in the vicinity of the southern Juan de Fuca Ridge and estimated values of process rate constants are used to limit the range of possible model outcomes. The results present a picture of water column distributions and fluxes of dissolved, fine particulate, and large-particle associated Mn in a plume advecting off axis. The model and best available parameter values suggest that more than 80% of the hydrothermal Mn is deposited within several hundred kilometers of the ridge crest, though dissolved Mn concentrations beyond that distance exceed background levels by many times. The residence time of hydrothermal Mn in the water column is of the order of several years. An off-axis component of advection of the order of 0.1–0.3 cm/s is needed to make similar the model and measured distributions of Mn depositing in the sediments.

Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area (∼60 m in diameter). Compositions range from a 300°C, gas-enriched (285 mmol/kg CO2), low-chlorinity (∼33% of seawater) fluid to a 328°C, relatively gas-depleted (50 mmol/kg CO2), high-chlorinity (∼116% of seawater) fluid. The entire range of measured compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. Other mechanisms proposed to produce chlorinity variations in hydrothermal fluids (precipitation/dissolution of a chloride-bearing mineral or crustal hydration) cannot produce the covariation of chlorinity and gas content observed at ASHES. There is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low-chlorinity fluids over levels predicted by the model are attributed to fluid-rock interaction in the upflow zone. Depletions in iron and calcium suggest that these elements have been removed by iron-sulfide and anhydrite precipitation at some time in the history of the low-chlorinity fluids. The distribution of low- and high-chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability. The relatively shallow depth of the seafloor (1540 m) and the observed chemistry of ASHES fluids are consistent with phase separation in the sub-critical or near-critical region.

We have mapped the distribution and intensity of submarine hydrothermal emissions from the summit caldera of Axial Volcano by means of hydrographic and chemical sampling of the water column during annual cruises in four consecutive years (1985–1988). These investigations were undertaken to ascertain the strength of hydrothermal venting relative to other vent fields on the Juan de Fuca Ridge and to gauge the importance of Axial Volcano as a source of hydrothermal emissions to the northeast Pacific Ocean. Measurable temperature anomalies are mostly restricted to a 200-m-thick water column within the caldera, where they range from a background level of ∼0.01°C to local maxima of 0.02–0.1°C above known high-temperature vent fields. The temperature anomaly plume is significantly smaller in both extent and intensity than plumes emitted by vent fields on other segments of the Juan de Fuca Ridge. A maximum estimate of the total heat flux from the summit, based on the total excess heat and advective velocity of the plume, is 8×108 W. Hydrothermal venting seems surprisingly small for an edifice that testifies to a long-term oversupply of magma at a single axial location. We speculate that additional heat may be lost from Axial Volcano by diffuse percolation of sea water over broad areas of its flanks and by magma eruption and venting along flank rift zones.

In 1986 and 1987, buoyant and neutrally buoyant hydrothermal plume particles from the ASHES vent field within Axial Volcano were sampled to study their variations in composition with height above the seafloor. Individual mineral phases were identified using standard X ray diffraction procedures. Elemental composition and particle morphologies were determined by X ray fluorescence spectrometry and scanning electron microscopy/X ray energy spectrometry techniques. The vent particles were primarily composed of sphalerite, anhydrite, pyrite, pyrrhotite, chalcopyrite, barite, hydrous iron oxides, and amorphous silica. Grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice, followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume. The neutrally buoyant plume consisted of a lower plume, which was highly enriched in Fe, S, Zn, and Cu, and an upper plume, which was highly enriched in Fe and Mn. The upper plume was enriched in Fe and Mn oxyhydroxide particles, and the lower plume was enriched in suspended sulfide particles in addition to the Fe and Mn oxyhydroxide particles. The chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles. Short-term sediment trap experiments indicate that the fallout from the ASHES vent field is not as large as some of the other vent fields on the Juan de Fuca Ridge.

Deep CTD/transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally-buoyant plumes emanating from each of two major vent fields on the Southern Symmetrical Segment (SSS) and Endeavour Segment (ES) of the Juan de Fuca Ridge. At both vent fields, emissions from point and diffuse hydrothermal sources coalesced into a single 200-m-thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0.02–0.05°C and light-attenuation anomalies of 0.01–0.08 m−1 (10–80 μg/l above background). Temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped (10–15 km). Downcurrent light-attenuation trends were non-uniform and differed between plumes, apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates. Vent fluids from the SSS are metal-dominated and mostly precipitate very fine-grained hydrous Fe-oxides that remain suspended in the plume. Vent fluids from the ES are sulfur-dominated and precipitate a high proportion of coarser-grained Fe-sulfides that rapidly settle from the plume. The integrated flux of each vent field was estimated from measurements of the advective transport of each plume. Heat flux was 1700 ± 1100 MW from the ES and 580 ± 351 MW from the SSS. Particle flux varied from 546 ± 312 g/s to 204 ± 116 g/s at the ES depending on distance from the vent field, and was 92 ± 48 g/s from the SSS.

During two Atlantis II/Alvin cruises to the Juan de Fuca Ridge in 1984 active high temperature (140°–284°C) vents were sampled for black smoker particulates using the Grassle Pump. Individual mineral phases were identified using standard X ray diffraction and petrographic procedures. In addition, elemental compositions and particle morphologies were determined by X ray energy spectrometry and scanning electron microscope/X ray energy spectrometry techniques. The vent particulates from the southern Juan de Fuca Ridge vent sites were highly enriched in S, Si, Fe, Zn, and Cu and were primarily composed of sphalerite, wurtzite, pyrite, pyrrhotite, barite, chalcopyrite, cubanite, hydrous iron oxides, and elemental sulfur. Two additional unidentified phases which were prevalent in the samples included an Fe-Si phase and a Ca-Si phase. The grain sizes of the individual particle phases ranged from 100 μm for the Fe-Si particles. Grain size and current meter data were used in a deposition model of individual phase dispersal. For many of the larger sulfide and sulfate particles, the model predicts dispersal to occur over length scales of only several hundreds of meters. The high-temperature black smokers from the more northerly Endeavour Segment vents were highly enriched in Fe, S, Ca, Cu, and Zn and were primarily composed of anhydrite, chalcopyrite, sphalerite, barite, sulfur, pyrite, and other less abundant metal sulfide minerals. The grain sizes of the individual particles ranged from < 10 μm to slightly larger than 500 μm. The composition and size distributions of the mineral phases are highly suggestive of high-temperature mixing between vent fluids and seawater. A series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals. The dissolution rates ranged over more than 3 orders of magnitude, from 3.2 × 10−8 cm s−1 for anhydrite to 1.2 × 10−12 cm s−1 for chalcopyrite. The results indicate that for some minerals, particularly anhydrite and marcasite, total dissolution occurs within a few hours to a few weeks of their formation. For other more stable minerals, including pyrite, sphalerite and chalcopyrite, the time required for total dissolution is much longer, and consequently, individual crystals may be expected to persist in the sediments for considerable periods of time after deposition.

Evidence of large-scale episodic venting of hydrothermal fluids was initially discovered in August 1986 in the form of a 130-km3 radially symmetric “megaplume” over the southern Juan de Fuca Ridge. We report here on the discovery in September 1987 of a second, smaller megaplume about 45 km north of the location of the first megaplume. The 3He/heat, 3He/dissolved Mn, and 3He/dissolved silica ratios in both megaplumes were typical of high-temperature vent fluids. Evidence from long-term records of current flow over the southern Juan de Fuca Ridge, and from the mineralogy and Mn chemistry of megaplume particles, makes it unlikely that the second megaplume was a reencounter of the first. A plume model that relates the heat flux to the observed plume rise height of ∼1000 m finds that the total heat content of the fluids that formed the megaplumes was 1016–1017 J, or equivalently a fluid volume of 3–8 × 107 m3 at 350°C. The geometry and suspended particle population of the first megaplume imply that such features are formed within a few days time. The extraordinary heat and volume fluxes associated with megaplumes (102–103 greater than ordinary vent fields), as well as their typical hydrothermal chemistry, suggest that they resulted from tectonic or hydraulic fracturing that suddenly increased the permeability of the hydrothermal fluid reservoir in the axial crust. The flux of hydrothermal heat from continuous venting and episodic megaplumes on the southern Juan de Fuca Ridge is presently 4 − 10 × 109 W, a factor of 5–10 greater than various geophysical model calculations for this ridge segment. This imbalance may be symptomatic of a recent surge in the local cycle of magmatic activity.