Seafloor eruptions and evolution of hydrothermal fluid chemistry

D. A. Butterfield,¹ I. R. Jonasson,² G. J. Massoth,³ R. A.Feely,³ K. K Roe,¹ R. E. Embley,⁴ J. F. Holden,⁵ R. E. McDuff,⁵ M. D. Lilley,⁵ and J. R. Delaney

¹Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Seattle, WA 98195
²Geological Survey of Canada, Ottawa, Ontario, Canada
³Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, WA 98115
⁴Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Newport, OR 97365
⁵School of Oceanography, University of Washington, Seattle, WA 98195

Philosophical Transactions of the Royal Society of London A 355, 369-386 (1997).
Copyright ©1997 by the Royal Society. Further electronic distribution is not allowed.

Abstract

1.  Introduction

It has been recognized since seafloor hydrothermal vents were first sampled that they represent an important part of the geochemical cycles of many elements (Edmond et al. 1979; Von Damm et al. 1985; Palmer & Edmond 1989). Attempts to extrapolate to global chemical fluxes (using independent estimates of MOR fluxes of heat or helium and their correlations to elemental concentrations) have been tempered by an ever-increasing range of endmember fluid concentrations and by the lack of information on how fluid concentrations vary with time (see recent discussion by Von Damm 1995).

Large changes over time in chemical composition of vent fluids were not seen until 1990, when hydrothermal systems affected by volcanic activity were first sampled and it became apparent that rapid and significant changes in the style of venting and fluid compositions occurred immediately following an eruption (Haymon et al. 1993; Butterfield & Massoth 1994; Von Damm et al. 1995). Seafloor volcanic eruptions clearly have dramatic and interconnected consequences including formation of megaplumes (event plumes), microbiological blooms and rapid evolution of the temperature and composition of vent fluids (Haymon et al. 1993; Embley & Chadwick 1994; Baker 1995; Baker et al. 1995; Embley et al. 1995; Lupton et al. 1995; Von Damm et al. 1995; Holden 1996; Charlou et al. 1996). With the evidence accumulating from seafloor eruption events at N. Cleft, EPR at 9°50N, CoAxial segment, and the EPR near 17°30S (Auzende et al. 1996), it is possible to describe how hydrothermal systems change immediately following a volcanic event and to estimate how hydrothermal fluxes are affected. In this paper, we present the first detailed chemical data on hydrothermal fluids from the CoAxial segment of the Juan de Fuca Ridge, and relate it to a general model of hydrothermal response and chemical evolution of fluids following a volcanic event.

2. Description of CoAxial site

Figure 1: Map of CoAxial area, showing regional setting (inset bottom), location of lava Flow site, Floc site, and Source site. Circles are event plumes labelled A, B and C in order of both discovery and increasing age at the time of sampling (Baker et al. 1995; Massoth et al. 1995). Arrows show current direction at 1800 m depth (Cannon et al. 1995). A detailed map of the Floc site (inset top) shows the location of markers referred to in the text. Bathymetric contour interval is 200 m.
 

3. Results

Table 1 contains the chemical data as analyzed in individual samples. Selected elements for Flow and Floc sites are shown in figure 2. Note that the measured Mg concentration in diffuse fluids is affected by subseafloor dilution of a Mg-depleted source fluid and by entrainment of ambient seawater at the vent orifice during sampling (i.e. sample quality).

No high-temperature fluids were found in the Flow or Floc areas. Observed post-eruption venting was strictly diffuse and low-temperature (maximum 51°C on 1 August 1993). Evidence for high-temperature (greater than 200°C) reactions is present in the diffuse fluids in the depleted Mg and elevated Li, Fe, Mn, Ca and Si concentrations. Flow, Floc, and Source sites each have distinctive chemical characteristics reflecting differences in hydrothermal conditions in the three locations.

Figure 2: Plots of (a) Cl, (b) HS, (c) Fe, (d) Mn, (e) Li, (f) SO, (g) NH, and (h) Si versus Mg for Flow and Floc diffuse fluids. Symbols: triangle, Flow site 1993; rectangle, Floc site 1993; circles, Floc site 1994; hourglass, Floc site 1995; open square, ambient seawater. Lines drawn to show range of element-magnesium trends: thin line, Flow site; dashed line, Floc 1993; dot-dashed line, Floc 1994; dotted line, Floc 1995. Arrow in (f) represents conservative mixing of seawater with a zero-sulphate, zero-magnesium endmember.

The Flow site vents appeared as shimmering water exiting cracks and interstices of a fresh lobate/pillow lava mound at ca. 2390 m depth. Extensive surface alteration and precipitation of hydrothermal sediment was already apparent within one week of the eruption, and globules of orange material could be seen coming directly out of cracks in some places. The initial samples recovered by ROPOS were depleted in Mg by only 0.5% relative to ambient bottom seawater (indicative of substantial seawater entrainment during sampling), did not differ significantly from seawater in chlorinity, but were significantly enriched in Li, HSiO, Ca, Fe, and Mn. HS was undetectable (below 0.5 µmol kg) in all samples collected by ROPOS and Alvin at the Flow site.

The Flow site was revisited approximately nine weeks after the first samples were taken, and some subtle changes in chemical composition had occurred. Recovered samples had Mg depletions ranging from 0.7-2.3% relative to ambient seawater, and temperatures ranged from 4-36°C. Though the October samples were more depleted in Mg, they had lower iron concentrations than the initial ROPOS samples, and the chlorinities were now clearly higher than seawater. Measured ammonia concentrations ranged from 1-11 µmol kg, compared to background seawater at less than 0.3 µmol kg (greater than 30 µmol kg nitrate in bottom seawater is more than sufficient as a nitrogen source for the ammonia). Slightly different element-magnesium trends are apparent in the different vents sampled (figure 2); vent 6a (46°31.609N, 129°34.713W) has higher Fe, Cl and temperature than vent 7a, located about 330 m to the southwest (46°31.451N, 129°34.811W). Two samples from lower temperature vents in the graben just south of the lava flow (46°30.581N, 129°35.33W) were closer to seawater in composition.

The Flow site was visited again by Alvin in July 1994. Venting had nearly ceased one year after the event. One vent (46°31.444N, 129°34.804W, marker 0) with very little flow and temperature of 9°C was sampled, and the samples did not differ significantly from ambient seawater in Mg or Cl. The fluids did have very slightly elevated silica, iron, manganese, and alkalinity, indicating that water-rock reaction and, possibly, microbial activity were still taking place at greatly reduced levels relative to the immediate post-eruption period. Flow site vents were not sampled in 1995, when water column surveys indicated no detectable thermal anomalies over Flow (E. T. Baker, personal communication 1995). (c) Floc site The seafloor vents giving rise to the water column 'Floc snowstorm' observed with ROPOS on 2 August were found with Alvin in October 1993, when fluids were collected from vents near marker 3A (16-18°C) and marker 11 (22°C), separated by a distance of ca. 580 m. The vents differed markedly in appearance from the Flow site, as they were associated with fissures in older basalt between 2220 and 2290 m depth, and were emitting white flocculent material thought to be bacterial in origin (Juniper et al. 1995). Also in contrast to the Flow site, Floc vent fluids had elevated levels of HS and were lower than seawater in chlorinity (figure 2), indicative of a vapour component. In contrast to the rapid exhaustion of heat and diminution of fluid flux seen at Flow site after one year, significant fluid fluxes were maintained at the Floc site up to two years after the volcanic event. Observations of the seafloor in the Floc area support a waning of venting at particular sites and a general decrease in the area of active venting over time, but our submersible surveys do not allow quantitative assessment of changes in fluid or heat flux. Fluid chemistry exhibits detectable temporal and spatial variation at Floc (figures 2 and 3). In October 1993, chlorinities were lower than seawater; in July 1994, more samples were recovered showing a range of chlorinities both higher and lower than the initial three samples indicated. There were slight changes in composition at reoccupied marker 11 and 3a sites. In July 1995, chlorinities of all the recovered samples from Floc were very close to seawater.

Figure 3: Temporal and spatial variation in diffuse fluids. Stacked bar graph of average element to heat ratios in nmol J (HS/heat values have been divided by three to match scales) and Fe/Mn ratio. Vent marker names (see figure 1 and  2) are given below the bars, which are ordered from N (left) to S (right) for each area. Absence of HS at Flow site indicates that the degree of subseafloor mixing caused the redox potential to be dominated by seawater. Iron oxidation is significant at the Flow site, as plentiful amorphous iron oxides and ferric phosphate coatings on bacterial sheaths (Juniper et al. 1995) attest. Note HS pulse in 1994 at HDV. Spatial variation in HS/heat (0.8-31 nmol J) and HS/Fe (10-340) in Floc vent fluids is enormous, with highest near HDV and marker 26. Lower values at the ends of observed venting may be due to a combination of precipitation and oxidation of HS toward the periphery of the Floc upflow zone. HS may be the biolimiting energy source for micro-organisms in some Floc vents (e.g. markers 11, 18 and 15 in 1995).

In October 1993, Floc samples had significant levels of HS and Fe (up to 55 and 27 µmol kg, respectively). In 1994, HS increased to a maximum of 380 µmol kg, while iron was below 3 µmol kg in all but one sample. In 1995, maximum HS was 230 µmol kg and iron was below 6 µmol kg. While iron was virtually unchaged at HDV vent from 1994-1995, hydrogen sulphide concentration apparently reached a peak in 1994, which coincides with maximum light attenuation in the plume over Floc (E. T. Baker, personal communication 1996).

The continued presence of HS, Li, Fe and Mn in the fluids indicates that high-temperature reactions continue beneath the Floc site. If the reaction zone produces a high-temperature fluid that has zero sulphate as well as zero magnesium, then sulphate reduction in the near-seafloor upflow zone is not important enough to cause significant depletion of sulphate below a conservative mixing line in the sulphate versus magnesium plot. Nearly all of the samples are slightly above the conservative mixing line, suggesting that oxidation of sulphide or dissolution of sulphate minerals (e.g., anhydrite or caminite) is quantitatively more important than sulphate reduction. (d) Source site There are four known high-temperature vents (Beard, Church, Mongo, and Twin Spires) at the Source site spread out over ca. 100 m along a fissure running ca. 020° NNE near the crest of a pillow lava ridge at ca. 2055 m depth. The Source site is at the southern end of the CoAxial neovolcanic zone, collinear with Flow and Floc sites. The chemistry of the vent fluids from this site indicates that it was unaffected by the 1993 eruptive activity and probably existed prior to the eruption.

Figure 4: Plots of (a) Cl, (b) Ca, (c) HS, (d) Li, (e) Fe, and (f) Mn versus Mg for source vent fluids. Symbols: triangle, 1993, rectangle, 1994, circle, 1995, open box, ambient seawater. Cl, Ca, and Li data show that there is a single primary endmember for this site that is not changing significantly over time. Lines drawn in (e) to show temperature dependence of Fe concentration: solid line for Twin Spires at 223°C dashed line for several vents in range of 250-275°C and dot-dashed line for a 294°C orifice at Church vent.

All four of the Source vents have virtually the same major element composition, which did not change significantly over the two years following the eruption (figure 4). Source fluids are moderate temperature (223-294°C) brines (695 mmol kg Cl) with low HS (1.0-1.7 mmol kg), iron (45-120 µmol kg), Fe/Mn (0.4-0.5), and zinc (6-17 µmol kg) relative to other MOR vents with similar temperature and chlorinity. The combination of low Fe and a pH of 4.5-4.8 means that significant iron sulphide precipitation has not occurred in the upflow zone near the vents (otherwise the pH would be lower) and implies that the fluids last equilibrated near the temperature of venting (Seyfried & Mottl 1995). These fluids have very high Ca (67 µmol kg), high Li (900 µmol kg), and low Na/Cl ratio (0.76 compared to 0.86 in seawater), indicative of extensive water-rock reaction (Li-derived water-rock ratio is 1.0, similar to many high-temperature MOR vents). These characteristics are consistent with a brine that formed at high temperatures (greater than 400°C), then remained in the crust, cooled or mixed with cooler seawater-derived fluid, and equilibrated at temperatures closer to 300°C before venting.

4. Discussion

Figure 5: Response of hydrothermal systems to a volcanic event. Relative intensity of heat flux (dot-dashed line) and vent fluid concentrations of chloride (thin line), iron (dotted line), and hydrogen sulphide (thick line) over time. Systems evolve from a vapour-dominated, high heat flux stage accompanied by phase separation, through a transition to brine-dominated discharge, and eventually decay back toward zero heat flux and seawater composition. Our observations suggest high Fe concentrations in immediate post-eruptive fluids. Response model is based on this work, Butterfield & Massoth (1994), Von Damm et al. (1995), Lupton (1995), and Baker (1995).

The two very different types of diffuse fluids seen at CoAxial (oxidized, Fe-rich fluids at Flow, and more reducing HS-rich fluids at Floc) can be explained by a general model of how vent fluids evolve following a volcanic event (figure 5), taking into account the likely differences in the location of the heat source within the oceanic crust (figure 6). Immediately following a volcanic eruption or dyke injection, heat flux increases greatly, triggering phase separation and preferential venting of the more buoyant vapour phase. Continuous venting of brines observed in some systems means the conjugate brine phase must be temporarily retained around the heat source (by virtue of higher density or other physical properties), while higher-enthalpy vapour-rich fluids with low chlorinities remove heat, volatiles (HS, CO, He and H from magmatic degassing and water-rock interaction partition into the vapour phase) and metals (figure 6). Iron concentration during the vapour-dominated period will be a function of temperature, pH, Cl concentration and possibly reaction kinetics, and cannot be assumed to be simply proportional to Cl (Seyfried & Ding 1995). As heat is removed and the system cools, phase separation slows and stops, and the fluids go through a transition from vapour-dominated to brine-dominated (chloride and metals increase while volatiles decrease). Brine content in the vented fluids may reach a peak and then decline as heat from the system runs out and fluid compositions decay back toward seawater.

Figure 6: Chemical and microbial processes in a diffuse upflow zone. Injection of a dyke results in delivery of reduced volatiles and metals through outgassing and water-rock interaction. Phase separation partitions volatiles and some metals into vapour phase, and brines accumulate around heat source. Thermal and redox gradient provides a zone for chemical and microbial oxidation of reduced gases and metals as circulating seawater is entrained (microbial methanogenesis and sulphur reduction are known to occur at 110°C).

The time scale in figure 5 is relative to the size of the volcanic event. Vents at 9°46.5N on the EPR showed an increase in chlorinity but were still below seawater chlorinity three years after a seafloor eruption (Von Damm et al. 1995). North Cleft segment diffuse and high-temperature fluids reached the brine emission stage at least three years after the volcanic event (Butterfield & Massoth 1994). High-temperature fluids are still evolving at North Cleft (Butterfield, unpublished data) and it is not clear how long the decay phase may last. We propose that cooling and evolution of fluid chemistry was so fast at the Flow site that we missed the vapour-dominated venting period entirely. It took only 3-12 weeks to reach the brine stage at Flow, and venting was virtually exhausted after one year, while venting of low-chlorinity fluids continued for at least a year at Floc.

We attribute this post-eruptive fluid evolution to cooling of magma injected into the permeable upper layer of oceanic crust. In the absence of recent volcanic perturbations, it appears that low-chlorinity volatile-rich fluids are associated with high heat flux systems driving deeper phase separation (e.g. Endeavour Main Field), while brine-dominated fluids are associated with lower heat flux systems (e.g. Cleft segment) (see heat flux estimates in Baker 1995). The model depicted in figure 5 may apply in a general sense to all hydrothermal systems insofar as intensity of heat output over time should correlate with fluid chemistry. The Source vent area at CoAxial could be interpreted to be in the decay phase in figure 5. So far, no hydrothermal system has been sampled before and after a known volcanic event, but we propose that the model for post-eruptive fluid evolution would apply in an established hydrothermal system as well as in areas with no pre-existing, active hydrothermal system.

Figure 7: Photograph of halite coating on basalt within a cavity from Alvin rock sample 2672-7, recovered in October from the crest of the July 1993 lava flow. Halite coating is intergrown with anatase (TiO), boehmite (AlO(OH)), and rare sphalerite (ZnS) needles (prominent in this photograph). Interpretation is eruption caused by immediate phase separation and halite precipitation, followed by a high-temperature reaction period. Mineral compositions were determined by XRD and EDS-SEM at the Geological Survey of Canada in Ottawa.

The Flow site is located at the distal end of the dyke injection (Dziak et al. 1995) and was characterized by shallow crustal activity relative to the Floc site ( Schreiner et al. 1995 ) and a pillow-lobate eruption (Embley et al. 1995). The chemistry of the event plumes, vent fluids and basalt alteration products at Flow site is consistent with an initial high-temperature venting period lasting less than three weeks after the end of the eruption, followed by rapid cooling of the lava mound and underlying dyke. The large inventory of Fe and Mn and the presence of ZnS particles in the event plumes (Massoth et al. 1995) and the high particulate Cu/Fe ratios in event plume A (0.002-0.005, compared to less than 0.0016 in the diffuse fluid-dominated chronic plumes) indicate that a high-temperature fluid contributed to event plume formation. (Plume particle chemistry determined by XRF.) Many of the freshly erupted basalt samples recovered showed evidence of an initial high-temperature reaction period: halite coatings precipitated by direct contact of seawater with very hot rock; fresh needles of sphalerite and small crystals of pyrite and chalcopyrite lodged on top of halite and other surfaces (figure 7), indicating flow of fluids from a hot (greater than 350°C) reaction zone; and pervasive staining consisting of oxides (amorphous iron oxide, boehmite, anatase), kaolinite and chlorite, indicating temperatures of 250°C and up to greenschist conditions. The precipitation of halite coatings on altered glass surfaces is consistent with heating seawater to temperatures of 440°C at 235 bar local seafloor pressure ( Bischoff & Pitzer 1989 ) and represents a short-term storage of chloride. Vapour-dominated fluids must have vented when halite was precipitating. Continued circulation of warm fluids through the lava flow should dissolve the halite and increase the chlorinity of vented fluids. Some of the halite coatings on basalts collected in October 1993 showed surface textures indicative of partial dissolution.

Figure 8: Geologic interpretation of CoAxial hydrothermal evolution, with depth below sea level in meters on vertical axis and latitude (°N) on horizontal axis. At the distal end of the dyke injection, heat is removed rapidly by formation of event plumes and circulation of seawater through the permeable lava mound, while a larger and deeper heat source near the magma supply continues to discharge for several years and provides a habitat for microbial communities. See discussion.

At the Floc site, we propose that the heat source was deeper, larger and not directly exposed to ambient seawater (figure 8), allowing the vapour-dominated period to last for at least one year. At two years after the event, fluid chlorinities were nearly identical to seawater, suggesting that this site was making the transition toward brine-like composition. We predict that Floc site fluids will evolve to become brine-dominated, eventually becoming totally depleted in HS and dominated by iron, as seen at North Cleft and at Flow.

Subseafloor chemical and microbial oxidation of sulphide has the potential to generate significant quantities of particulate elemental sulphur, which we have observed in direct association with biogenic particles in the plume over Floc. Our data and general model strongly suggest that methane (possibly from subseafloor methanogens) and biogenic particles vented from post-eruptive vapour-dominated diffuse vents contribute significantly to the high CH/Mn and particulate S/Fe ratios observed in hydrothermal plumes over magmatically active ridge segments (Lupton et al. 1993).

We hypothesize that nearly all of the potential heat to be extracted at the Flow site is contained in the seafloor lava flow and shallow underlying dyke and conduits, and that a significant part of this heat was extracted and formed event plumes during the eruptive event. The decline in observable fluid flux and maximum measured vent fluid temperature from 51°C on 1 August 1993, to 36°C in mid-October, to only 9°C in July 1994 shows that the heat source at the Flow site was nearly exhausted within a year. Using the volume of extruded lava (5.4 × 10 m) estimated by Chadwick et al. (1995) and the basalt properties found in Sleep et al. (1983), we calculate that the heat available in the lava mound through latent heat of crystallization and cooling to ambient seawater temperatures is 2.7 × 10 J. The estimate of Baker et al. (1995) for the heat contained in the three event plumes (1.8 × 10 J) is equivalent to 2/3 of the heat within the lava mound. (Diffuse venting was seen north and south of the lava mound, and consideration of the potential volume of dyke beneath the entire area of known venting at Flow site (1 m × 8 km × 2 km) increases the available heat by a factor of three.) There is no evidence for hydrothermal discharge at the Flow site immediately preceding the 1993 eruption, so it is difficult to invoke a significant shallow crustal hydrothermal fluid reservoir, as required in some event plume models (Cann & Strens 1989). High-temperature sulphide minerals clearly precipitated from the early fluids exiting the lava mound (figure 7) and therefore provide a source for the sulphide minerals observed in the event plumes. Because the CoAxial volcanic event was pulsed and episodic over several weeks, the initial penetration of a dyke in the eruption area creates a high-temperature reaction zone which could be disrupted and emptied during later eruptive pulses. The very large permeability required to generate event plumes by dyke injection (Lowell & Germanovich 1995) is provided here by the presence of lava at, and directly below, the seafloor. The published plume data (Baker et al. 1995; Cannon et al. 1995; Lavelle 1995; Lupton et al. 1995; Massoth et al. 1995) and our evidence for very short-lived, high-temperature venting support the hypothesis that the observed CoAxial event plumes resulted directly from rapid high-temperature water-rock interaction during the seafloor eruption and shallow dyke injection. The presence of erupted lava mounds near the North Cleft megaplume site provides some support that this is a general mechanism of event plume formation but questions remain regarding the relationship of event plume chemistry to event plume formation mechanisms.

Acknowledgements. We thank the ROPOS and Alvin Deep Submergence teams, Ed Baker and John Lupton for comments on the manuscript, Joe Cann for editing, Ryan Whitney and Martha Jackson for word processing, Aries Galindo for graphics, and My Nguyen for assistance with chemical analysis. This research was supported by the NOAA VENTS Program, the National Undersea Research Program, and the U.S. National Science Foundation. PMEL contribution number 1731. JISAO contribution number 362.

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Discussion

D. Pyle (Department of Earth Sciences, University of Cambridge, UK). In Dr Butterfield's model he considers the interaction of the hot lava-flow body with the overlying water column as a mechanism for producing high-temperature fluids. Is it not possible that much of this hot fluid might be produced by heating of seawater trapped in the shallow fractured water-saturated crust which was engulfed by the rapidly advancing flow-front?

D. A. Butterfield. Indeed, we believe that most of the heat is removed by fluids circulating around and through the lava mound and underlying dyke, and that hot fluids are primarily generated below the interface between the open water-column and the lava surface. Fluid flow is facilitated by the high porosity of the upper ca. 100 m of ocean crust in this area, and by the high permeability of the lava mound itself, as indicated by the extensive venting of warm fluids from cracks and interstices between pillows. Rapid removal of heat from the portion of the dyke and lava flow very near the seafloor can contribute to event plume formation, while heat from the deeper portions of the dyke would probably be removed more slowly.
 

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