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.
Figure 2: Plots of (a) Cl, (b) H
S, (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.
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).
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.
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).
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).
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.
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.
Table 1 vent fluid chemical composition: HS, pH, alkalinity and ammonia in hydrothermal fluids collected in 755
ml titanium major samplers were determined within 12 h of submersible recovery, and
dissolved silica was analysed within 48 h on diluted aliquots. In 1994 and 1995, anions were
analysed shipboard using a Dionex DX500 ion chromatograph. Other methods are the same as
in Butterfield & Massoth (1994) except that Li, Na, K, Mg and Ca were also measured by ion
chromatography. For diffuse fluid samples, high-precision (less than 0.3% rsd) titration
methods were used for Mg, Ca, and Cl, with IAPSO seawater as standard. Alk is in units of meq
l; HS, NH, Li, Fe
and Mn are in units of µmol kg; Cl, SO, Mg, Ca and Si are in units of mmol kg.
Table 1. Cont.
Table 1. Cont.
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