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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 ∆pH 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 ∆pH (-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 (∆pH 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 3 He/ 4 He 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 subcropping 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. 210 Pb/ 226 Ra dating of one chimney gives an age of 27 ± 6 years; 226 Ra/Ba dating of other mineralization indicates ages up to 1,200 years. Formation temperatures derived from ∆ 34 Ssulfate-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, ...

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47ø57'N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have .......... '--• ,,,e,,• are of the •'•'• of ltm oeen analyzeu for major and minor elements and gases. •v•, •. ,, ...... 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 fluidcomposition, 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 >400øC are consistent with supercritical phase separation and loss of brine phase below the seafloor.The factor-of-2 variation in CO 2 (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. Honolulu 4pacific Marine Environmental Laboratory, NOAA, Newport, Oregon 5pacific Marine Environmental Laboratory, NOAA, Seattle, Washington Paper number 93JB03132. 0148-0227/94/93JB03132505.00 assemblage may be approached or reached at depth in a hydrothermal system [Bowers et al., 1988; Berndt et al., 1989]. However, the extremely wide range in the measured composition of hydrothermal fluids cannot be explained by equilibration between seawater and basalt at high temperatures. In particular, the range of reported chloride concentrations in hydrothermal fluids (30% to >200% of the seawater concentration) clearly requires a process other than chemical exchange between seawater and oceanic crust, because there are no known, adequate mineral sources or sinks for chloride. The variability in hydrothermal chloride concentrations has received much attention because chloride is the major anion in hydrothermal fluids and affects the solubility of many minerals and the transport of metals in solution by forming complexes with dissolved cations. It is important to understand what processes cause the large changes in chlorinity of hydrothermal fluids and how these processes affect the overall fluid composition. The two proposed mechanisms for generating large variations in the chlorinity of hydrothermal fluids are phase separation and precipitation of a chloride-bearing mineral. While the evidence for...

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