The Chemical Composition of Shallow-Water Hydrothermal Fluids in Tutum Bay, Ambitle Island, Papua New Guinea and Their Effect on Ambient Seawater

Marine Chemistry 64Ž. 1999 229±252

The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater

Thomas Pichler a,), Jan Veizer a,b, Gwendy E.M. Hall c a Ottawa-Carleton Geoscience Centre, UniÕersity of Ottawa, Ottawa, Ontario, Canada, K1N 6N5 b Institut fuer Geologie, Ruhr UniÕersitaet, 44780 Bochum, Germany c Geological SurÕey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, K1A 0E8 Received 27 January 1998; accepted 1 September 1998

Abstract

Submarine, hydrothermal venting occurs at Tutum Bay in shallowŽ. 5±10 m water along the inner shelf that contains a patchy distribution of coral±algal reefs. Two types of venting are observed.Ž. 1 Focused discharge of a clear, two-phase fluid from discrete orifices, 10±15 cm in diameter. Discharge temperatures are between 89 and 988C and estimated flow rates are as high as 300 to 400 lrmin.Ž. 2 Dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous in the area. The composition of the gas is mainly CO222Ž. 92.6±97.9% with minor amounts of N Ž.Ž. 2.2±4.7% , O 0.43±0.73% , ; CH4 Ž. 0.6±2% and He Ž0.01±0.02% . . Based on their geographic position and chemical composition, the vents have been divided into two groups, A and B. Area B vents have higher K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, and Sr concentrations. Their chemical difference is likely caused by subsurface mixing of a CO2 -rich water with a deep reservoir neutral chloride fluid in varying proportions. A two- or possibly three-step process controls fluid evolution and final chemical composition:Ž. 1 phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO2 -rich water of approximately 150±1608C.Ž. 2 The steep topography causes lateral movement of this CO2 -rich fluid towards the margin of the hydrothermal system where it mixes with the marginal upflow from the deep reservoir. This produces a dilute chloride water of approximately 1658C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B±RbrCs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. Compared to seawater, the hydrothermal fluids are depleted in Cl, Br, SO4 , Na, K, Ca, Mg, and Sr and enriched in HCO3 , B, Si, Li, Mn, Fe, Rb, Cs, Sb, Tl and As. Although some elements are significantly enriched, they do not have a clear impact on ambient seawater composition because their concentration is buffered by mixing and uptake into secondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Tutum Bay, Papua New Guinea; hydrothermal fluid; seawater composition; shallow water

) Corresponding author. Tel.: q1-613-562-5800 ext. 6639; Fax: q1-613-562-5192; E-mail: [email protected]

1. Introduction Here we report on the chemistry of shallow-water hydrothermal fluids in Tutum Bay and their effect on The chemical composition of seawater is con- ambient seawater composition. In this near-shore, trolled through the combined effort of numerous shallow-water setting, physicochemical conditions processes, such as low and high temperature seafloor may not be drastically different from on-shore hy- alteration, fluvial and atmospheric input, and sedi- drothermal systems, for example, in terms of pres- mentation. Most studies of seafloor hydrothermal sure and temperature, but their discharge into seawa- activity have focused primarily on deep-sea, hy- ter sets them apart. Mixing with cold oxidizing drothermal systems found along volcanically active seawater provides a mechanism for precipitation of portions of the mid-ocean ridges or in deep back-arc dissolved constituents in the hydrothermal fluids. basins. Submarine hydrothermal activity and hy- Mixing may occur right at the seawater±seafloor drothermal alteration of oceanic crust, however, is interface for an impermeable seafloor, or in the not confined to deep-water environments. Additional subsurface if local sediments are saturated with sea- locations are found in much shallower water at the water. As a result, Eh, pH and temperature change in flanks of volcanic islands and on the tops of the hydrothermal fluid, in pore water and in ambient seamountsŽ e.g., Vidal et al., 1978; Vidal et al., seawater. 1981; Sarano et al., 1989; Butterfield et al., 1990; Varnavas and Cronan, 1991; Dando and Leahy, 1993; Hodkinson et al., 1994; Pichler et al., 1995; Heikoop 2. Location and geological setting et al., 1996; Sedwick and Stuben,È 1996. . The sub- merged flanks of island arc volcanoes provide an Our study area lies along the southwest margin of exceptional opportunity to study the essential differ- Ambitle Island, one of the Feni islands in the south- ences and similarities between subaerial and subma- ernmost island group of the Tabar±Feni chain, Papua rine venting and the transition from one to the other. New GuineaŽ. Fig. 1 . The islands of the Tabar±Feni

Fig. 1. Location of Ambitle Island, one of the Feni islands in eastern Papua New GuineaŽ modified after Licence et al., 1987 and Pichler and Dix, 1996. . Geothermal areas indicated in dark are primarily along the western side of the island. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 231 chain are Pliocene-to-Recent alkaline volcanoes that western coast and in the western part of the caldera occur in the forearc region of the former ensimatic near breaches in the caldera wallŽ. Fig. 1 . Hot mud New Hanover±New Ireland±Bougainville island arc pools, springs of chloride and acid sulfate waters, Ž.Fig. 1 . The volcanoes erupt an unusual suite of and a few low temperature fumaroles are present, silica-undersaturated, high-K calc-alkaline lavas gen- with temperature and pH values ranging from 67 to erated by adiabatic decompression melting during 1008C and 1.9 to 9.1, respectivelyŽ Wallace et al., the uplift of the New Ireland forearc basinŽ McInnes 1983. . and Cameron, 1994. . The Tabar±Feni volcanoes stand apart from the surrounding arc volcanoes of New Ireland and New Britain in that they have a 3. Field observations proclivity towards producing Au mineralization as epitomized by the world-class Ladolam Au deposit Submarine, hydrothermal venting occurs at Tutum Ž.40 million oz on Lihir island Ž Davies and Ballan- BayŽ.Ž. Figs. 1 and 2 in shallow 5±10 m water along tyne, 1987. . the inner shelf that contains a patchy distribution of Ambitle Island is part of a Quaternary stratovol- coral±algal reefs surrounded by medium to coarse- cano with a central eroded caldera built on poorly grained mixed carbonate±volcaniclastic sand and exposed Oligocene marine limestoneŽ Wallace et al., gravel. The site is located along a fault trace that 1983.Ž . Volcanic strata interbedded lava flows, lahar intersects several of the on-land geothermal areas deposits, tuffs, and scoriae. dip radially from the Ž.Fig. 1 . island, presumably extending beneath the shelf. Sev- Two types of venting are observed.Ž. 1 Focused eral geothermal areas are located primarily along the discharge of a clear, two-phase fluid occurs at dis-

Fig. 2. Location of hydrothermal vents within Tutum Bay on the west side of Ambitle Island. The bold dashed line indicates the extent of gaseous discharge. 232 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

Fig. 3. Underwater photographs of focused discharge in Tutum Bay.Ž. a Vent 1 and Ž. b Vent 3. The field of view is ;18 m at a water depth of 8 m forŽ. a . T. Pichler et al.rMarine Chemistry 64() 1999 229±252 233 crete ports, 10±15 cm in diameter, with minor phase charge where sediment covers are thickerŽ e.g., separationŽ. boiling at the sea floor. There is no O'Hara et al., 1994. . As a result, there is clear associated topographic edifice. This type of dis- separation between liquid and gaseous discharges in charge produces a roaring sound underwater and has Tutum Bay. an estimated flow rate as high as 300 to 400 lrmin Four locations of focused venting are present in Ž.i.e., fire hose . Shimmering and a whitish col- Tutum BayŽ. Fig. 2 . Vents 1 and 2 are single vents oration, indicative of hot water and water vapor, with hydrothermal discharge from only one orifice extends several meters above vent portalsŽ. Fig. 3a . Ž.Fig. 3a and vent 3 is an assemblage of three vents Fluid temperatures at portal entrances are between that are very close together in an area of approxi- 89 and 988C.Ž. 2 Dispersed or diffuse discharge mately 2=2 m2 Ž. Fig. 3b . Vent 4 is located some ) consists of streams of gas bubbles Ž.95% CO2 distance north of the other three ventsŽ. Fig. 2 and it emerging directly through the sandy to pebbly un- is closely associated with an area of massive gas consolidated sediment and through fractures in vol- emanationŽ. Fig. 4 . Vent 4 is slightly different from canic rocksŽ. Fig. 4 . This type of venting appears to the other vents; here shimmering hot water dis- be itinerant, with shifts in locations on the order of charges at low rates through several small orificesŽ 1 tens of centimeters, possibly related to tortuous mi- to 5 cm é. in an area of approximately 1=3m.2 gration through the surface sediment. Based on their geographic position, the vents have Gases that discharge through unconsolidated sedi- been divided into two groups, A and B; area A ment often carry an equivalent amount of water with consists of vents 1, 2 and 3 and area B of vent 4 them. The sediment cover in Tutum Bay, however, is Ž.Fig. 2 . very thinŽ. 10±30 cm which does not allow for an Hydrothermal precipitates accumulate on dead extensive gas seep induced interstitial water circula- coral substrate and rock fragments surrounding vent tion. This is in contrast to other areas of gas dis- portals and include euhedral aragonite crystals and

Fig. 4. Underwater photograph of the diffuse gaseous discharge around vent 4. The field of view is ;8 m at a water depth of 9 m. 234 T. Pichler et al.rMarine Chemistry 64() 1999 229±252 microcrystalline crusts of Fe-oxyhydroxide, arago- tal letter following the sample number, i.e., 1B. Two nite, and ferroan calciteŽ. Pichler and Dix, 1996 . of the three orifices at vent 3 were sampled, i.e., 3-1 Two of the on-shore hot springs, Waramung and and 3-2. A duplicate of 3-1 was taken. At vent 4, Kapkai were sampled in addition to the Tutum Bay three samples were taken from three different ori- vents for comparison. A detailed description of these fices within a single mound, i.e., 4, 4B, 4C. A sites can be found elsewhereŽ Wallace et al., 1983; sample of ambient seawaterŽ. SW and the samples Licence et al., 1987. . W-1 and W-2 from the on-shore hot springs were sampled by submerging a 1-l high density polyethylene bottle. These sample were treated exactly as the Tutum Bay vent waters. 4. Sampling and analytical procedures Seventy-five seawater samples were collected for chemical analyses throughout the bay at the bottom, Generally the chemical analysis of water samples in mid-water and at the surface at varying distances is more accurate under well-controlled laboratory from vent sites in order to reconstruct the dissipation conditions than in the field. Some parameters, how- of the hydrothermal plume and its influence on the ever, are unstable and, therefore, must be measured seawater composition in Tutum Bay. Samples were immediately after sampling. In this study we used pumped to the surface through a Nalgenew tube that sample containers, preservation techniques and hold- was lowered to the depth of sampling. In order to ing times recommended for water analysis by the US avoid contamination, several tube volumes were dis- Federal RegistryŽ. HACH, 1992 . Vent temperatures carded before taking the actual sample. Horizontal were measured with a maximum thermometer. Due distance from vent sites was determined with ropes to inherent difficulties when working underwater, and buoys. At the time of sampling no water move- however, these measurements should be regarded as mentŽ. current other than that caused by the hy- minimum temperatures. Vent samples were collected drothermal vents themselves was observed. These over a period of 10 days either by inserting a Teflonw samples were filtered to -0.45 mm, acidified with tube as far as possible into the vent orifice or cover- ultrapure HNO3 for later laboratory analyses and ing the vent orifice with a Teflonw funnel. Sixty-mil- stored in high-density polyethylene bottles. liliter medical syringes were connected to the Teflon Throughout the sampling period several samples tube or funnel via a three-way stopcock. Using this of deionized water were treated in the same way as set-up, generally 15 syringes were filled at a time the vent samples, i.e., filtered, acidified and filled and brought to the surface. While sampling vents 2, into high density polyethylene bottles. These samples 3 and 4 minor amounts of water vapor, indicative of were later analyzed together with the vent samples in phase separation, were observed during filling of the order to check for possible contamination. syringes. On-board, aliquots were taken and immedi- Major cations, trace elements, ultra trace elements ately analyzed for pH, conductivity and alkalinity. and rare earth elementsŽ. REE were determined at Total alkalinity Ž AT . was measured as equivalent the Geological Survey of Canada: Li, Rb, Sr, Sb, Cs, y CaCO33 and expressed as HCO , using a Hach digi- and Tl by direct inductively coupled plasma mass tal titrator and titrating to an endpoint of pH 4.5 spectrometryŽ. ICP-MS and Fe and Mn by chelation Ž.HACH, 1992 . The remaining solution was filtered ICP-MS following a tenfold dilution with deionized to -0.45 mm and subsequently separated into sub- water. REEs were analyzed by chelation ICP-MS sets for later analyses. Samples were stored in high without dilution. A more detailed description of the density polyethylene bottles and aliquots for cation direct and chelation ICP-MS methodology and asso- and trace element analyses were acidified with ultra- ciated figures of merit can be found elsewhereŽ Hall 2q 2qq q pure HNO3 . Additionally, 60 ml aliquots of selected et al., 1995, 1996. . Ca , Mg , Na and K were samples were acidified with double distilled HCl measured by flame atomic absorption spectrometry Ž.35% for determination of As3qrAs5q . Ž.FAA . Prior to analyses for Ca2q , Mg2qq , Na and The single orifice vents 1 and 2 were sampled Kq,aCsrLa buffer solution was added to a suitable twice and duplicate samples are indicated by a capi- aliquot of the sample to suppress ionization. Si and B T. Pichler et al.rMarine Chemistry 64() 1999 229±252 235 T mol m q 3 mol m mol m mol m mol m mol m ram mol m mol m mol m 17%. mol m s 16%; As s 7%; Sb s 5%; K - a a alkalinity. 4 y 43 pH Cl Br SO HCO B Si Na K Ca Mg Li Mn Fe Rb Sr Cs Sb Tl As As C mmolmmolmmolmmolmmolmmolmmolmmolmmolmmol 8 T Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž. Hot Spring Hot Spring Not determined. s Values represent total alkalinity because they include HS Table 1 Location, temperature, pH and major elementNo. composition of of Tutum Location unit Bay vent waters, ambient seawater and1 onshore geothermal springs in1B moles per kilog 2 Vent 1 Vent2B 13-1 Vent 2 Vent3-1B 2 90 Vent 90 33-2 Vent 3 6.2 6.24 80 34 Vent 80 35 34B 6.3 88 6.2 884C 0.1 Vent 66 4 6.3 0.1 Vent 81W-1 4 6.2 89 87 Vent 4 49 0.1 11 Waramung 0.2 6.2 11W-2 90 0.1 98 90 54 0.1 12 Kapkai 6.2 13 91 8.8 12 6.4 13SW 337 46 12 0.1 6.7 54 12 13 Seawater 0.8 92 83 13n.d. 0.4 0.7 0.1 11 0.1a 13 8.4 13 28 390 0.2 0.8 3.7 n.d. 66 3.7 10 8.0 11 13 0.7 548 0.3 0.8 12 3.4 3.3 54 13 52 9.2 14 0.6 3.3 0.7 74 3.6 13 74 2.5 83 4.1 0.8 2.4 29 3.5 0.8 88 11 60 0.7 2.7 5.1 3.0 2.8 3.5 5.2 3.5 64 2.5 2.9 5.1 2.5 3.2 5.6 2.5 420 5.7 2.7 59 0.4 63 5.8 2.6 3.7 146 5.5 36 5.6 145 84 6.6 0.0 2.8 7.2 5.5 8.4 3.1 490 135 3.7 132 8.1 3.2 0.2 127 450 4.9 40 29 4.1 7.9 135 5.0 7.9 28 5.3 0.04 7.7 135 3.5 9.1 8.6 28 4.1 26 619 4.0 0.1 3.9 6.6 27 136 10.1 8.7 134 30 77 0.04 3.8 0.03 76 125 3.7 50.8 6.5 29 6.4 732 3.7 79 0.4 0.4 3.9 79 6.1 0.4 20 18 80 17 0.03 32.4 81 3.9 0.1 0.4 17 0.4 0.1 0.03 24 80 0.4 0.4 4.1 4.1 0.4 0.02 0.1 0.1 0.02 0.3 3.9 39.1 73 3.4 0.1 73 0.4 10.0 0.1 0.02 10.7 0.02 74 1.2 30 10.9 0.4 1.3 0.02 0.1 10.3 0.4 10.7 0.02 91 9.4 0.4 4.1 10.4 0.1 0.10 9.2 0.02 0.1 9.5 9.4 0.004 0.1 0.3 20.7 0.02 9.2 9.8 0.004 0.02 9.8 20.7 0.02 0.001 0.08 10.8 10.1 9.9 n.d. 38.8 11.7 9.6 11.1 38.8 0.05 11.8 Repeat standard deviations in % are as follows: Cl, Br, SO , B, Si, Na, Ca, Fe, Mn, Rb, Sr, 236 T. Pichler et al.rMarine Chemistry 64() 1999 229±252 were analyzed by ICP-ES. The total arsenicŽ AsT . ical and Nuclear Sciences in Lower Hutt, New concentration was measured by AAS at a solution Zealand following Giggenbach and GoguelŽ. 1989 . pH-2 as As-hydride and As3q was determined by AAS as As3q -hydride, generated at pH 5Ž citrate buffer.Ž Driehaus and Jekel, 1992 . . Finally, As5q 5. Results was calculated from the difference between AsT and 3q yy 2y As . AnionŽ Cl , Br , SO4 . concentrations were 5.1. General statement measured at the University of Ottawa with a Dionex 100 High Pressure Liquid Chromatograph. Calibra- The samples collected from the submarine vents tion curves were constructed using a control blank in Tutum Bay are mixtures of hydrothermal fluid and and DIONEX standard solutions. The overall com- seawaterŽ. Table 1 . Entrainment of ambient cool pleteness and quality of each analyses was checked seawater may have happened in the shallow seafloor by performing a charge balance according to: Ýan- or during sampling. Assuming conservative behavior ionsŽ. meq sÝcations Ž. meq . The difference was during mixing, several chemical species can be used always less than 5%. to trace the mixing process if the concentration Gas samples were collected into 600-ml bottles difference between seawater and hydrothermal fluid through an inverted Teflonw funnel that was placed is sufficient. The best case scenario is if the element over the vent. In order to estimate the total gas flux under consideration is absent in one of the mixing from Tutum Bay, more than 100 flux measurements partners. The early laboratory study of Bishoff and were carried out by timing the replacement of a DicksonŽ. 1975 showed that high temperature Ž) known water volume from a graded cylinder. All 2008C. hydrothermal fluids have lost all their Mg flux measurements were normalized to 1 barŽ i.e., due to precipitation of Mg-rich minerals in the sub- atmospheric pressure at sea level. . The chemical surface. Although the fluids that was discharged in composition was analyzed at the Institute of Geolog- Tutum Bay are quite different from those studied by

Fig. 5.Ž. a Dissolved Mg vs. Cl in Tutum Bay vent waters and seawater. Ž. b Dissolved Si vs. Cl in Tutum Bay vent waters and seawater. Ž. c Dissolved Mg vs. Cl for individual vents.Ž. d Dissolved Si vs. Cl for individual vents. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 237 mol m mol m mol m mol m mol m mol m mol m mol m mol m 144 6.9 19 4 71 0.1 0.5 0.03 12 a a a a a a a a a a 4 Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž.Ž. Mg concentration for these samples is 0 by convention. Table 2 Calculated endmember composition for pH, majorSample and selected Cl minor elements in Tutum Bay vent waters1 in moles Br per1B kilogram 2 SO2B 7.4 6.83-1 0.05 B3-1B 6.3 12 0.08 113-2 10 0.12 10 8.14 Si 0.164B 0.06 10 8.7 0.09 0.84C 0.8 9 Na 10 0.07 8.8 10 10a 0.8 3.9 3.9 11 n.d. 0.09 K 9 0.8 0.10 0.8 3.9 34 29 0.21 3.8 9 3.8 Ca 0.8 9 3.8 27 2.1 2.0 9 28 0.8 28 3.8 Mg 29 0.8 2.0 4.9 4.9 0.8 1.9 3.8 Li 29 1.8 3.8 2.0 5.1 0 0 3.7 5.0 29 5.1 2.0 Mn 29 5.1 0 153 29 0 152 2.4 0 Fe 5.1 0 2.5 8.9 150 8.6 2.3 4.5 149 0 Rb 4.5 145 144 8.9 30 29 4.5 9.1 0 Sr 8.9 0 9.3 145 32 4 0 4 29 32 145 Sb 9.4 33 4 76 4 141 75 7.0 32 4 Cs 4 77 0.1 7.0 19 0.1 78 4 Tl 78 80 0.1 0.5 19 0.5 4 0.1 As 79 0.1 0.1 0.5 0.02 4 0.02 0.5 72 0.5 12 0.1 0.5 0.02 11 71 0.02 0.1 0.02 12 0.5 0.02 10 0.1 10 0.5 11 0.02 0.5 11 0.03 13 0.02 14 unit mmol mmol mmol mmol mmol mmol mmol mmol mmol 238 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

Bishoff and DicksonŽ. 1975 , we use Mg as an indica- sampling pressure, steam composition andror reser- tor for seawater entrainment. Given their chemical voir conditions have to be knownŽ Henley et al., composition and proximity to shore, our hydrother- 1984. . This information can generally be obtained mal fluids most likely derived from meteoric waters when sampling in explored on-land geothermal sys- which are already much lower in Mg when com- tems, but not when sampling underwater hot-springs. pared to seawater. In addition, most studies of deep The effect on chemical composition by phase separa- circulating meteoric derived geothermal waters show tion in Tutum Bay vent waters, however, seems to be very low concentrations of Mg Ž.Ž-1mgrl e.g., negligible. Only minor amounts of steam were ob- Nicholson, 1992. and as confirmed by our analyses served while sampling vents 2, 3 and 4 and no steam of samples W-1 and W-2Ž. Table 1 . The assumption of complete Mg absence in the vent fluids is most likely not correct, but given the enormous concentration difference between seawater and vent fluid, it is well within any analytical uncertainty. Another op- tion to correct for seawater entrainment is to use Si as a tracer of mixingŽ e.g., Sedwick and Stuben,È 1996. . Both alternatives have been explored in Fig. 5, where we have plotted Si and Mg 2q vs. Cly with their respective linear regression fits. The Si values, although consistently decreasing with an increase of seawater componentŽ i.e., Cly concentration. , deviate barely beyond their analytical errorŽ. Fig. 5b,d . Relative to ambient seawater, Mg 2q is depleted and Si is enriched and both show almost perfect correlation Žr 2 )0.99. with Cly Ž. Fig. 5a,b . This high correlation, however, is due to the outlier effect Ž.Swan and Sandilands, 1995 caused by the seawater data point. Excluding this outlierŽ. Fig. 5c,d , the correlation, although, still high, is somewhat less for Si Žr 2 s0.928. than for Mg 2q Žr 2 s0.987. . There- fore, we assume that for our samples, Mg 2q is a more reliable tracer of mixing. The poorer correlation for Si may be due to its higher susceptibility to changes in temperature and its greater inclination to post-sampling concentration changes due to adsorption onto container walls. The data presented in Table 2 are endmember compositions that have been calculated according to the following formula: y r XmswmX Ž.Mg Mg sw X s 1 hf y r Ž. 1 Ž.Mg mswMg

Fig. 6. Major anionsŽ. Cl, SO and HCO vs. Mg in Tutum Bay where Xhf is the calculated endmember concentra- 43 vent waters and seawater. r 2 is the correlation coefficient for the tion, Xmswis the measured concentration and X is the concentration in seawater. No correction of linear regression taking into account the whole data set including seawaterŽ. Mg;51 mmol and r 2 in brackets is the correlation chemical composition for phase separation was at- coefficient for the linear regression excluding seawater. See text tempted. In order to account for phase separation, the for more details. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 239 was observed at vent 1. The impression of heavy correlation for all three species Ž.)0.986 , but ex- 2y boiling seems to be an effect caused by simultaneous cluding the seawater outlier the correlation for SO4 2 s y discharge of non-condensable gas and water. is much less Žr 0.77. and for HCO3 there is no The samples, W-1 and W-2, from the on-land more correlation Žr 2 s0.165. . Endmember composi- 2y thermal areas were taken from the same springs as tions are listed in Table 2. For all samples, SO4 samples WA1 and KP in Licence et al.Ž. 1987 . Their values are within their analytical uncertainty. Cly chemical composition is in excellent agreement, indi- and Bry, however, show a significant spread except cating stable conditions for these springs during more for vent 1. than 12 years. The alkaline metals Liq,Rbq and Csq are significantly enriched over ambient seawater and nega- 5.2. Chemical composition of Tutum Bay Õent waters tively correlated with Mg2q Ž. Table 1 and Fig. 7 . Naq and Kq are significantly depleted relative to The vent waters from Tutum Bay are depleted in seawater and positively correlated with Mg2q Ž Table y 2y y q Br ,SO4 and Cl relative to ambient seawater 1 and Fig. 7. . The Na depletion is similar to the 2y y y and SO4 and Cl are positively correlated with depletion observed for Cl . Again, excluding the Mg2q Ž. Fig. 6 . There are no distinct trends for seawater outlier, only Naq maintains the high degree individual vents; all samples either follow the same of correlationŽ. Fig. 7 . Endmember concentrations vent water±seawater mixing trend or plot in one are listed in Table 2. Except for the Naq value in y cluster. HCO3 is enriched relative to ambient seawa- sample 1, which is slightly higher than the rest, all ter and negatively correlated with Mg 2q. The linear Naq and Liq analyses are within their analytical regression analysis shows a very high degree of error. The Kq values fall into two distinct groups for

Fig. 7. Li, Na, K and Rb vs. Mg in Tutum Bay vent waters and seawater. Explanation of correlation coefficients as in Fig. 6. See text for more details. 240 T. Pichler et al.rMarine Chemistry 64() 1999 229±252 area A and area B, with higher concentrations in B. The Rbq and Csq concentrations are within their analytical error, the values for area B, however, are consistently higher than those for area A. All Tutum Bay vent waters have Ca2q concentrations that are significantly lower than ambient seawa- terŽ. Table 1 and Fig. 8 . Values for vents in area A and area B plot in two distinct groups that are both positively correlated with Mg2q Ž. Fig. 8 . Linear regression analysis shows a high degree of correlation Žr 2 )0.99. for both groups, which is main- tained Žr 2 )0.93. after exclusion of the seawater outlierŽ. Fig. 8 . The Sr2q concentration in vent waters is only slightly lower than in ambient seawater and there is no apparent correlation between Sr 2q and Mg 2q. Endmember concentrations for both areas are within their respective analytical uncertainty, but area A vents have slightly higher valuesŽ. Table 2 . The transition metals Fe and Mn are highly enriched over ambient seawaterŽ. Table 1 and Fig. 9 .

Fig. 9. Fe and Mn vs. Mg in Tutum Bay vent waters and seawater. Explanation of correlation coefficients as in Fig. 6. See text for more details.

Values for vents in area A are higher than those from area B, but both are positively correlated with Mg 2q Ž.Fig. 9 . After exclusion of the seawater value, correlation for area A drops substantially while for area B it remains high. The values for the individual areas, however, do not deviate beyond their analytical error. Endmember concentrations for both areas are within their respective analytical uncertainty, but area A vents have significantly higher valuesŽ Table 2.. Relative to ambient seawater, Sb, As and Tl are significantly enriched in all ventsŽ. Table 1 . Arsenic concentrations are up to 275 times above seawater and it is dominantly present in its trivalent state. Endmember concentrations for Sb, As and Tl fall 2q Fig. 8. Ca and Sr vs. Mg in Tutum Bay vent waters and seawater. into groups, as observed for Ca , Fe and Mn, Explanation of correlation coefficients as in Fig. 6. See text for except that here values for area B are higher than more details. those for area A. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 241

Fig. 10. North American Shale CompositeŽ.Ž NASC Haskin et al., 1968 . normalized rare earth element plots for vents 1, 2, 3 and 4. REE concentrations for vent waters are multiplied by 106. The bold pattern without labels represents the average REE composition of volcanic rocks from Ambitle Island; data are from Wallace et al.Ž. 1983 . Raw data are available from: [email protected].

REE concentrations in Tutum Bay vent waters are 5.3. Gas compositions generally one order of magnitude higher than those reported for average seawaterŽ. Fleet, 1984 . North The chemical compositions of two samples col- American Shale CompositeŽ. NASC normalized raw lected from submarine gas vents in Tutum Bay are and endmember concentrations are plotted in Fig. 10. listed in Table 3. In all the gases, CO2 is the The pattern geometry for raw and endmember values is effectively the same, except that the endmember Table 3 patterns plot slightly higher. The samples from vents Chemical compositions of dry gases from Tutum Bay in 1 and 3Ž. 1, 3-1 and 3-2 and their duplicates Ž 1B and r r mmol mol; T Ž.8C is the CH 42CO equilibration temperature 3-1B. plot very closely together, showing the same Sample CO22 H S He H 2 O 222 N N CH 4T pattern geometry: an initial drop from La to Ce Ž.c Ž8C . followed by rise from the Ce minimum to an Eu TB-1 927 -0.3 0.021 -0.01 7.3 44 29 20 257 maximum and a slight decrease towards an interme- TB-2 966 -0.3 0.012 -0.01 5.4 22 11 6 291 diate Lu. The pattern geometry for samples from TB-3 979 -0.3 -0.01 -0.01 6.1 36 24 14 268 vent 4Ž. 4, 4B and 4C is quite different; REE concen- TB-4 949 -0.3 -0.01 -0.01 4.3 35 23 14 267 - - trations initially drop from La to a Ce minimum TB-5 926 0.3 0.011 0.01 5.8 47 35 20 257 which is followed by a rise to a Dy maximum and a s y Nc222Ž. N 2O . slight decrease towards Lu. n.d.sNot determined. 242 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

1997. . All vent fluids were found to be reducing and have similar Eh values; vent 1sy0.175 V, vent 2sy0.171 V, vent 3sy0.173 V and vent 4s y0.175 V. These values are similar to those obtained from As3qrAs5q calculationsŽ. Fig. 11 .

Fig. 11. Eh±pH diagram for the system As±O±H at 258CŽ dashed lines.Ž. and 1008C solid lines at a pressure of 1.013 bars. Activi- yy y3 y4 ties of AsŽ. OH43 and HCO are assumed to be 10 and 10 , respectively. Thermodynamic data are from BrookinsŽ. 1988 and references therein. dominant component and its approximate flux is 100 dm3rs. All the submarine gases contain significant amounts of O2 , either representing an inherent component of these gases, dissolved air stripped from seawater by the rising gas bubbles, or being introduced during sampling. In the absence of any information on the origin of O22 and N , their concentrations can be corrected by subtracting 2O2 from measured values of N22 to obtain NŽ.Ž c corrected . Ž.W. Giggenbach, personal communication . The fac- tor of 2 is the approximate value of the N22 ±O ratio in air-saturated water.

5.4. Redox potential

In addition to the sampling for chemical analyses, we also sampled a limited amount of water from each vent during a single dive for simultaneous on-board Eh and pH measurements in order to assess if there is a significant redox difference between the individual vent fluids. The pH measurement was Fig. 12. Model of hydrothermal component inŽ. A surface and Ž. B used to monitor seawater contamination and the bottom water in Tutum Bay. The contour lines were calculated with Eq.Ž.Ž 2 see text . and represent the concentration of hy- quality of the Eh measurement was checked against drothermal component in %. The darker colors indicate higher the Quinhydrone solutionŽ e.g., Clark and Fritz, hydrothermal component. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 243

6. Hydrothermal fluid dissipation and its effect on from 0.05 and 0.30%Ž. Fig. 12 . The small area just Tutum Bay seawater below vent 2 shows the highest concentrations, but this is an area where hydrothermal fluids seep through The contour maps of the hydrothermal component the sandy bottom at very low discharge rates. The Ž concentration in Tutum Bay seawater Figs. 12 and low concentrations of hydrothermal fluid in the bot- . 13 were constructed using silica as a tracer and tom water are not surprising given the physico- assuming conservative behavior and linear mixing. chemical conditions of fluid discharge. The vent Relative to seawater, silica showed the highest con- fluids have a salinity of -3½, which combined Ž. centration in the hydrothermal fluids Table 1 . The with their discharge temperature of ;908C, makes hydrothermal component in Tutum Bay seawater them buoyant relative to seawater. As a result the was calculated using a simple mass balance equa- hydrothermal fluids quickly rise to the surfaceŽ Fig. tion: 3. and the surface water shows an up to 10 times s q CTBWxC SWyC HF Ž.2 higher hydrothermal component concentration than where CTBW is the silica concentration in Tutum Bay the bottom waterŽ. Fig. 12 . Only little mixing occurs seawater, CSW is the silica concentration in reference with ambient seawater in the immediate proximity of seawater and CHF is the silica concentration in the vent orifices. The overall distribution of hydrother- hydrothermal fluid. mal component is most likely controlled by two The bottom water in Tutum Bay shows varying processes:Ž. 1 diffuse low rate discharge of hy- degrees of hydrothermal component and values range drothermal fluid andŽ. 2 the development of hy-

Fig. 13. Cross-section model of hydrothermal component in Tutum Bay. The contour lines were calculated with Eq.Ž.Ž 2 see text . and represent the concentration of hydrothermal component. The darker colors indicate higher hydrothermal component. Suggested hydrothermal convection cells are indicated by dashed lines. 244 T. Pichler et al.rMarine Chemistry 64() 1999 229±252 drothermal convection cells within the bay due to cipitate in Tutum Bay can contain more than 50 000 temperature differencesŽ. Fig. 13 . ppm AsŽ. Pichler and Dix, 1996; Pichler et al., 1996 It is important, however, to realize that the fluid and are by an order of magnitude the highest As dissipation model presented in Figs. 12 and 13 is values found in a natural system. only a snapshot in space and time of a very dynamic system. At times discharge rates may be much higher or lower resulting in a different concentration of 7. Discussion hydrothermal component. Nearshore currents, although none were noticed while sampling, may be The discharge composition of thermal springs is present at other times, modifying the plume disper- controlled by two sets of processes:Ž. 1 deep reser- sion pattern. voir conditionsŽ deep reservoirsreaction zone im- Of potentially poisonous trace metals that are mediately above the heat source.Ž. , and 2 secondary released into Tutum Bay, only arsenic has a signifi- processes during ascent. In the deep reservoir, host cantly higher concentration than seawater. In particu- rock composition, temperature, direct magmatic con- lar, AsIII-compounds are known to have a genotoxic tributions and residence time are the controlling fac- potentialŽ. Gebel et al., 1997 . The hydrothermal tors. During ascent, a drop in pressure and tempera- fluids contain extremely high arsenic concentrations ture can initiate phase separation and mineral pre- of more than 11 mmol and the combined discharge cipitation, causing a dramatic change in fluid compo- of all vents is estimated to be more than 1500 g of sition. Mixing with other hydrothermal fluids andror arsenic per day into an area of approximately 50 m groundwater is possible at any depth. In near-shore by 100 m with an average depth of 6 m. These and submarine environments, mixing with seawater values are the highest arsenic concentration reported cannot be ruled out. The chemical composition of a from any marine setting, including black smoker hydrothermal fluid, sampled at the surface, generally fluids from mid-ocean ridges. Despite the amount of contains an imprint of its subsurface history. Chemi- arsenic released into the bay, corals, clams and fish cally inert constituentsŽ. tracers provide information do not show any direct response to the elevated about their source, whereas chemically reactive values. Fish have been observed to hover over vent speciesŽ. geoindicators record physicochemical orifices, bathing in the hydrothermal fluid. The di- changesŽ e.g., Ellis and Mahon, 1977; Giggenbach, versity and health of the coral reef is indiscernible 1991; Nicholson, 1992. . Examples of widely used from reefs that are not exposed to hydrothermal solute tracers and geoindicators are Cl, B, Li, Rb, Cs discharge. Pichler et al.Ž. for submission analyzed and Na, K, Mg, Ca, SiO2 , respectively. The bound- the skeletons of scleractinian corals and the shells of ary between the two groups, however, is not rigid Tridacna gigas clams but they did not find elevated and depending on the physicochemical conditions of concentrations of arsenic or other trace metals when the hydrothermal system, tracers may participate in compared to specimens collected from outside Tu- chemical reactions and geoindicators can behave tum Bay. inertly. Two mechanisms efficiently control and buffer In Tutum Bay waters, the ratios of all alkalis, the arsenic concentration:Ž. 1 dilution by seawater except for Li, are almost identical to those in sam- andŽ. 2 incorporation in and adsorption on Fe- ples W-1 and W-2. This would argue that at least oxyhydroxides that precipitate when the hydrother- some of the relative Li increase has to be attributed mal fluids mix with ambient seawater. The hy- to shallow leaching, a proposition contrary to the drothermal fluids have a pH of ;6 and are slightly belief that Li is the alkali probably least affected by reducing. Under these conditions Mn stays in its secondary processesŽ. e.g., Giggenbach, 1991 . These reduced form and, therefore, in solution, whereas As elements are added to the hydrothermal fluid under is rapidly oxidized and scavenged by Fe-oxyhydro- deep reservoir conditions due to water±rock interac- xides. Oxidation of As3q to its pentavalent state tion and they are generally not affected by shallow significantly decreases its reactivity and genotoxicity processes. All samples plot away from the composi- Ž.Gebel et al., 1997 . The Fe-oxyhydroxides that pre- tional field of crustal rocks, indicating that secondary T. Pichler et al.rMarine Chemistry 64() 1999 229±252 245 processes must have affected their composition. The trivalent stateŽ. Goldberg et al., 1963 . In addition, Ce crystallization of illite at high temperature has re- forms colloidal ceric hydroxideŽ Carpenter and Grant, moved some of the initial Rb from fluids W-1 and 1967. which is readily scavenged by Fe-oxyhydro- W-2Ž. Fig. 14 . The LirCs ratios are dependent on xides. The slight difference in pattern geometry be- lithology and the intermediate position between basalt tween area A and B could be caused as well by and rhyolite of W-1 and W-2 is in good agreement varying degrees of subsurface mixing. This, how- with the andesitic composition of the Ambitle vol- ever, does not account for the difference in vent 2 canoŽ. Wallace et al., 1983 . The relatively higher Li samples. content of the Tutum Bay fluids may be a result of Li-uptake due to shallow processes or mixing with meteoric waterŽ. Fig. 14 . The loss of Cs to zeolites is 7.1. Origin of the hydrothermal fluid and probable most likely not of any importance considering that mixing trends the RbrCs ratios in Tutum Bay and on-shore sam- plesŽ. W-1 and W-2 more or less remain constant. A fluid in a hydrothermal system may be derived The REE patternsŽ. Fig. 10 are as expected for from any or any combination of the following hydrothermal fluids with a pH of ;6Ž Michard, sources: meteoric water, seawater, connate water, 1989. . The pattern geometry is similar to that of the magmatic water and juvenile water. Mixing of wa- host rocksŽ. Fig. 10 but Ce, Pr, Nd and Sm are ters from different sources affects many aspects of relatively depleted which reflects their greater immo- the geochemistry of a hydrothermal system, such as bility during hydrothermal alterationŽ Thompson, chemical composition, isotopic composition, temper- 1991. . Some of the Ce may also have been lost due ature profile and gas content. Deep sea hydrothermal to adsorption onto or co-precipitation with Fe- systems active along mid-oceanic ridges, in back-arc oxyhydroxides prior to sampling. Ce3q is easily basins and on the flanks of seamounts likely derive oxidized to Ce4q and may directly precipitate as all their fluid from seawater, although a minor mag-

CeO2 while the other REEs still remain in their matic contribution cannot be completely ruled out Ž.De Ronde, 1995 . In contrast, on-land hydrothermal areas derive most of their fluid from meteoric sources, but a significant magmatic contribution is likely Ž.e.g., Giggenbach, 1992 . Determination of the fluid origin is an important step in order to determine subsurface processes and reservoir conditions. While the source determination is relatively straightforward in deep marine and inland settings, the subject be- comes more complex in coastal regionsŽ off-shore and on-shore. where seawater incursion can significantly alter the fluid composition in a hydrothermal system. For example, seawater mixing is present in the SavusavaŽ. Fiji , Puna Ž Hawaii . and Reykjanes Ž.Iceland hydrothermal systems Ž Nicholson, 1992 . . Similarly, meteoric water may be the source for submarine hydrothermal systems in coastal areas Fig. 14. Relative Li, Rb and Cs contents of Tutum Bay, Wara- where a steep topography and ample supply of rain- mung and Kapkai hot-springs. The two arrowsŽ. a indicate fluid water can force the Ghyben±HerzbergŽ freshwater± evolution due to Rb-uptake during precipitation of illite at t) seawater.Ž boundary substantially offshore Nahm, 3008C and the two arrowsŽ. b indicate fluid evolution due to 1966; Chuck, 1967. . Tropical and subtropical island- Cs-uptake during precipitation of zeolites at t-3008C. The dashed line indicates the relative Li uptake by the hydrothermal fluid due arc volcanoes are good examples for these condi- to shallow processesŽ. see text and possible entrainment of mete- tions. Subsurface sealing of a hydrothermal system, oric water. as argued for White Island, New ZealandŽ Giggen- 246 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

shifting the samples away from their original field

along the Cl±ÝCO3 mixing lineŽ. Fig. 15 . Two other elements, Cl and B, are widely used to trace the origin and subsurface flow direction of hydrothermal fluidsŽ. e.g., Nicholson, 1992 . In the case of the Tutum Bay samples, however, Cl cannot be applied successfully because the concentration gradient between seawater and hydrothermal fluid is too large and, therefore, the error introduced due to seawater contamination is substantial. Boron, on the other hand, seems to be an excellent tracer for subsurface conditions at Ambitle Island. The subsurface flow direction from W-2 over W-1 to Tutum Bay is indicated by decreasing B and alkali contents Fig. 15. Relative Cl, SO4 and ÝCO3 contents of Tutum Bay, Ž.Fig. 16 which record dilution of the deep reservoir Waramung and Kapkai hot-springs. Empty symbols indicate SO4 fluid by either groundwater or bicarbonate water. corrected positionŽ. see text . ÝCO has been corrected for seawa- 3 This is in accord with their geographic and topo- ter contamination and includes H23 CO and HCO 3 . bach et al., 1989. , can be another mechanism to prevent seawater from entering the nearshore environment, thus allowing for a meteoric water domi- nated submarine hydrothermal system. The hydrothermal fluids in Tutum Bay, although discharging in a submarine environment, are most likely derived from a meteoric source. Total dissolved solidsŽ. TDS are rather low Ž-3000 mgrl . and molecular ratios indicative of seawater dilution, r r r r such as Cl Mg, Cl Br, Cl SO4 and Ca Mg Ž.Nicholson, 1992 , are quite different from those caused by seawater mixing. Their origin and physicochemical history can be explored in a Cl,

SO43 and HCO ternary diagramŽ e.g., Chang, 1984; Giggenbach, 1991; Nicholson, 1992; Giggenbach, 1997. . Hydrothermal waters are generally divided into neutral chloride, acid sulfate and bicarbonate waters, but mixtures between the individual groups are commonŽ. Fig. 15 . Tutum Bay vents plot close to the ÝCO3 corner along the upper boundary for steam heated waters. Waters that plot in this area generally form by underground absorption of vapors separated from a deeper neutral chloride water into cooler ground water, where based on the gas content of the vapor and redox conditions either acid sulfate or bicarbonate waters are formedŽ e.g., Ellis and Ma- Fig. 16.Ž. a Li, Rb and Cs vs. B in Tutum Bay, Waramung Ž W-1 . hon, 1977; Henley and Ellis, 1983; Hedenquist, 1990; Ž. 2 ) 2y and Kapkai W-2 hot-springs. r 0.99 for all linear regression Giggenbach, 1997. . The relatively high SO4 con- lines.Ž. b Na and K vs. B in Tutum Bay, Waramung Ž W-1 . and 2 tent can be attributed to oxidation of H2 S, thus KapkaiŽ. W-2 hot-springs. r )0.91 for all linear regression lines. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 247 graphic location. The Tutum Bay vents are topo- and W-2 that also have elevated concentrations of K, graphically lowest and furthest away from the center Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, of the hydrothermal system, whereas the sample and Sr concentrationsŽ. Tables 1 and 2 . Thus the from the Kapkai thermal areaŽ. W-2 is relatively slight chemical difference in area A and B vents may higherŽ. approximately 30 m above sea level and be attributed to varying subsurface addition of a closer. The linear regression lines for Rb and Cs and hydrothermal fluid similar in composition to W-1 for Na and K converge almost exactly at the point and W-2. where they intercept the x-axisŽ. Fig. 16 . Assuming that the regression lines represent mixing between 7.2. Geothermometry deep neutral chloride and shallow bicarbonate water, the intercept of the x-or y-axis represents one Solute geothermometers, such as listed in Table 4, endmember. This endmember, if based on Rb and provide powerful tools to estimate subsurface condi- Cs, would have a boron excess, of approximately tions. Their successful application has been exten- 0.27 mmol, whereas the one based on Li would have sively discussed in the geothermal literature and a Li excess of approximately 0.04 mmolŽ. Fig. 16 . relies on five basic assumptions:Ž. 1 exclusively The former is a much more likely alternative for a temperature dependent mineral±fluid reaction;Ž. 2 process that forms bicarbonate water. Boron occurs abundance of the mineral andror solute;Ž. 3 chemi- largely as boric acidŽ. H33 BO , its volatile form, and cal equilibrium;Ž. 4 no re-equilibration; and Ž. 5 no can be easily carried in the vapor phase even at mixing or dilutionŽ. Nicholson, 1992 . The no mixing lower temperaturesŽ. Tonani, 1970 . Using a simple or dilution assumption, however, can be circum- mass balance equation: vented if their extent andror influence on solute ratiosŽ.Ž. e.g., NarK is known see above . C sxC qyC Ž.3 TBVF DR BW The calculated temperatures in Table 4 differ where CTBVF is the average boron concentration in quite drastically. This is not surprising considering Tutum Bay vent fluids, CDR is the estimated boron that most problems in the use of geothermometers concentration in the deep reservoir fluid and CBW is arise from application to unsuitable samples. Differ- the calculated boron concentration in the bicarbonate ent geothermometers, however, record different equi- water. Based on the 0.27 mmol boron excess, bicar- libria and disagreement does not immediately elimi- bonate and chloride water fractions were calculated nate the use of one or the other. Careful application to be 0.89 and 0.11, respectively. Based on the and evaluation of calculated temperatures may pro- Na±K boron excess, bicarbonate and chloride water vide important clues to the overall hydrology of the fractions would be approximately 0.93 and 0.07, hydrothermal system. respectively. The deep reservoir fraction, present in The temperatures calculated with the silica Tutum Bay samples, is in good agreement with that geothermometersŽ. Fournier, 1977 are too low to found in waters of similar origin in other hydrother- represent a reliable estimate of the reservoir condi- mal systems. HedenquistŽ. 1990 , for example, esti- tion. This is to be expected in particular for the mated that at the Broadlands±Ohaaki geothermal samples W-1 and W-2, since these springs are fed system, most of the marginal upflow mixes with from reservoirs with temperatures in excess of ; steam-heated waters and only a small amount of 2308C and have, therefore, lost some of their initial deep fluid Ž.;10% rises to the surface. silica due to precipitation of quartz, chalcedony, or The division into area A and B that was initially amorphous silica during ascentŽ Rimstidt and Barnes, based solely on geographic location has been rein- 1980; Fournier, 1985. . The application of the silica forced by chemical composition. Although the chem- thermometer to Tutum Bay samplesŽ. V-1 to V-4 is ical composition of all Tutum Bay vents is quite problematic because they are a mixture of at least alike, compared to area A, area B vents have higher two fluids and the thermometer is based on absolute K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, silica concentration. and Sr concentrationsŽ. Table 2 . This seems to be The quality of reservoir temperatures calculated directly related to the composition of samples W-1 with the thermometers listed in Table 4 depends 248 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

Table 4 Calculated reservoir temperatures for Kapkai, Waramung and Tutum Bay vent fluids Thermometer T Ž.8C Reference Vent 1 Vent 2 Vent 3 Vent 4 W-1 W-2 Amorphous silica 9887y3 6 FournierŽ. 1977 Chalcedony 100 99 99 98 86 97 FournierŽ. 1977 Quartz 128 127 127 126 114 125 FournierŽ. 1977 Quartz steam loss 125 124 124 123 113 122 FournierŽ. 1977 KrMg n.d. n.d. n.d. n.d. 306 310 GiggenbachŽ. 1988 LirMg n.d. n.d. n.d. n.d. 203 209 Kharaka and MarinerŽ. 1989 NarLi 166 172 169 168 111 112 Kharaka and LicoŽ. 1982 NarK 246 253 252 273 276 271 FournierŽ. 1979 NarK 240 245 244 262 264 260 GiggenbachŽ. 1988 Na±K±Ca 191 192 192 206 312 320 Fournier and TruesdellŽ. 1973

strongly on the equilibrium between fluid and host identical temperatures to W-1 and W-2Ž. Table 4 . rock lithology. Attainment of deep reservoir equilib- This would confirm that the discharge from vent 4 rium has been verified for samples W-1 and W-2 has a relatively greater proportion of the deep reser- based on their relative Na, K and Mg concentrations voir fluidŽ. see above . The notable disagreement Ž.e.g., Giggenbach, 1988 . Unfortunately, this method between Tutum Bay and on-land Na±K±Ca tempera- cannot be directly applied to the Tutum Bay samples tures is a result of the high pCO2 and, therefore, because they have zero-Mg by definition, but equi- high Ca concentration in Tutum Bay fluids. librium conditions would apply for Mg concentra- The NarLi temperatures obtained for Tutum Bay tions between approximately 0.00083 and 0.083 samples may be representative of shallow reservoir mmol. Temperatures obtained from the LirMg and conditions that post date mixing between deep reser- NarLi thermometers are substantially lower than voir and bicarbonate fluid. When compared to Mg, those from the other ionic solute thermometersŽ Ta- Na, K, Rb and Cs, Li seems to be the element most ble 4. . Exchange reactions with Mg seem to be affected by shallow reservoir processes, but the cal- expeditious at lower temperatures and Mg-based culated NarLi temperatures appear to be in good thermometers may, therefore, record the last equilib- agreement with theoretical considerations. Let us rium prior to dischargeŽ. e.g., Nicholson, 1992 . The consider one more time the process that leads to the r low Li Mg temperature, however, is in contrast with formation of a CO2 or bicarbonate water. Isenthalpic the KrMg thermometer. Considering that the rela- expansion of steam, separated from a deeper liquid tive Mg, Na and K concentrations of W-1 and W-2 phase to close to atmospheric pressure, is accompa- indicate deep reservoir equilibrium, it is rather the Li nied by a drop in temperature to about 1608CŽ Gig- concentration that must have been affected during genbach, 1997. . Bicarbonate waters formed due to ascent or at a shallower level. The temperatures interaction between groundwater and this 1608C obtained from KrMg, NarK and Na±K±Ca are in steam, generally have temperatures in the vicinity of good agreement and indicate a reservoir temperature 1508CŽ e.g., Cioni and D'Amore, 1984; Hedenquist, of approximately 3008C. NarK temperatures are 1990. . Mixing of this 1508C bicarbonate water with slightly lower, which is to be expected given the a 3008C deep reservoir fluid at a ratio of ;9:1, as slightly peripheral location of the thermal areas. The obtained from the boron mixing modelŽ. see above , NarK ratio of a hydrothermal fluid generally in- leads to a final shallow reservoir temperature of creases during lateral flowŽ. Ellis and Mahon, 1977 . 1658C which is in perfect agreement with the calcu- The Tutum Bay samples, although mixed with water lated NarLi temperaturesŽ. Table 4 . of a different origin, still carry a significant NarK Additional insight into potential processes control- imprint of the deep reservoir. Temperatures for vents ling the fluid composition and subsurface processes 1, 2 and 3 are slightly lower, while vent 4 has may be provided by the chemical composition of the T. Pichler et al.rMarine Chemistry 64() 1999 229±252 249

r gases, as listed in Table 3. An examination of the CH 42CO ratios in the samplesŽ Giggenbach and gases is carried out in terms of relative concentra- Matsuo, 1991. , the CH42 ±CO equilibrium tempera- tions of N22Ž. c , He and CO contents. On the basis of tures are between 257 and 2918CŽ. Table 3 . Depth of gas discharges from a wide range of tectonic envi- separation, therefore, should be at approximately ronments, the compositions of volcanic and geother- 1000 m, provided that the deep reservoir fluid is on mal vapors were found to represent essentially mix- the boiling curve. These CH42 ±CO equilibrium tem- tures of two endmember components: a mantle de- peratures are in good agreement with reservoir fluid r r r rived component, very low in N22 with CO He estimates based on the Mg Li, Na K and Na±K±Ca ratios of 20 000 to 40 000, and an arc-type compo- solute geothermometersŽ. Table 4 . r ) 6 nent with much higher CO2 He ratios of 10 and r " a quite uniform CO22N ratios of 100 60Ž Gig- genbach, 1995. . The data points for the gases plot at the upper 8. Summary and conclusions boundary of the compositional area outlined for volcanic and geothermal vaporsŽ. Fig. 17 and across the middle of the triangle, suggesting a major input of The shallow-water hydrothermal fluids from Tu- mantle-derived gases. The increased relative N2 con- tum Bay seem to be dominantly of meteoric origin, tents may reflect increased stripping of N2 during the although they discharge in a marine environment. rise of the gases through air-saturated seawater or This confirms the work of Pichler and DixŽ. 1996 groundwater. Increased interaction of the gases with who, based on fluid inclusions and d18 Oinhy- water can be expected to lead to removal of soluble drothermal aragonite, also proposed their meteoric components, an assumption supported by the absence origin. Their final chemical composition is the out- of H2 S, the most soluble componentŽ. Table 3 which come of a two- or possibly three-step processŽ Fig. r leads to the higher SO4 Cl in Tutum Bay vent fluids 18.Ž. : 1 Phase separation in the deep reservoir be- when compared to samples W-1 and W-2. neath Ambitle Island produces a high temperature Assuming equilibration of the gases dissolved in a vapor that rises upward and subsequently reacts with liquid phase and preservation of equilibrium cooler ground water to form a low pH, CO2 -rich water of approximately 150±1608C. This fluid is highly reactive and pH±Eh sensitive elements such as Fe, Mn, Ca and Sr are leached from the host rock in the shallow reservoir.Ž. 2 Due to the steep topog-

raphy, this CO2 -rich fluid moves laterally towards the margin of the hydrothermal system where it mixes with the marginal upflow of the deep reservoir fluid. This produces a dilute chloride water of approximately 1658C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B±RbrCs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. The definite chemical composition of Tutum Bay vent fluids may be slightly different from the values reported in Table 2, because of phase separation during ascent from the shallow reservoir and Mg- Fig. 17. Relative CO22 , N and He contents of non-condensable correction for seawater contamination. Thermometry gases collected from submarine discharges in Tutum Bay. The shaded area indicates the relative position for mantle and arc type and mixing calculations, however, are not affected, because they are based on element ratios rather than gases. The arrow indicates N2 uptake due to sample contamination with air. on absolute concentrations. 250 T. Pichler et al.rMarine Chemistry 64() 1999 229±252

Fig. 18. Hypothetical cross section through Tutum Bay and the west side of Ambitle Island. The circled numbersŽ. 1, 2 and 3 indicate the approximate location of subsurface reaction zones:Ž. 1 Formation of a steam heated bicarbonate water, Ž. 2 gravity driven lateral flow and mixing with marginal upflow from deep reservoir neutral chloride water, andŽ. 3 possible mixing with seawater near the Ghyben±Herzberg boundary.

Compared to seawater, the hydrothermal fluids grateful to Donna Switzer and Yannick Beaudoin for are depleted in Cl, Br, SO4 , Na, K, Ca, Mg, and Sr help underwater, to Ajaz Karim and Johannes Barth and enriched in HCO3 , B, Si, Li, Mn, Fe, Rb, Cs, for sharing their thoughts on water sampling and to Sb, Tl, and As. Although some elements are signifi- Werner Giggenbach for the gas analyses. We wish to cantly enriched, they do not have a clear impact on thank Drs. David Butterfield and Paul Dando for ambient seawater composition because their concen- their constructive reviews. An earlier version of this tration is buffered by mixing and uptake into sec- paper benefited from comments by Ian Clark. ondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids. References Bishoff, J.L., Dickson, F.W., 1975. Seawater basalt interaction at 2008C and 500 bars: implications for the origin of seafloor Acknowledgements heavy metal deposits and the regulations of seawater chemistry. Earth and Planetary Science Letters 25, 385±397. Most of this research was funded by an American Brookins, D.G., 1988. Eh ±pH Diagrams for Geochemistry. Ž Springer-Verlag, New York, 178 pp. Chemical Society, Petroleum Research Grant No. Butterfield, D.A., Massoth, G.J., McDuff, R.E., Lupton, J.E., 31585-AC8. and Natural Sciences and Engineering Lilley, M.D., 1990. Geochemistry of hydrothermal fluids from Research Council of Canada operating grant to Jan Axial Seamount hydrothermal emissions study vent field, Juan Veizer. Thomas Pichler acknowledges the support of de Fuca Ridge: subseafloor boiling and subsequent fluid±rock two Geological Society of America Student Research interaction. Journal of Geophysical Research 95Ž. B8 , 12895± Ž.12921. Grants Nos. 5681-95 and 5904-96 . We all would Carpenter, J.H., Grant, V.E., 1967. Concentration and state of like to thank John Loop, Judy Vaive and Jean Pelchat cerium in coastal waters. Journal of Marine Research 25, for help with the water analyses. Thomas Pichler is 228±238. T. Pichler et al.rMarine Chemistry 64() 1999 229±252 251

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