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Chemical characteristics of newly discovered black smoker fluids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge
Earth and Planetary Science Letters 193 (2001) 371^379 www.elsevier.com/locate/epsl
Chemical characteristics of newly discovered black smoker £uids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge Toshitaka Gamo a; *, Hitoshi Chiba b , Toshiro Yamanaka c , Takamoto Okudaira d , Jun Hashimoto e , Shinji Tsuchida e , Jun-ichiro Ishibashi f , Satoshi Kataoka f , Urumu Tsunogai a , Kei Okamura g , Yuji Sano h;1 , Ryuichi Shinjo i a
d
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Sapporo 060-0810, Japan b Institute for Study of the Earth's Interior, Okayama University, 827 Yamada, Misasa, Tohaku, Tottori 682-0193, Japan c Institute of Geoscience, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8571, Japan Department of Geosciences, Faculty of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan e Marine Ecosystems Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan f Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan g Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan h Department of Earth and Planetary Science, Hiroshima University, Higashi, Hiroshima 739-8526, Japan i Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan Received 10 May 2001; received in revised form 13 September 2001; accepted 13 September 2001
Abstract The chemical characteristics of hydrothermal fluids in the Indian Ocean have been revealed for the first time. A hydrothermal field (called the Kairei field), including active black smoker chimneys, was discovered by ROV Kaiko dives at 25³19.17PS, 70³02.40PE (V2450 m depth) on the southwestern flank of an off-axis knoll located V15 miles north of the Rodriguez Triple Junction, Central Indian Ridge, after detailed tow-yo surveys of the hydrothermal plume distribution. The temperature of the fluid expelled from the most active chimney was almost stable (359^360³C) throughout three successive days of fluid sampling. The endmember fluid has a chemical composition similar to the hydrothermal fluids sampled from sediment-starved mid-ocean ridges in the Pacific and the Atlantic oceans, suggesting typical interactions between hot fluid and mid-ocean ridge basalts in the subseafloor reaction zone. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: hydrothermal vents; geochemistry; black smokers; plumes; Rodriguez Triple Junction; Central Indian Ridge
* Corresponding author. Fax: +81-11-746-0394. 1
E-mail address: [email protected] (T. Gamo).
Present address: Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan.
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 1 1 - 8
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1. Introduction Hydrothermal activity at plate spreading centers of mid-oceanic ridges, back-arc basins, and hot spot volcanoes has been extensively surveyed in the world's oceans since the ¢rst discovery of warm springs at the Galapagos spreading center in the eastern Paci¢c Ocean [1]. Although many active hydrothermal sites have been located in the Paci¢c and Atlantic oceans, no sites with actively venting hydrothermal £uids have previously been located in the Indian Ocean, due to limited surveys using research vessels and submersibles. A knowledge of the chemical composition of vent £uids from the Indian Ocean is imperative for the evaluation of chemical hydrothermal £uxes on a global scale. In addition, the Indian Ocean is regarded as the crucial link between the Atlantic and Paci¢c oceans for the understanding of the distribution and migration of biological communities characteristic of submarine hydrothermal activity. The existence of hydrothermal activity in the Indian Ocean is indicated by the presence of hydrothermal plumes in various locations: the Gulf of Aden [2], the Central Indian Ridge [3^5], Rodriguez Triple Junction [6], the southeastern Indian Ridge [7], and the southwestern Indian Ridge [8]. Particularly detailed plume data are available on the ¢rst segment of the Central Indian Ridge, just north of the Rodriguez Triple Junction (25³35PS, 70³00PE; Fig. 1), where CTD hydrocasts and tow-yo surveys have mapped anomalies of light transmission and chemical tracers (CH4 , Mn, Fe) in 1993 [6]. These surveys suggested that hydrothermally active sites should exist somewhere around an o¡-axis knoll (called Hakuho Knoll), V4 miles northeastward from the ridge axis of the Central Indian Ridge [6]. The knoll was revisited in 1998 by the JAMSTEC submersible Shinkai 6500, but although more intense light transmission anomalies were detected relative to those observed in 1993, no actively venting hydrothermal £uids were discovered [9]. In August 2000, detailed tow-yo and deep sea camera surveys were conducted over almost all of the Hakuho Knoll during the KR-00-05 cruise of R/V Kairei of JAMSTEC (chief scientist : J. Ha-
Fig. 1. A topographic map (depth in meters) of the Hakuho Knoll obtained by the multi-narrow beam echo sounder (SeaBeam 2100) of Kairei, where tracks of the tow-yo observations (TY02-10) and those of the bottom camera surveys are shown in solid and broken lines, respectively.
shimoto). Having pinpointed the most likely region of active venting, the ROV Kaiko was launched and a ¢eld of black smoker chimneys was discovered for the ¢rst time in the Indian Ocean [10]. This paper presents (i) the results of light transmission and CH4 measurements for hydrothermal plumes above the Hakuho Knoll, and (ii) the ¢rst chemical data of hydrothermal £uids taken from a black smoker chimney. 2. Hydrothermal plume surveys Hydrothermal plumes were mapped using the JAMSTEC 4000 m class Deep Tow camera system equipped with a CTD (SeaBird, 9-plus), a transmissometer (SeaTech, 25 cm light path), and an acoustic transponder for SSBL (super short base line) positioning. The system was also equipped with 12 Niskin bottles (General Oceanics, 2.5 l in volume) for water sampling. The system was continually raised and lowered (towyoed) above the Hakuho Knoll while slowly streaming [11]. Nine tracks (TY02^TY10) were dedicated to the tow-yo observations (Fig. 1). Seawater samples were taken during tracks TY02, 03 and 04 between 2120 and 2282 m water depth (a few hundred meters above the bottom)
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Fig. 2. Areal extent of light transmission anomaly observed a few hundred meters above the Kakuho Knoll. An arrow shows the location where the maximum light transmission anomaly (1.1%) was observed (see Fig. 3). An `X' mark stands for the Kairei hydrothermal ¢eld.
where transmission anomalies (0.1^0.8%) were detected. The system was also used for taking video pictures of the sea bottom; video survey tracks are also shown in Fig. 1. The seawater samples, including a background sample with no transmission anomaly, were analyzed for dissolved methane (CH4 ) on board the ship. An automated system was used for purging and trapping of CH4 which was equipped with a FID gas chromatograph. As shown in Fig. 2, light transmission anomalies ( s 0.1%) were detected all over the northwest-
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ern part of the Hakuho Knoll. Particularly high values of s 0.8% were observed along the ridge of the knoll. The highest transmission anomaly (1.1%) was observed close to the summit of the knoll, the location of which is indicated by the arrow in Fig. 2. A temperature anomaly of V+0.04³C was also observed at this point (Fig. 3). Fig. 4 shows the relationship between light transmission anomaly (vT) and CH4 concentration. Although the CH4 /vT ratio shows considerable variation, it is obvious that larger light transmission anomalies are accompanied by higher CH4 concentrations. This con¢rms that the transmission anomalies denote a hydrothermal plume because CH4 is an unambiguous tracer of hydrothermal activity on the sea£oor. The ¢rst ROV Kaiko dive (#167) along the southwestern slope of the Hakuho Knoll found an active hydrothermal venting ¢eld (called the Kairei Field) at 25³19.17PS, 70³02.40PE [10]. At least seven active venting sites associated with black smoker complexes with the maximum height of s 10 m were observed in a 40 m by 80 m ¢eld. The location is indicated by the `X' mark in Fig. 2, several hundred meters south of the maximum transmission anomaly point. 3. Chemical analyses of hydrothermal £uid Black smoker hydrothermal £uids were taken using titanium syringe samplers with a volume of 755 cm3 [12] during Kaiko dives #168 and
Fig. 3. Vertical pro¢les of (a) light transmission, (b) potential temperature, and (c) salinity measured during the tow-yo observation (TY06) with the CTD system at the location marked by the arrow in Fig. 2.
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tively. Table 1 summarizes chemical components measured and the analytical methods used in this study. The results are shown in Table 2. Mg concentrations of the two £uid samples (169A1 and 169A2) were extremely low, showing that they are almost pure endmember £uids with a mixing of ambient seawater (including the dead volume seawater) of less than a few percent. There are linear relationships between Mg and other components. They represent simple two-component mixing lines between the hydrothermal endmember ([Mg] = 0) and ambient seawater ([Mg] = 53 mM), giving the endmember values by extrapolating the lines to [Mg] = 0. 4. Discussion Fig. 4. Relationship between light transmission anomaly and CH4 concentration in seawater samples taken during the tow-yo observations (TY02, 03, and 04) above the Hakuho Knoll. Broken lines were drawn arbitrarily to show bi-modal characteristics of hydrothermal plumes.
#169. The dead volume of the sampler (3.8 cm3 ) was ¢lled with ambient bottom water. Three £uid samples (168A, 169A1, and 169A2) were obtained for chemical analyses from the same chimney, which was the most active black smoker observed during this cruise. The temperature of the venting £uid, measured with a Pt resistance thermometer, was fairly constant over three successive days of measurement (between 359³C and 360³C). An ambient bottom water sample (169M) was also taken for comparison. The samples were analyzed for pH, alkalinity and silica on board the Kairei, and for major cations, anions, trace elements and stable isotopes in shore-based laboratories. Subsamples for major and trace element analyses were passed through 0.45 Wm ¢lters to remove entrained solid particles, and then aliquots for major cation and trace element measurements were acidi¢ed with ultra-pure HCl before storage. Subsamples for N13 C(CH4 ) and 3 He/4 He measurements were taken without air contamination in 100 cm3 glass vial bottles with HgCl2 for disinfection, and in 40 cm3 Pbglass tubes with stopcocks at both ends, respec-
Table 3 lists the endmember chemical composition of the black smoker £uid obtained in this study, together with those of other representative black smoker £uids from mid-ocean ridges sampled to date. In general, the Kairei £uid has similar characteristics to those of other black smoker £uids. The acidity (pH 3.5) and the lack of Mg and SO4 , as well as the high temperature (360³C) of the venting £uid, which are the most common characteristics for submarine hydrothermal activity, indicate the existence of a typical hydrothermal £uid circulation associated with £uid^MORB interaction at high temperature below the Kairei Field. The helium isotope ratio (7.9 R/Ra ) of the vent £uids is similar to that of typical MORB (8.2 þ 0.7 R/Ra ) [34]. Fresh glasses of basaltic rocks from the Indian Ridge axis close to the Rodriguez Triple Junction (between 25³14PS and 25³49PS) also showed similar He isotope ratios of between 8.0 and 8.6 R/Ra [35]. The systematics of ND(H2 O) and N18 O(H2 O) (vDHTÿSW : +4.8 and v18 OHTÿSW : +1.9, where vXHTÿSW symbolizes the di¡erence of N-values between the hydrothermal £uid and ambient seawater [36]) gives a £uid/MORB ratio as low as V0.3 according to incremental reaction models [16,36]. The 87 Sr/86 Sr ratio of the £uids (0.7041) is almost the same as or slightly higher than those of the other black smoker £uids, possibly re£ect-
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Table 1 Chemical components measured, together with their analytical methods and analytical errors Component Shipboard analysis pH Alkalinity Si Major components Mg Ca Sr Na K Cl SO4 Metals Fe Mn Gases CH4 H2 S Isotopes ND(H2 O) N18 O(H2 O) N13 C(CO2 ) N13 C(CH4 ) N34 S(H2 S) 87 Sr/86 Sr 3 He/4 He
Analytical method
Anal. error ( þ c)
pH meter Potentiometric titration Colorimetry
0.01 1 (%) 2 (%)
ICP-AES ICP-AES ICP-AES Flame emission spectrometry Flame emission spectrometry Titration Ion chromatography
1 (mM) 1 (mM) 1 (WM) 10 (mM) 0.5 (mM) 5 (mM) 1 (mM)
ICP-AES ICP-AES
0.01 (mM) 0.01 (mM)
Gas chromatography Gravimetry (as BaSO4 )
10 (WM) 0.1 (mM)
Mass Mass Mass Mass Mass Mass Mass
1 (x) 0.1 (x) 0.1 (x) 0.5 (x) 0.3 (x) 0.000027 0.1 (R/Ra )
spectrometry spectrometry spectrometry spectrometry spectrometry spectrometry spectrometry
(Okayama University) (Okayama University) (Hokkaido University) (Hokkaido University) (Okayama University) (University of the Ryukyus) (Hiroshima University)
The 3 He/4 He ratio (expressed as R/Ra , where Ra is the atmospheric 3 He/4 He ratio of 1.4U1036 ), the N13 C(CH4 ), and the N34 S(H2 S) were measured according to Sano et al. [13], Tsunogai et al. [14], and Yanagisawa and Sakai [15], respectively. Sr isotope ratios were statically measured relative to the NBS-987 standard and fractionation corrected to 86 Sr/88 Sr = 0.1194.
ing higher 87 Sr/86 Sr of Indian Ocean MORB than those of Atlantic and Paci¢c MORBs [37]. By combining Sr concentrations (99^135 ppm) and 87 Sr/86 Sr ratios (0.70304^0.70329) reported for MORB glasses from the Rodriguez Triple Junction (between 24³59PS and 25³47PS) [37] with the £uid data obtained in this study, £uid/MORB ratios are estimated to be 2.1^3.7 according to an isotope equilibrium model [21]. The uncertainty of the £uid/MORB ratio (0.3^3) may be partly due to the di¡erence of assumed conditions between the models, and a more precise ratio should be estimated hereafter by accumulating more data. The N34 S(H2 S) value, +6.9x, is the second highest among the black smoker £uids from sediment-starved mid-ocean ridges (Table 3), suggesting a higher contribution of seawater sulfate (N34 S = 21x) in the formation of H2 S [17]. The
high N34 S values of 6.50^8.84x observed for the TAG sul¢de minerals were interpreted as caused by a supply of heavy sulfur from an anhydriterich zone beneath the TAG mound [38]. A similar situation may exist beneath the Hakuho Knoll. The N13 C(CH4 ), 38.7x, is heavier than the values observed at EPR 21³N (Table 3), indicating less contribution of organic matter derived CH4 . The endmember SiO2 concentration (15.8 mM) nearly equals the quartz solubility for T = 360³C and P = 250 bar [39], which suggests that the hydrothermal circulation is very shallow, if we assume the dissolution equilibrium between quartz and the Kairei black smoker £uid. However, the endmember Cl concentration (642 mM) is signi¢cantly higher than the background bottom water value of 553 mM. If the high Cl value is caused by phase separation [40], the £uid temperature would
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38.6 38.7 0.68 1.56 1.62 30.18 31.1 1.0 1.2 33.8 0.41 30.24 30.42 1.82 81 83
(R/Ra ) (mM)
5.23 3.84 3.42 6.66 1.6 4.0 4.0 0.0
(mM) (mM)
20.4 1.3 0.6 29.7 587 634 645 553
(mM) (mM)
6.90 14.90 15.60 0.14 0.38 0.87 0.86 0.00
(mM)
2.01 5.60 5.42 0.00
(mM)
22.0 29.7 28.7 10.2 31.0 2.5 1.3 53.3 11.7 14.6 14.1 9.6
(mM) (mM) (mM) (mM)
531 567 550 493 360 360 360 2 168A 169A1 169A2 169M
(³C)
86.4 76.1 73.6 85.1
(mM)
7.8 8.0
0.707015 0.704365 0.704228 0.709140
36.3 36.1
N13 C (CH4 ) PDB (x) N13 C (gCO2 ) PDB (x) Sr/86 Sr 87
He/4 He 3
N18 O (H2 O) VSMOW (x) pH NBS Alkalinity ND (H2 O) VSMOW 25³C (mM) (x) CH4 H2 S SO4 Cl Si Mn Fe Sr Mg
Ca K Temp. Na Sample No.
Table 2 Results of chemical and isotope analyses
6.8 7.0
T. Gamo et al. / Earth and Planetary Science Letters 193 (2001) 371^379 N34 S (H2 S) CDT (x)
376
previously have been higher than 360³C, which would increase SiO2 solubility. Alternative reasons for higher Cl concentrations, such as hydration reactions [12] and dissolution of Cl-bearing minerals [41], may have to be considered. The wide variation of CH4 /vT values in the hydrothermal plumes over the Hakuho Knoll (Fig. 4) implies that there are at least two hydrothermal endmembers in the study area, between CH4 -rich (gas-rich) endmember (trend A) and CH4 -poor endmember (trend B). Using the data in Fig. 3, the hydrothermal plume peak with the transmission anomaly of 1.1% indicates a dilution of the hydrothermal endmember with ambient bottom water with the dilution factor of 360³C/ 0.04³C = V104 . The 360³C £uid analyzed in this study belongs to the trend B endmember, because the CH4 concentration of 82/104 (WM) with a transmission anomaly of 1% is almost on the trend B line. Since CH4 is consumed by bacterial activity in hydrothermal plumes, other hydrothermal endmembers with higher CH4 content must be introduced in order to explain the CH4 data along the trend A. The high ( s 0.8%) transmission anomaly zone (Fig. 2), extending 0.5^1 mile north to northwest of the Kairei Field, may be a target for future investigations, although the camera surveys in this zone (Fig. 1) failed to ¢nd any active venting site. CH4 -rich £uids could be formed by phase separation and/or serpentinization of ultrama¢c rocks. Serpentinite pieces have ever been dredged from the ridge axis (25³29PS, 70³00PE) V9 miles south of the Hakuho Knoll [42]. It is uncertain at the present stage whether ultrama¢c outcrops exist on the Hakuho Knoll. 5. Conclusions 1. The ¢rst high temperature (360³C) £uid sampled from the Indian Ocean (Kairei hydrothermal ¢eld, 25³19.17PS, 70³02.40PE) has chemical characteristics similar to those of other sediment-starved black smoker £uids sampled in the Paci¢c and Atlantic oceans to date. 2. The 3 He/4 He ratio (7.9 R/Ra ) of the hydrother-
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Table 3 Chemical and isotope compositions of endmember £uids from sediment-starved mid-ocean ridges Name of site
Indian Ridge RT Junction Kairei Field
East Paci¢c Rise 21³N
East Paci¢c Rise 13³N
Mid-Atlantic Ridge Mid-Atlantic Ridge MARK TAG
Depth (m) Location (Lat.) Location (Lon.) Temp. (³C) pH (25³C) SiO2 (mM) Alkalinity (mM) Cl (mM) SO4 (mM) Na (mM) K (mM) Ca (mM) Sr (WM) Mn (WM) Fe (WM) H2 S (mM) CH4 (WM) N13 C(CH4 ) N13 C(CO2 ) vD(H2 O) v18 O(H2 O) 3 He/4 He(R/Ra ) 87 Sr/86 Sr 34 N S(H2 S) References
2450 25³19.17PS 70³02.40PE 360 3.5 15.8 30.46 642 0.3 560 14.3 30 77 840 5400 4.0 82 38.7 36.2 4.8 1.9 7.9 0.7041 6.9 This study
2600 20³50PN 109³07PW 273^355 3.3^4.0 15.6^19.5 30.54 to 30.19 489^579 0^0.6 432^510 23.2^25.8 11.7^20.8 62^97 699^1024 750^2429 6.6^8.4 51^65 317.6 to 315.0 37.0 þ 0.1 2.4^4.3 1.5^1.8 7.8 0.7030^0.7036 1.3^5.5 [12,16^21]
2600 12³50PN 103³57PW 354^381 3.1^3.3 17.9^21.9 30.74 to 30.40 712^760
3700 23³22PN 44³57PW 335^350 3.7^3.9 18.2^19.2 30.56 to 30.06 559^563 0 510^546 23^24 9.9^10.5 48^51 440^490 1830^2560 2.7^6.1 17^65
3700 26³08PN 44³49PW 320^369 3^3.8 20.8^21.4 32.7 to 30.45 636^675 30.9 549^557 17.1^20 30.8 99^103 670^680 4280^5590 2.5^4.0 150^620
3.1 1.9 7.9^8.6 0.7028^0.7039 4.9^5.0 [25^33]
36.9 to 313.0 3.0 1.5 7.5 0.7029^0.7038 8.6 [25^33]
mal £uid is almost the same as that of MORB, suggesting £uid^MORB reaction below sea£oor. The water/rock ratio is estimated to be 0.3^3 from the ND(H2 O) and N18 O(H2 O) systematics and the 87 Sr/86 Sr data of the black smoker £uid and basalts. 3. While the Si concentration of the £uid (which is assumed to be in equilibrium with quartz) suggests shallow hydrothermal circulation at V360³C, the Cl concentration (16% higher than the ambient seawater value) argues for phase separation at higher temperatures. More data are required for further discussion. 4. The relationship between the CH4 concentration and the light transmission anomaly for the hydrothermal plume samples suggests the existence of other £uid endmembers with higher CH4 concentrations than that observed in this study.
551^596 27.5^29.8 44.6^54.8 168^182 1689^2932 3980^10760 2.9^8.2 27^54 35.5 to 34.1 2.2^3.4 0.4^0.7 7.6^7.9 0.7041 [22^24]
Acknowledgements The authors wish to thank M. Miyamoto and the Deep Tow operation team, K. Hirata and the ROV Kaiko operation team, S. Ishida and the crew of R/V Kairei, and the shipboard scienti¢c party for their invaluable collaboration. Thanks are also due to F. Kouzuma and A. Hirota for the measurement of CH4 and its N13 C. Comments from R. James, J. Alt, and an anonymous reviewer were useful for revising the initial manuscript. This study was partly supported by a grant-in-aid for Scienti¢c Research (A) from JSPS (No. 11308020) and by a Special Coordination Fund of MEXT `Archaean Park' project. [AH] References [1] J.B. Corliss, J. Dymond, L.I. Gordon, J.M. Edmond,
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plume, Myojin Knoll Caldera, Izu-Bonin arc: implications for microbial methane oxidation in the oceans and applications to heat £ux estimation, Geochim. Cosmochim. Acta 64 (2000) 2439^2452. F. Yanagisawa, H. Sakai, Thermal decomposition of barium sulfate-vanadium pentaoxide-sillica glass mixtures for preparation of sulfur dioxide in sulfur isotope ratio measurements, Anal. Chem. 55 (1983) 985^987. T.S. Bowers, H.P. Taylor Jr., An integrated chemical and stable isotopic model of the origin of mid-ocean ridge hot spring systems, J. Geophys. Res. 90 (1985) 12583^12606. L.G. Woodru¡, W.C. Shanks III, Sulfur isotope study of chimney minerals and vent £uids from 21³N, East Paci¢c Rise: hydrothermal sulfur sources and disequilibrium sulfate reduction, J. Geophys. Res. 93 (1988) 4562^4572. J.A. Welhan, H. Craig, Methane, hydrogen and helium in hydrothermal £uids at 21³N on the East Paci¢c Rise, in: P.A. Rona, K. Bostrom, L. Laubier, K.L. Smith (Eds.), Hydrothermal Processes at Sea£oor Spreading Centers, Plenum Press, New York, 1983, pp. 391^409. J.E. Lupton, G.P. Klinkhammer, W.R. Normark, R. Haymon, K.C. Macdonald, R.F. Weiss, H. Craig, Helium-3 and manganese at the 21³N East Paci¢c Rise hydrothermal site, Earth Planet. Sci. Lett. 50 (1980) 115^127. A.C. Campbell, T.S. Bowers, C.I. Measures, K.K. Falkner, M. Khadem, J.M. Edmond, A time series of vent £uid compositions from 21³N, East Paci¢c Rise (1979, 1981, 1985) and the Guaymas Basin, Gulf of California (1982, 1985), J. Geophys. Res. 93 (B5) (1988) 4537^4549. F. Albare©de, A. Michard, J.F. Minster, G. Michard, 87 Sr/ 86 Sr ratios in hydrothermal waters and deposits from the East Paci¢c Rise at 21³N, Earth Planet. Sci. Lett. 55 (1981) 229^236. T.S. Bowers, A.C. Campbell, C.I. Measures, A.J. Spivack, M. Khadem, J.M. Edmond, Chemical controls on the composition of vent £uids at 13^11³N and 21³N, East Paci¢c Rise, J. Geophys. Res. 93 (B5) (1988) 4522^4536. L. Merlivat, F. Pineau, M. Javoy, Hydrothermal vent waters at 13³N on the East Paci¢c Rise: isotopic composition and gas concentration, Earth Planet. Sci. Lett. 84 (1987) 100^108. G. Michard, F. Albare©de, A. Michard, J.-F. Minster, J.-L. Charlou, N. Tan, Chemistry of solutions from the 13³N East Paci¢c Rise hydrothermal site, Earth Planet. Sci. Lett. 67 (1984) 297^307. A.C. Campbell, M.R. Palmer, G.P. Klinkhammer, T.S. Bowers, J.M. Edmond, J.R. Lawrence, J.F. Casey, G. Thompson, S. Humphris, P. Rona, J.A. Karson, Chemistry of hot springs on the Mid-Atlantic Ridge, Nature 335 (1988) 514^519. P. Jean-Baptiste, J.L. Charlou, M. Stievenard, J.P. Donval, H. Bougault, C. Mevel, Helium and methane measurements in hydrothermal £uids from the mid-Atlantic ridge: the Snake Pit site at 23³N, Earth Planet. Sci. Lett. 106 (1991) 17^28. J.M. Edmond, A.C. Campbell, M.R. Palmer, G.P. Klinkhammer, C.R. German, H.N. Edmonds, H. Elder¢eld, G.
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Thompson, P. Rona, Time series studies of vent £uids from the TAG and MARK sites (1986, 1990) Mid-Atlantic ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies, in: Hydrothermal Vents and Processes, Geological Society Special Publication No. 87, 1995, pp. 77^86. M.D. Rudnicki, H. Elder¢eld, Helium, radon and manganese at the TAG and Snakepit hydrothermal vent ¢elds, 26³ and 23³N, Mid-Atlantic Ridge, Earth Planet. Sci. Lett. 113 (1992) 307^321. T. Gamo, H. Chiba, H. Masuda, H.N. Edmonds, K. Fujioka, Y. Kodama, H. Nanba, Y. Sano, Chemical characteristics of hydrothermal £uids from the TAG mound of the mid-Atlantic Ridge in August 1994: implications for spatial and temporal variability of hydrothermal activity, Geophys. Res. Lett. 23 (1996) 3483^3486. H.N. Edmonds, C.R. German, D.R.H. Green, Y. Huh, T. Gamo, J.M. Edmond, Continuation of the hydrothermal £uid chemistry time series at TAG, and the e¡ects of ODP drilling, Geophys. Res. Lett. 23 (1996) 3487^3489. J.-L. Charlou, J.-P. Donval, Hydrothermal methane venting between 12³N and 26³N along the mid-Atlantic Ridge, J. Geophys. Res. 98 (B6) (1993) 9625^9642. J.-L. Charlou, J.-.P. Donval, P. Jean-Baptiste, A. Dapoigny, P.A. Rona, Gases and helium isotopes in high temperature solutions sampled before and after ODP Leg 158 drilling at TAG hydrothermal ¢eld (26³N, MAR), Geophys. Res. Lett. 23 (1996) 3491^3494. H. Chiba, H. Masuda, S.-Y. Lee, K. Fujioka, Chemistry of hydrothermal £uids at the TAG active mound, MAR 26³N, in 1998, Geophys. Res. Lett. 28 (2001) 2919^ 2922. D.R. Hilton, K. Hammerschmidt, G. Loock, H. Friedrichsen, Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: evidence of crust/mantle interactions in a back-arc basin, Geochim. Cosmochim. Acta 57 (1993) 2819^2841. Y. Nishio, T. Ishii, T. Gamo, Y. Sano, Volatile element
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isotopic systematics of the Rodriguez Triple Junction Indian Ocean MORB: implications for mantle heterogeneity, Earth Planet. Sci. Lett. 170 (1999) 241^253. W.C. Shanks III, J.K. Bo«hlke, R.R. Seal II, Stable isotopes in mid-ocean ridge hydrothermal systems: interactions between £uids, minerals, and organisms, in: S.E. Humphris, R.A. Zierenberg, L.S. Mullineaux, R.E. Thomson (Ed.), Sea£oor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions, Geophysical Monograph 91, American Geophysical Union, 1995, pp. 194^221. R.C. Price, A.K. Kennedy, M. Riggs-Sneeringer, F.A. Frey, Geochemistry of basalts from the Indian Ocean triple junction: implications for the generation and evolution of Indian Ocean ridge basalts, Earth Planet. Sci. Lett. 78 (1986) 379^396. H. Chiba, N. Uchiyama, D.A.H. Teagle, Stable isotope study of anhydrite and sul¢de minerals at the TAG hydrothermal mound, mid-Atlantic Ridge, 26³N, Proc. ODP, Sci. Results 158 (1998) 85^90. K.L. Von Damm, J.L. Bischo¡, R.J. Rosenbauer, Quartz solubility in hydrothermal seawater: an experimental study and equation describing quartz solubility for up to 0.5 M NaCl solutions, Am. J. Sci. 291 (1991) 977^1007. K.L. Von Damm, J.L. Bischo¡, Chemistry of hydrothermal solutions from the southern Juan de Fuca Ridge, J. Geophys. Res. 92 (1987) 11334^11346. W.E. Seyfried Jr., M.E. Berndt, D.R. Janecky, Chloride mobility during hydrothermal alteration of basalt in the low-pressure supercritical region, Geochim. Cosmochim. Acta 50 (1986) 469^475. T. Fujii, S. Yamashita, T. Ishii, Y. Tainosho, C.H. Langmuir, S. Nakada, S. Matsumoto, Y. Harada, N. Takahashi, K. Hirose, Petrological sampling at the Rodriguez Triple Junction in the Indian Ocean, in: K. Tamaki, H. Fujimoto (Eds.), Preliminary Cruise Report of R/V Hakuho-maru KH93-3 Research Cruise, Ocean Research Institute, University of Tokyo, 1995, pp. 68^87.
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Chemical characteristics of newly discovered black smoker £uids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge Toshitaka Gamo a; *, Hitoshi Chiba b , Toshiro Yamanaka c , Takamoto Okudaira d , Jun Hashimoto e , Shinji Tsuchida e , Jun-ichiro Ishibashi f , Satoshi Kataoka f , Urumu Tsunogai a , Kei Okamura g , Yuji Sano h;1 , Ryuichi Shinjo i a
d
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Sapporo 060-0810, Japan b Institute for Study of the Earth's Interior, Okayama University, 827 Yamada, Misasa, Tohaku, Tottori 682-0193, Japan c Institute of Geoscience, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8571, Japan Department of Geosciences, Faculty of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan e Marine Ecosystems Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan f Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan g Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan h Department of Earth and Planetary Science, Hiroshima University, Higashi, Hiroshima 739-8526, Japan i Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan Received 10 May 2001; received in revised form 13 September 2001; accepted 13 September 2001
Abstract The chemical characteristics of hydrothermal fluids in the Indian Ocean have been revealed for the first time. A hydrothermal field (called the Kairei field), including active black smoker chimneys, was discovered by ROV Kaiko dives at 25³19.17PS, 70³02.40PE (V2450 m depth) on the southwestern flank of an off-axis knoll located V15 miles north of the Rodriguez Triple Junction, Central Indian Ridge, after detailed tow-yo surveys of the hydrothermal plume distribution. The temperature of the fluid expelled from the most active chimney was almost stable (359^360³C) throughout three successive days of fluid sampling. The endmember fluid has a chemical composition similar to the hydrothermal fluids sampled from sediment-starved mid-ocean ridges in the Pacific and the Atlantic oceans, suggesting typical interactions between hot fluid and mid-ocean ridge basalts in the subseafloor reaction zone. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: hydrothermal vents; geochemistry; black smokers; plumes; Rodriguez Triple Junction; Central Indian Ridge
* Corresponding author. Fax: +81-11-746-0394. 1
E-mail address: [email protected] (T. Gamo).
Present address: Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan.
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 1 1 - 8
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1. Introduction Hydrothermal activity at plate spreading centers of mid-oceanic ridges, back-arc basins, and hot spot volcanoes has been extensively surveyed in the world's oceans since the ¢rst discovery of warm springs at the Galapagos spreading center in the eastern Paci¢c Ocean [1]. Although many active hydrothermal sites have been located in the Paci¢c and Atlantic oceans, no sites with actively venting hydrothermal £uids have previously been located in the Indian Ocean, due to limited surveys using research vessels and submersibles. A knowledge of the chemical composition of vent £uids from the Indian Ocean is imperative for the evaluation of chemical hydrothermal £uxes on a global scale. In addition, the Indian Ocean is regarded as the crucial link between the Atlantic and Paci¢c oceans for the understanding of the distribution and migration of biological communities characteristic of submarine hydrothermal activity. The existence of hydrothermal activity in the Indian Ocean is indicated by the presence of hydrothermal plumes in various locations: the Gulf of Aden [2], the Central Indian Ridge [3^5], Rodriguez Triple Junction [6], the southeastern Indian Ridge [7], and the southwestern Indian Ridge [8]. Particularly detailed plume data are available on the ¢rst segment of the Central Indian Ridge, just north of the Rodriguez Triple Junction (25³35PS, 70³00PE; Fig. 1), where CTD hydrocasts and tow-yo surveys have mapped anomalies of light transmission and chemical tracers (CH4 , Mn, Fe) in 1993 [6]. These surveys suggested that hydrothermally active sites should exist somewhere around an o¡-axis knoll (called Hakuho Knoll), V4 miles northeastward from the ridge axis of the Central Indian Ridge [6]. The knoll was revisited in 1998 by the JAMSTEC submersible Shinkai 6500, but although more intense light transmission anomalies were detected relative to those observed in 1993, no actively venting hydrothermal £uids were discovered [9]. In August 2000, detailed tow-yo and deep sea camera surveys were conducted over almost all of the Hakuho Knoll during the KR-00-05 cruise of R/V Kairei of JAMSTEC (chief scientist : J. Ha-
Fig. 1. A topographic map (depth in meters) of the Hakuho Knoll obtained by the multi-narrow beam echo sounder (SeaBeam 2100) of Kairei, where tracks of the tow-yo observations (TY02-10) and those of the bottom camera surveys are shown in solid and broken lines, respectively.
shimoto). Having pinpointed the most likely region of active venting, the ROV Kaiko was launched and a ¢eld of black smoker chimneys was discovered for the ¢rst time in the Indian Ocean [10]. This paper presents (i) the results of light transmission and CH4 measurements for hydrothermal plumes above the Hakuho Knoll, and (ii) the ¢rst chemical data of hydrothermal £uids taken from a black smoker chimney. 2. Hydrothermal plume surveys Hydrothermal plumes were mapped using the JAMSTEC 4000 m class Deep Tow camera system equipped with a CTD (SeaBird, 9-plus), a transmissometer (SeaTech, 25 cm light path), and an acoustic transponder for SSBL (super short base line) positioning. The system was also equipped with 12 Niskin bottles (General Oceanics, 2.5 l in volume) for water sampling. The system was continually raised and lowered (towyoed) above the Hakuho Knoll while slowly streaming [11]. Nine tracks (TY02^TY10) were dedicated to the tow-yo observations (Fig. 1). Seawater samples were taken during tracks TY02, 03 and 04 between 2120 and 2282 m water depth (a few hundred meters above the bottom)
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Fig. 2. Areal extent of light transmission anomaly observed a few hundred meters above the Kakuho Knoll. An arrow shows the location where the maximum light transmission anomaly (1.1%) was observed (see Fig. 3). An `X' mark stands for the Kairei hydrothermal ¢eld.
where transmission anomalies (0.1^0.8%) were detected. The system was also used for taking video pictures of the sea bottom; video survey tracks are also shown in Fig. 1. The seawater samples, including a background sample with no transmission anomaly, were analyzed for dissolved methane (CH4 ) on board the ship. An automated system was used for purging and trapping of CH4 which was equipped with a FID gas chromatograph. As shown in Fig. 2, light transmission anomalies ( s 0.1%) were detected all over the northwest-
373
ern part of the Hakuho Knoll. Particularly high values of s 0.8% were observed along the ridge of the knoll. The highest transmission anomaly (1.1%) was observed close to the summit of the knoll, the location of which is indicated by the arrow in Fig. 2. A temperature anomaly of V+0.04³C was also observed at this point (Fig. 3). Fig. 4 shows the relationship between light transmission anomaly (vT) and CH4 concentration. Although the CH4 /vT ratio shows considerable variation, it is obvious that larger light transmission anomalies are accompanied by higher CH4 concentrations. This con¢rms that the transmission anomalies denote a hydrothermal plume because CH4 is an unambiguous tracer of hydrothermal activity on the sea£oor. The ¢rst ROV Kaiko dive (#167) along the southwestern slope of the Hakuho Knoll found an active hydrothermal venting ¢eld (called the Kairei Field) at 25³19.17PS, 70³02.40PE [10]. At least seven active venting sites associated with black smoker complexes with the maximum height of s 10 m were observed in a 40 m by 80 m ¢eld. The location is indicated by the `X' mark in Fig. 2, several hundred meters south of the maximum transmission anomaly point. 3. Chemical analyses of hydrothermal £uid Black smoker hydrothermal £uids were taken using titanium syringe samplers with a volume of 755 cm3 [12] during Kaiko dives #168 and
Fig. 3. Vertical pro¢les of (a) light transmission, (b) potential temperature, and (c) salinity measured during the tow-yo observation (TY06) with the CTD system at the location marked by the arrow in Fig. 2.
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tively. Table 1 summarizes chemical components measured and the analytical methods used in this study. The results are shown in Table 2. Mg concentrations of the two £uid samples (169A1 and 169A2) were extremely low, showing that they are almost pure endmember £uids with a mixing of ambient seawater (including the dead volume seawater) of less than a few percent. There are linear relationships between Mg and other components. They represent simple two-component mixing lines between the hydrothermal endmember ([Mg] = 0) and ambient seawater ([Mg] = 53 mM), giving the endmember values by extrapolating the lines to [Mg] = 0. 4. Discussion Fig. 4. Relationship between light transmission anomaly and CH4 concentration in seawater samples taken during the tow-yo observations (TY02, 03, and 04) above the Hakuho Knoll. Broken lines were drawn arbitrarily to show bi-modal characteristics of hydrothermal plumes.
#169. The dead volume of the sampler (3.8 cm3 ) was ¢lled with ambient bottom water. Three £uid samples (168A, 169A1, and 169A2) were obtained for chemical analyses from the same chimney, which was the most active black smoker observed during this cruise. The temperature of the venting £uid, measured with a Pt resistance thermometer, was fairly constant over three successive days of measurement (between 359³C and 360³C). An ambient bottom water sample (169M) was also taken for comparison. The samples were analyzed for pH, alkalinity and silica on board the Kairei, and for major cations, anions, trace elements and stable isotopes in shore-based laboratories. Subsamples for major and trace element analyses were passed through 0.45 Wm ¢lters to remove entrained solid particles, and then aliquots for major cation and trace element measurements were acidi¢ed with ultra-pure HCl before storage. Subsamples for N13 C(CH4 ) and 3 He/4 He measurements were taken without air contamination in 100 cm3 glass vial bottles with HgCl2 for disinfection, and in 40 cm3 Pbglass tubes with stopcocks at both ends, respec-
Table 3 lists the endmember chemical composition of the black smoker £uid obtained in this study, together with those of other representative black smoker £uids from mid-ocean ridges sampled to date. In general, the Kairei £uid has similar characteristics to those of other black smoker £uids. The acidity (pH 3.5) and the lack of Mg and SO4 , as well as the high temperature (360³C) of the venting £uid, which are the most common characteristics for submarine hydrothermal activity, indicate the existence of a typical hydrothermal £uid circulation associated with £uid^MORB interaction at high temperature below the Kairei Field. The helium isotope ratio (7.9 R/Ra ) of the vent £uids is similar to that of typical MORB (8.2 þ 0.7 R/Ra ) [34]. Fresh glasses of basaltic rocks from the Indian Ridge axis close to the Rodriguez Triple Junction (between 25³14PS and 25³49PS) also showed similar He isotope ratios of between 8.0 and 8.6 R/Ra [35]. The systematics of ND(H2 O) and N18 O(H2 O) (vDHTÿSW : +4.8 and v18 OHTÿSW : +1.9, where vXHTÿSW symbolizes the di¡erence of N-values between the hydrothermal £uid and ambient seawater [36]) gives a £uid/MORB ratio as low as V0.3 according to incremental reaction models [16,36]. The 87 Sr/86 Sr ratio of the £uids (0.7041) is almost the same as or slightly higher than those of the other black smoker £uids, possibly re£ect-
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Table 1 Chemical components measured, together with their analytical methods and analytical errors Component Shipboard analysis pH Alkalinity Si Major components Mg Ca Sr Na K Cl SO4 Metals Fe Mn Gases CH4 H2 S Isotopes ND(H2 O) N18 O(H2 O) N13 C(CO2 ) N13 C(CH4 ) N34 S(H2 S) 87 Sr/86 Sr 3 He/4 He
Analytical method
Anal. error ( þ c)
pH meter Potentiometric titration Colorimetry
0.01 1 (%) 2 (%)
ICP-AES ICP-AES ICP-AES Flame emission spectrometry Flame emission spectrometry Titration Ion chromatography
1 (mM) 1 (mM) 1 (WM) 10 (mM) 0.5 (mM) 5 (mM) 1 (mM)
ICP-AES ICP-AES
0.01 (mM) 0.01 (mM)
Gas chromatography Gravimetry (as BaSO4 )
10 (WM) 0.1 (mM)
Mass Mass Mass Mass Mass Mass Mass
1 (x) 0.1 (x) 0.1 (x) 0.5 (x) 0.3 (x) 0.000027 0.1 (R/Ra )
spectrometry spectrometry spectrometry spectrometry spectrometry spectrometry spectrometry
(Okayama University) (Okayama University) (Hokkaido University) (Hokkaido University) (Okayama University) (University of the Ryukyus) (Hiroshima University)
The 3 He/4 He ratio (expressed as R/Ra , where Ra is the atmospheric 3 He/4 He ratio of 1.4U1036 ), the N13 C(CH4 ), and the N34 S(H2 S) were measured according to Sano et al. [13], Tsunogai et al. [14], and Yanagisawa and Sakai [15], respectively. Sr isotope ratios were statically measured relative to the NBS-987 standard and fractionation corrected to 86 Sr/88 Sr = 0.1194.
ing higher 87 Sr/86 Sr of Indian Ocean MORB than those of Atlantic and Paci¢c MORBs [37]. By combining Sr concentrations (99^135 ppm) and 87 Sr/86 Sr ratios (0.70304^0.70329) reported for MORB glasses from the Rodriguez Triple Junction (between 24³59PS and 25³47PS) [37] with the £uid data obtained in this study, £uid/MORB ratios are estimated to be 2.1^3.7 according to an isotope equilibrium model [21]. The uncertainty of the £uid/MORB ratio (0.3^3) may be partly due to the di¡erence of assumed conditions between the models, and a more precise ratio should be estimated hereafter by accumulating more data. The N34 S(H2 S) value, +6.9x, is the second highest among the black smoker £uids from sediment-starved mid-ocean ridges (Table 3), suggesting a higher contribution of seawater sulfate (N34 S = 21x) in the formation of H2 S [17]. The
high N34 S values of 6.50^8.84x observed for the TAG sul¢de minerals were interpreted as caused by a supply of heavy sulfur from an anhydriterich zone beneath the TAG mound [38]. A similar situation may exist beneath the Hakuho Knoll. The N13 C(CH4 ), 38.7x, is heavier than the values observed at EPR 21³N (Table 3), indicating less contribution of organic matter derived CH4 . The endmember SiO2 concentration (15.8 mM) nearly equals the quartz solubility for T = 360³C and P = 250 bar [39], which suggests that the hydrothermal circulation is very shallow, if we assume the dissolution equilibrium between quartz and the Kairei black smoker £uid. However, the endmember Cl concentration (642 mM) is signi¢cantly higher than the background bottom water value of 553 mM. If the high Cl value is caused by phase separation [40], the £uid temperature would
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38.6 38.7 0.68 1.56 1.62 30.18 31.1 1.0 1.2 33.8 0.41 30.24 30.42 1.82 81 83
(R/Ra ) (mM)
5.23 3.84 3.42 6.66 1.6 4.0 4.0 0.0
(mM) (mM)
20.4 1.3 0.6 29.7 587 634 645 553
(mM) (mM)
6.90 14.90 15.60 0.14 0.38 0.87 0.86 0.00
(mM)
2.01 5.60 5.42 0.00
(mM)
22.0 29.7 28.7 10.2 31.0 2.5 1.3 53.3 11.7 14.6 14.1 9.6
(mM) (mM) (mM) (mM)
531 567 550 493 360 360 360 2 168A 169A1 169A2 169M
(³C)
86.4 76.1 73.6 85.1
(mM)
7.8 8.0
0.707015 0.704365 0.704228 0.709140
36.3 36.1
N13 C (CH4 ) PDB (x) N13 C (gCO2 ) PDB (x) Sr/86 Sr 87
He/4 He 3
N18 O (H2 O) VSMOW (x) pH NBS Alkalinity ND (H2 O) VSMOW 25³C (mM) (x) CH4 H2 S SO4 Cl Si Mn Fe Sr Mg
Ca K Temp. Na Sample No.
Table 2 Results of chemical and isotope analyses
6.8 7.0
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376
previously have been higher than 360³C, which would increase SiO2 solubility. Alternative reasons for higher Cl concentrations, such as hydration reactions [12] and dissolution of Cl-bearing minerals [41], may have to be considered. The wide variation of CH4 /vT values in the hydrothermal plumes over the Hakuho Knoll (Fig. 4) implies that there are at least two hydrothermal endmembers in the study area, between CH4 -rich (gas-rich) endmember (trend A) and CH4 -poor endmember (trend B). Using the data in Fig. 3, the hydrothermal plume peak with the transmission anomaly of 1.1% indicates a dilution of the hydrothermal endmember with ambient bottom water with the dilution factor of 360³C/ 0.04³C = V104 . The 360³C £uid analyzed in this study belongs to the trend B endmember, because the CH4 concentration of 82/104 (WM) with a transmission anomaly of 1% is almost on the trend B line. Since CH4 is consumed by bacterial activity in hydrothermal plumes, other hydrothermal endmembers with higher CH4 content must be introduced in order to explain the CH4 data along the trend A. The high ( s 0.8%) transmission anomaly zone (Fig. 2), extending 0.5^1 mile north to northwest of the Kairei Field, may be a target for future investigations, although the camera surveys in this zone (Fig. 1) failed to ¢nd any active venting site. CH4 -rich £uids could be formed by phase separation and/or serpentinization of ultrama¢c rocks. Serpentinite pieces have ever been dredged from the ridge axis (25³29PS, 70³00PE) V9 miles south of the Hakuho Knoll [42]. It is uncertain at the present stage whether ultrama¢c outcrops exist on the Hakuho Knoll. 5. Conclusions 1. The ¢rst high temperature (360³C) £uid sampled from the Indian Ocean (Kairei hydrothermal ¢eld, 25³19.17PS, 70³02.40PE) has chemical characteristics similar to those of other sediment-starved black smoker £uids sampled in the Paci¢c and Atlantic oceans to date. 2. The 3 He/4 He ratio (7.9 R/Ra ) of the hydrother-
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377
Table 3 Chemical and isotope compositions of endmember £uids from sediment-starved mid-ocean ridges Name of site
Indian Ridge RT Junction Kairei Field
East Paci¢c Rise 21³N
East Paci¢c Rise 13³N
Mid-Atlantic Ridge Mid-Atlantic Ridge MARK TAG
Depth (m) Location (Lat.) Location (Lon.) Temp. (³C) pH (25³C) SiO2 (mM) Alkalinity (mM) Cl (mM) SO4 (mM) Na (mM) K (mM) Ca (mM) Sr (WM) Mn (WM) Fe (WM) H2 S (mM) CH4 (WM) N13 C(CH4 ) N13 C(CO2 ) vD(H2 O) v18 O(H2 O) 3 He/4 He(R/Ra ) 87 Sr/86 Sr 34 N S(H2 S) References
2450 25³19.17PS 70³02.40PE 360 3.5 15.8 30.46 642 0.3 560 14.3 30 77 840 5400 4.0 82 38.7 36.2 4.8 1.9 7.9 0.7041 6.9 This study
2600 20³50PN 109³07PW 273^355 3.3^4.0 15.6^19.5 30.54 to 30.19 489^579 0^0.6 432^510 23.2^25.8 11.7^20.8 62^97 699^1024 750^2429 6.6^8.4 51^65 317.6 to 315.0 37.0 þ 0.1 2.4^4.3 1.5^1.8 7.8 0.7030^0.7036 1.3^5.5 [12,16^21]
2600 12³50PN 103³57PW 354^381 3.1^3.3 17.9^21.9 30.74 to 30.40 712^760
3700 23³22PN 44³57PW 335^350 3.7^3.9 18.2^19.2 30.56 to 30.06 559^563 0 510^546 23^24 9.9^10.5 48^51 440^490 1830^2560 2.7^6.1 17^65
3700 26³08PN 44³49PW 320^369 3^3.8 20.8^21.4 32.7 to 30.45 636^675 30.9 549^557 17.1^20 30.8 99^103 670^680 4280^5590 2.5^4.0 150^620
3.1 1.9 7.9^8.6 0.7028^0.7039 4.9^5.0 [25^33]
36.9 to 313.0 3.0 1.5 7.5 0.7029^0.7038 8.6 [25^33]
mal £uid is almost the same as that of MORB, suggesting £uid^MORB reaction below sea£oor. The water/rock ratio is estimated to be 0.3^3 from the ND(H2 O) and N18 O(H2 O) systematics and the 87 Sr/86 Sr data of the black smoker £uid and basalts. 3. While the Si concentration of the £uid (which is assumed to be in equilibrium with quartz) suggests shallow hydrothermal circulation at V360³C, the Cl concentration (16% higher than the ambient seawater value) argues for phase separation at higher temperatures. More data are required for further discussion. 4. The relationship between the CH4 concentration and the light transmission anomaly for the hydrothermal plume samples suggests the existence of other £uid endmembers with higher CH4 concentrations than that observed in this study.
551^596 27.5^29.8 44.6^54.8 168^182 1689^2932 3980^10760 2.9^8.2 27^54 35.5 to 34.1 2.2^3.4 0.4^0.7 7.6^7.9 0.7041 [22^24]
Acknowledgements The authors wish to thank M. Miyamoto and the Deep Tow operation team, K. Hirata and the ROV Kaiko operation team, S. Ishida and the crew of R/V Kairei, and the shipboard scienti¢c party for their invaluable collaboration. Thanks are also due to F. Kouzuma and A. Hirota for the measurement of CH4 and its N13 C. Comments from R. James, J. Alt, and an anonymous reviewer were useful for revising the initial manuscript. This study was partly supported by a grant-in-aid for Scienti¢c Research (A) from JSPS (No. 11308020) and by a Special Coordination Fund of MEXT `Archaean Park' project. [AH] References [1] J.B. Corliss, J. Dymond, L.I. Gordon, J.M. Edmond,
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