Helium Isotopes at the Rainbow Hydrothermal Site (Mid-Atlantic Ridge, 36‡14PN) Philippe Jean-Baptiste A;�, Elise Fourre¤ A, Jean-Luc Charlou B, Christopher R

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Earth and Planetary Science Letters 221 (2004) 325^335 www.elsevier.com/locate/epsl

Helium isotopes at the Rainbow hydrothermal site (Mid-Atlantic Ridge, 36‡14PN) Philippe Jean-Baptiste a;, Elise Fourre¤ a, Jean-Luc Charlou b, Christopher R. German c, Joel Radford-Knoery b

a IPSL/LSCE, UMR CEA-CNRS 1572, CE Saclay, 91191 Gif-sur-Yvette Cedex, France b IFREMER, DRO-GM, Centre de Brest, P.O. Box 70, 29280 Plouzane¤, France c Southampton Oceanography Centre, Empress Dock, Southampton S014 3ZH, UK

Received 1 September 2003; received in revised form 22 January 2004; accepted 28 January 2004

Abstract

The 3He/4He ratio and helium concentration have been measured in the vent fluids and the dispersing plume of the 3 4 3 Rainbow hydrothermal site, on the Mid-Atlantic Ridge (MAR). The He/ He ratio (7.51 Ra) and He end-member concentration (25 pmol/kg) are in the range of observed values elsewhere on mid-ocean ridges, pointing to the relative homogeneity of the upper mantle with respect to helium isotope geochemistry. 3He is linearly correlated with methane 3 3 and manganese throughout the plume, with CH4/ He and Mn/ He ratios identical to those measured in the hot fluids. The bulk residence time of the plume in the rift valley estimated from the plume 3He budget is V20 days. The 3He flux transported by the plume, calculated from current-meter data, is 12.3 þ 3 nmol/s, requiring a flux of 490 kg/s of high-temperature fluid. The scaling of the heat flux emitted by the Rainbow site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (9.3 þ 2U10318 mol/J), indicates a heat output of 1320 MW. With a regional spreading rate of V25 mm/year, we calculate that the annual 3He flux for this section of the MAR is of the order of 0.5 þ 0.2 mmol per kilometre of ridge per millimetre of newly formed crust. This figure compares well with the flux calculated for the neighbouring Lucky Strike segment. Although the uncertainties remain large, both fluxes are V40^50% above the world average (0.33 mmol/km/mm), thus supporting earlier suggestions that the intensity of the hydrothermal activity south of the Azores is higher than expected from the regional spreading rate. ß 2004 Elsevier B.V. All rights reserved.

Keywords: helium; hydrothermal vents; isotopes; heat £ux; Mid-Atlantic Ridge

1. Introduction 1994 from the detection of optical back-scatter anomalies [1]. In the year 1997, a detailed survey The Rainbow hydrothermal plume (Mid-Atlan- of the site was undertaken as part of the EU tic Ridge (MAR), 36‡14PN) was ¢rst identi¢ed in Amores project, including physical oceanography and geochemical study of the hydrothermal plume dispersal. Sea£oor investigations were also carried Nautile * Corresponding author. Tel.: +33-169087714; out with the submersible , which led to the Fax: +33-169087716. discovery and study of the active vents [2]. Here E-mail address: [email protected] (P. Jean-Baptiste). we report helium isotope results for the high-tem-

EPSL 7030 25-3-04 Cyaan Magenta Geel Zwart 326 P. Jean-Baptiste et al. / Earth and Planetary Science Letters 221 (2004) 325^335 perature £uids and the hydrothermal plume. In along the western £ank of a topographic high (the the following, the helium 3He/4He isotope ratio, Rainbow ridge) located at the western end of a R, is expressed either relative to the atmospheric non-transform discontinuity between the South 36 ratio (Ra = 1.38U10 ) or using the delta nota- AMAR and AMAR segments (Fig. 1). The hy- 3 tion, N He (%) = (R/Ra31)U100. With regard to drothermal site consists of a 100U200 m active hydrothermal processes, the helium isotopes rep- vent ¢eld, between 2270 m and 2330 m depth, resent a useful tracer in two di¡erent ways. First, displaying extensive stockwork mineralisation. owing to the clearly distinct 3He/4He signature of Ten major groups of active black smokers emit the di¡erent solid earth reservoirs, including the hot (T = 365‡C) acidic £uids. Unlike the previ- 3 4 depleted upper mantle ( He/ He = 7^9 Ra), the ously discovered Menez Gwen and Lucky Strike 3 4 lower mantle ( He/ He s 30 Ra) and the continen- hydrothermal sites south of the Azores, which are 3 4 tal crust ( He/ He 6 0.1 Ra), the helium isotope located in a basaltic environment, the Rainbow ratio is a good indicator of the source of the hy- site is nested in serpentinised ultrama¢c rocks. drothermal £uids. Second, due to its high concen- This speci¢c geological environment strongly in- tration in vent £uids compared to seawater and to £uences the chemistry [3] and gas content of the its chemical inertness, 3He provides a faithful £uids, which show exceptionally high hydrogen tracer of £uid dilution and plume dispersion. concentrations attributed to the serpentinisation process [4]. Physical oceanography data, including CTD, 2. Geological and hydrographic settings LADCP and long-term current-meter moorings [5,6], as well as the spatial distribution of optical The Rainbow site lies at 36‡13.8PN, 33‡54.12PW back-scatter anomalies, reveal that the hydrother-

Fig. 1. Bathymetric 3-D map of the Rainbow area.

EPSL 7030 25-3-04 Cyaan Magenta Geel Zwart P. Jean-Baptiste et al. / Earth and Planetary Science Letters 221 (2004) 325^335 327 mal plume is advected to the northeast along the equipment is monitored by miniature digital sen- rift valley of the AMAR segment as a single spa- sors. The measured pressure after the drying col- tially and temporally coherent structure, detect- umn makes it possible to determine the exact able over a distance of several tens of kilometres amount of dissolved gases [10^12]. In parallel away from its source [6,7]. with the stainless steel container, gas aliquots were also taken in small copper tubes tightly sealed with special clamps for helium isotope 3. Sample collection and analytical methods analysis. In the laboratory, the copper tubes were directly connected to the inlet system of Water samples were taken in the rift valley on the mass spectrometer for isotopic analysis [8]. each side of the Rainbow ridge from a series of The uncertainty in the isotopic ratio is within CTD rosette vertical and ‘tow-yo’ casts designed to obtain a 3-D picture of the hydrothermal plume. The water samples were stored in sealed copper tubes and returned to the laboratory for subsequent gas extraction and mass spectrometry measurement with a VG 3000 mass spectrometer using routine analysis procedures [8]. The accura- cy in the N3He (%) excess is better than þ 0.3, and the relative standard deviation of the absolute 4He concentration is 0.8%. Helium isotope analyses were also performed on high-temperature vent samples collected in 750-ml gas-tight titanium syringes. These sam- pling devices, developed at IFREMER for the French submersibles Cyana and Nautile, have been used routinely since the early 1980s for hydrothermal gas studies. Leak tests carried out at various pressures in the high-pressure facility at IFREMER have shown that these syringes, if properly maintained, are totally reliable to recov- er gassy samples such as those found in hot hydrothermal systems. The consistency of the Mn/ 3 He and Mn/CH4 ratios recorded in the plume and in the hot £uids (see Fig. 4) is further evidence that the gas phase was adequately sampled. On deck, the £uid samples were transferred immediately in a specially designed evacuated degassing tank ensuring 100% gas recovery. During the transfer procedure, the syringes were kept under Fig. 2. Selected N3He vertical pro¢les along the dispersing water to avoid any contamination by atmospheric plume (solid lines). The location of the stations (a^d) is air. The degassing tank is a development of the shown in Fig. 3. N3He vertical pro¢les at nearby stations out- prototype used in previous studies [9]. The main side the central rift (dashed lines) are also displayed to de¢ne improvement consists in the addition of a mem- the regional 3He background: NATS (1981) stations 60 brane pump which ensures the complete transfer (36‡25N/34‡38W), 61 (36‡25N/33‡67W), 65 (36‡25N/30‡25W) and 66 (36‡25N/29‡27W) [13], and TOPOGULF (1983) sta- of the dissolved species from the degassing tank tion 83 (40‡00N/35‡00W) [14]. (N.B. the N3He maximum in to a small stainless steel container through a dry- the upper 1000 m re£ects the transient signal produced by ing column. The pressure in various parts of the the radioactive decay of bomb tritium.)

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þ 0.05 Ra, with a relative error in the total helium depths o¡set by 40^50 m (Fig. 2), consistent concentration 6 5%. with the nephelometry data [6]. This interval nearly corresponds to the depth interval spanned by the active vents on the Rainbow ridge so that 4. Results this double plume is likely due to discrete contri- butions from the di¡erent vents. However, it has 4.1. Plumedata been observed in the TAG hydrothermal site [15] that a plume rise height can vary by more than The vertical structure of the plume (Fig. 2) 100 m as a function of tidal cycling. Therefore, shows pronounced N3He anomalies in a near-con- this double peak structure could also be due to a stant depth range between 2000 and 2300 m, with similar phenomenon. The stations southwest of increasing values up to 37% close to the vent site. the ridge show no N3He anomaly (Fig. 3). This This value is high compared to the background is in agreement with the structure of the deep value of the deep Atlantic waters, around 5% at currents recorded by long-term moorings (V400 this latitude and depth [13,14]. The near-¢eld days), which indicate a persistent northeastward plume exhibits two maxima at two di¡erent advection of the plume [5]. The N3He horizontal

Fig. 3. Horizontal distribution of the N3He (%) maximum associated with the Rainbow hydrothermal plume in the 2000^2300 m range (coordinates in decimal degree). Letters a^d indicate the location of the stations displayed in Fig. 2. The dashed line shows the position of the Rainbow sill, northeast of the vent site. Darkly shaded regions are less than 2000 m.

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section at plume height (Fig. 3) shows that the plume is detectable over a distance of about 30 km. Throughout the dispersing plume, 3He is linearly correlated with manganese and methane (Fig. 4), with slopes of (110 þ 10)U106 and (131 þ 16) 6 3 3 U10 for Mn/ He and CH4/ He ratios respectively.

Fig. 5. 3He^4He relationship (a) and 4He concentration ver- sus magnesium (b) in the vent £uids.

4.2. Vent £uid data

The £uids are characterised by a rather uniform chemical composition indicating a single deep supply for the various vents [3,4].The3He/4He ratio, R, obtained from the slope of the 3He^ 4 He diagram (Fig. 5a), is 7.51 þ 0.02 Ra, which Fig. 4. Manganese (a) and methane (b) as a function of 3He is within the 7^9 Ra range of mid-ocean ridge in the hydrothermal plume. basalt (MORB) values. Helium concentrations

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Table 1 3 4 Rainbow vent £uid helium isotope data (the uncertainty in the He/ He ratio is within þ 0.05 Ra, with a relative error in the helium concentration 6 5%) 4 Vent marker Sample Temperature R/Ra [ He] Mg (‡C) (Wmol/kg) (mmol/kg) 2 PL03-G2 ^ 6.80 0.22 45.5 3 PL04-D3 351 7.75 1.00 29.6 3 PL07-G2 351 7.18 2.04 11.2 4 PL03-D3 354 7.02 0.70 33.3 4 PL03-G3 354 5.71 0.23 48.3 4 PL05-EG 354 7.34 1.93 9.5 5 PL04-G2 362 6.73 0.12 46.1 5 PL09-D2 362 7.66 2.49 3.3 7 PL06-D2 ^ 7.61 1.55 3.7 9 PL10-D2 364 7.56 2.89 2.1 SW deep water 3^4 1.05 0.0017 52.9

(Table 1) are plotted in Fig. 5b against magnesi- End-member hydrothermal tracer ratios in the um, used as an indicator of the dilution factor of hot £uids are (94 þ 20)U106 and (108 þ 37)U106 3 3 the hydrothermal end-member with ambient sea- for Mn/ He and CH4/ He ratios respectively. water. The data de¢ne a helium (4He) end-mem- The 3He/heat ratio corresponding to the £uid ber of 2.41 þ 0.5 Wmol/kg, corresponding to a 3He output temperature of 365‡C is (9.3 þ 2)U10318 concentration of 25 þ 5 pmol/kg. mol/J.

Table 2 Compilation of helium isotopes and heat data for hydrothermal vents from the MAR and East Paci¢c Rise 3 3 Hydrothermal site [ He] R/Ra He/heat Reference (pmol/kg) (10318 mol/J) MAR Logatchev (14.45‡N) ^ 7.2 ^ P. Jean-Baptiste, unpublished data Snake Pit (23‡N) 23.8 þ 1.6 8.4 12.4 þ 1.2 [9] Snake Pit (23‡N) 12.5 þ 1.5 7.9 6.5 þ 0.8 [37] TAG (26‡N) 20.1 þ 2.2 7.5 9.3 þ 1.1 [37] TAG (26‡N) 16.7 þ 1.4 8.2 7.7 þ 0.8 [38] Broken Spur (29‡N) ^ 8.9 ^ P. Jean-Baptiste, unpublished data Rainbow (36‡14N) 25.0 þ 5 7.51 9.3 þ 2 this work Lucky Strike (37‡17N) 10 þ 3.5 8.13 6.5 þ 1.2 [36] Menez Gwen (37‡50N) 20 þ 4 8.65 17.8 þ 3.5 [11] EPR Galapagos ^ 7.8 5.2 þ 0.4 [39] EPR 21‡N12.4 þ 2.3 7.8 5.2 þ 0.9 [21] EPR 13‡N21 þ 7 7.6 11.0 þ 3.7 [40] EPR 11‡N17.9 þ 8 8.32 9.5 þ 4 [41] EPR 18‡S 9.2 þ 3 8.3 6.2 þ 2 [42] Global average for mature sites 17.1 8.0 8.9 Standard deviation (c) (5.4) (0.5) (3.6) Juan de Fuca: event plume ^ 7.9 3.2^4.3 [19] associated deep plume ^ 7.9 14^47 [19] over fresh lava £ows ^ 22^75 [19]

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5. Discussion related to magmatic events on the Juan de Fuca Ridge [16^19] or directly associated with fresh One of the main achievements of the Amores lava £ows (see for instance ¢g. 2 in [19]). With a project was to deliver a coherent integrated data value of (9.3 þ 2)U10318 mol/J, the Rainbow site set of the physical and geochemical variables rel- exhibits a 3He/heat ratio typical of that observed evant to the study of an active venting site and its for mature sites, characterised by a well-estab- plume dispersal. In addition to presenting new lished steady-state circulation. 3 data for hydrothermal tracers in a rather unusual Large variations in the CH4/ He ratio are ob- (ultrama¢c) environment that permits comparison served along mid-ocean ridges. Even for basaltic with other sites, this combined vent site/plume environments, the variability can be substantial: 3 approach allows us: (1) to compare plume tracer the CH4/ He ratio at EPR-13‡Nand EPR-21‡N data to their source composition, (2) to establish is (1.7 þ 0.4)U106 [20] and (5.0 þ 1.5)U106 [21] the 3He and thermal budget for a single vent site, respectively while it is 65U106 at Lucky Strike and (3) to put the results in perspective with re- and 85U106 at Menez Gwen on the MAR [11]. spect to global hydrothermal £uxes. Mn/3He ratios also display a large variability: for example, the Mn/3He ratio of the EPR-13‡N 5.1. Comparison of the vent £uid results with other £uids is 13.7U106 [22] while it is more than hydrothermal sites 160U106 at the Galapagos spreading center [23]. Due to the relative homogeneity of the 3He end- The 3He end-member (25 þ 5 pmol/kg) lies in member in mid-ocean ridge hydrothermal sys- 3 the range of previously studied hydrothermal sites tems, the variations in the CH4/ He and Mn/ on mid-ocean ridges (Table 2). Unlike major and 3He ratios essentially re£ect the variations in the trace elements and reactive gases such as CH4, end-member values of these tracers. In the Rain- H2,H2S, whose concentrations in the hydrother- bow £uids, CH4 concentrations are indeed among mal £uids strongly depend on the geological na- the highest for abiogenic methane [4] and re£ect ture of the substratum and local thermodynamic the production of methane by the serpentinisation conditions, the mean 3He end-member concentra- reactions in the ultrama¢c substratum. Manga- tion averaged over the MAR and East Paci¢c nese and other transition metal concentrations Rise (EPR) sites where helium isotope data are are also unusually high [3]. A comprehensive dis- available (Table 2) is relatively constant (17.1 cussion of the geochemistry of these tracers in pmol/kg, with a standard deviation of 5.4 pmol/ relation to the geological setting of the Rainbow kg, or 32% in relative terms). This appears thus to site is available in [3,4]. re£ect primarily the relative homogeneity of the upper mantle source with respect to helium geo- 5.2. 3Heand heatplumebudgets chemistry. This is all the more true for the 3He/ 4He ratio in hot £uids which varies in a very nar- Direct current observations, spanning the time row range of 8.0 þ 0.5 Ra (relative standard devi- when the plume samples were collected, were car- ation V6%), in line with the range of most ried out with lowered ADCP. The results indicate 3 4 MORB He/ He data (7^9 Ra). that the local deep velocity ¢eld is strongly con- 3He/heat ratios for mature hydrothermal vents trolled by the topography [6] and show that the ^ mean value = (8.9 þ 3.6)U10318 mol/J (Table 2) entire plume is advected across the Rainbow sill ^ exhibit a higher variability than that of the he- situated immediately northeast of the vent site lium end-member, presumably in connection with (see Figs. 1 and 3). LADCP measurements made variations in the water^rock ratio which translate in the saddle of the sill are in good agreement into a variable rate of extraction of heat and 3He with those of the current-meter moorings de- from the hot basalt [16]. However, anomalously ployed during the cruise to monitor the water large short-timescale variations of the 3He/heat £ow across the valley section on the sill and re- ratio at a single vent site have been observed, covered in 1998 (i.e. after V400 days of opera-

EPSL 7030 25-3-04 Cyaan Magenta Geel Zwart 332 P. Jean-Baptiste et al. / Earth and Planetary Science Letters 221 (2004) 325^335 tion). This long-term monitoring shows that be- 3He concentration, we calculate that a £ux of low 2000 m, the direction and the intensity of the 490 þ 220 kg/s of high-temperature £uid is re- deep current advected over the sill are remarkably quired to sustain the 3He plume we observe. stable, corresponding to a mean water £ux of The above 3He £ux can also be combined with x = (0.070 þ 0.012)U106 m3/s [5]. This cross-valley the 3He/heat ratio of the £uids to derive a thermal section on the Rainbow sill is an ideal location for output of the Rainbow site of 1320 þ 600 MW. calculating the 3He £ux advected with the plume. This calculation assumes that 3He/heat ratios in The best estimate of the mean N3He integrated the focussed and di¡use vent £uids are identical. vertically across the sill between 2000 m and As a matter of fact, since both helium and heat are the sill depth is N = (12.3 þ 0.5)% (Fig. 6). Hence, conservative tracers, they may locally occur in con- 3 the hydrothermal He £ux Q3He transported stant proportions. Thus we think that the above 4 across the sill is given by Q3He = xU[ He]URaU assumption is valid. This value is comparable to 4 (N3NBG)/100, where [ He] is the helium concen- that for the Endeavour vent ¢eld on the Juan de 3 tration and NBG is the N He background. With Fuca Ridge, estimated around 1700 þ 1100 MW a mean measured helium concentration [4He] = based on advective transport of heat [24]. 3 3 1.75 þ 0.1 Wmol/m and NBG = 5%, the He £ux transported by the plume is Q3He =(12.3þ3)U 5.3. Hydrothermal tracer dispersal 1039 mol/s. Knowing the end-member vent £uid Most plume tracer data were obtained within a distance of V20 km from the vent site. This por- tion of the plume, which corresponds roughly to the isoline N3He = 7.5%, contains 0.021 3He moles in excess of the regional background (this ¢gure was computed from the grid ¢le used for the plot of Fig. 3, with a plume shape between 2000 m and 2300 m depth approximated by triangles). Dividing this plume 3He inventory by the 3He £ux transported by the dispersing plume, Q3He = (12.3 þ 3) nmol/s, leads to a bulk residence time of the plume in the rift valley of V20 days. Over this 3 3 period of time, the CH4/ He and Mn/ He ratios calculated for the dispersing plume (Fig. 4) and those measured in the vent £uids are identical, within the experimental uncertainties. This linear correlation among the tracers and the consistency of the ratios in the hot £uids and in the diluted plume show that, on this timescale, the di¡erent hydrothermal tracers behave conservatively with respect to one another in the plume and with respect to their source £uids. This is in agreement with available observations which show that the 2þ oxidation kinetics of Mn and CH4 are such that they behave conservatively close to the vents Fig. 6. N3He (%) vertical section across the sill located north- [25,26]. east of the vent site. Due to the lack of stations at the exact location of the sill, the section was reconstructed from the 5.4. Comparison with global hydrothermal £uxes horizontal grid ¢le used for plotting Fig. 3 (on the vertical, the shape of the pro¢le in the range 2000^2300 m was approximated by a triangle). The spatial pattern of 3He excess in the deep

EPSL 7030 25-3-04 Cyaan Magenta Geel Zwart P. Jean-Baptiste et al. / Earth and Planetary Science Letters 221 (2004) 325^335 333 ocean strongly points to the prominent role evidence for an average spacing between adjacent played by the mid-ocean ridges with respect to vent sites of 25^30 km along-axis [35]. Assigning 3He mantle degassing, with major water column this length scale to the Rainbow site leads to a N3He anomalies being centred above spreading 3He £ux of 0.5 þ 0.2 mmol per kilometre of ridge centres in the East Paci¢c [27,28], in the Indian per millimetre of spreading rate. This value is Ocean [29,30] and in the Atlantic Ocean [31].To comparable to that for the Lucky Strike segment the ¢rst order, 3-D oceanic general circulation (0.45 mmol/km/mm) [36]. Both sites are V40^ models make a good job at simulating the 3He 50% above the world average (0.33 mmol/km/ distribution with the simple assumption that the mm), thus supporting an earlier suggestion [35] 3He source intensity is linearly correlated with the that hydrothermal activity along this section of spreading rate [32,33]. The slope of the correlation ridge axis close to the Azores Triple Junction is is V330 3He mol/km2 of crustal spreading, trans- more intense than has been reported elsewhere on lating to a world average 3He annual £ux of 0.33 slow spreading ridges. mmol per kilometre of ridge and per millimetre/ year of full spreading rate (Fig. 7). The section of the MAR to which the Rainbow site belongs has 6. Conclusion a spreading rate close to 25 mm/year [34]. In order to plot the Rainbow site in the diagram dis- The main results of this helium isotopic study played in Fig. 7, one has to de¢ne the length of of the Rainbow hydrothermal site are as follows: ridge involved. While Lucky Strike and Menez 1. The Rainbow site, although hosted in serpenti- Gwen venting sites, situated further north, are in nised diapir, i.e. a rather unusual geological segment centres, the Rainbow site occurs within a environment compared to most already studied non-transform discontinuity between the South sites, displays a helium isotope geochemistry AMAR and the AMAR segment. Therefore, it very similar to ‘classic’ mid-ocean ridge sites. cannot directly be assigned a single segment 2. From the compilation of mid-ocean ridge sites length. On the other hand, surveys of hydrother- for which helium isotope data are available, we mal plumes on the MAR 36^38‡Nhave yielded come to the more general conclusion that, as far as helium geochemistry is concerned, the variability among hydrothermal sites is rather limited, with relative standard deviations of 34% and 6% for the 3He end-member concentration and 3He/4He ratio respectively. This re- sult points to the relative homogeneity of the helium isotope composition of the upper mantle, also noted in MORB studies. 3. The 3He budget leads to a plume residence time of V20 days in the rift valley. On this timescale, 3He correlates linearly with methane and manganese hydrothermal tracers throughout the dispersing plume. Furthermore, CH4/ 3He and Mn/3He ratios are identical in the plume and in the hot £uids showing that the Fig. 7. 3He annual £ux per kilometre of ridge as a function di¡erent hydrothermal tracers behave conser- of spreading rate. The dashed line is the linear relationship vatively with respect to one another in the used in general circulation model simulations [32,33]. Lucky plume and with respect to their source £uids. Strike and Red Sea values are calculated from [36] and [43]. 3He £ux transported by the plume could The global average corresponds to the classic value of 4 þ 1 4. The 3He atom/cm2/s estimated by Harmon Craig and his col- be deduced from the measurement of the water leagues 30 years ago [44]. transport across the sill which marks the en-

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trance of the AMAR segment, allowing us to Lane-Ser¡, K.G. Speer, Flow and mixing in the rift valley infer a thermal output of 1320 MW. Trans- of the Mid-Atlantic Ridge, J. Phys. Oceanogr. 32 (2002) 1763^1778. lated into a £ux per kilometre of ridge and [6] C.R. German, K.J. Richards, M.D. Rudnicki, M.M. 3 per millimetre of spreading rate, this He £ux Lam, J.L. Charlou, the FLAME Scienti¢c Party, Topo- (0.5 mmol/km/mm) is comparable to the £ux graphic control of a dispersing hydrothermal plume, calculated for the neighbouring Lucky Strike Earth Planet. Sci. Lett. 156 (1998) 267^273. segment (0.45 mmol/km/mm). Although the [7] H. Bougault, M. Aballea, J. Radford-Knoery, J.L. Char- V lou, P. Jean-Baptiste, P. Appriou, D. Needham, C. Ger- uncertainties are large, both £uxes are 40^ man, M. Miranda, FAMOUS and AMAR segments on 50% above the world average (0.33 mmol/km/ the Mid-Atlantic Ridge: ubiquitous hydrothermal Mn, 3 mm), thus showing that the MAR south of the CH4, N He signals along the rift valley walls and o¡sets, Azores discharges substantially more than its Earth Planet. Sci. Lett. 161 (1998) 1^17. share of the global hydrothermal input to the [8] P. Jean-Baptiste, F. Mantisi, A. Dapoigny, M. Stievenard, Design and performance of a mass spectrometric facility ocean. for measuring helium isotopes in natural waters and for low-level tritium determination by the 3He ingrowth method, Appl. Radiat. Isot. 43 (1992) 881^891. Acknowledgements [9] P. Jean-Baptiste, J.L. Charlou, M. Stievenard, J.P. Don- val, H. Bougault, C. Me¤vel, Helium and methane measurements in hydrothermal £uids from the Mid-Atlantic We would like to thank D. Pinti, M.D. Rud- Ridge: the Snake Pit at 23‡N, Earth Planet. Sci. Lett. 106 nicki, and one anonymous reviewer for their con- (1991) 17^28. structive comments on the manuscript. This re- [10] J.L. Charlou, Y. Fouquet, J.P. Donval, J.M. Auzende, P. search was undertaken as part of the Amores Jean-Baptiste, M. Stievenard, S. Michel, Mineral and gas project supported by the European Commission chemistry of hydrothermal £uids on an ultrafast spreading ridge: East Paci¢c Rise, 17‡S to 19‡S (Naudur cruise) ^ (MAST programme under Contract MAS3- phase separation processes controlled by volcanic and tec- CT95-0040).[BARD] tonic activity, J. Geophys. Res. 101 (1996) 15899^15919. [11] J.L. Charlou, J.P. Donval, E. Douville, P. Jean-Baptiste, J. Radford-Knoery, Y. Fouquet, A. Dapoigny, M. Stie- References venard, Compared geochemical signatures and evolution of Menez Gwen (37‡50N) and Lucky Strike (37‡17N) hy- [1] C.R. German, G.P. Klinkhammer, M.D. Rudnicki, The drothermal £uids, south of the Azores Triple junction on Rainbow hydrothermal plume, 36‡15PN, MAR, Geophys. the Mid-Atlantic Ridge, Chem. Geol. 171 (2000) 49^75. Res. Lett. 23 (1996) 2979^2982. [12] J.L. Charlou, J.P. Donval, T. Zitter, N. Roy, P. Jean- [2] Y. Fouquet, F. Barriga, J.L. Charlou, H. Elder¢eld, C.R. Baptiste, J.P. Foucher, J. Woodside, MEDINAUT Scien- German, H. Ondreas, L. Parson, J. Radford-Knoery, J. ti¢c Party, Evidence of methane venting and geochemistry Relvas, A. Ribeiro, A. Schulz, R. Apprioual, P. Cambon, of brines on mud volcanoes of the eastern Mediterranean I. Costa, J.P. Donval, E. Douville, J.Y. Landure¤, A. Nor- Sea, Deep-Sea Res. I 50 (2003) 941^958. mand, H. Pelle¤, E. Ponzevera, S. Riches, H. Santana, M. [13] W. Jenkins, Tritium and helium-3 results: TTO-NAS and Stephan, FLORES diving cruise with the Nautile near the NATS 1981, data release #3, WHOI Helium Isotopes Azores ^ ¢rst dives on the rainbow ¢eld: hydrothermal Laboratory, 1989. seawater/mantle interaction, InterRidge News 7 (1998) [14] C. Andrie¤, P. Jean-Baptiste, L. Merlivat, Tritium and he- 24^28. lium-3 in the northeastern Atlantic ocean during the 1983 [3] E. Douville, J.L. Charlou, E.H. Oelkers, P. Bienvenu, Topogulf cruise, J. Geophys. Res. 93 (1988) 12511^12524. C.F. Jove Colon, J.P. Donval, Y. Fouquet, D. Prieur, [15] M.D. Rudnicki, R.H. James, H. Elder¢eld, Near-¢eld P. Appriou, The Rainbow vents £uids (36‡14PN, MAR): variability of the TAG non-buoyant plume, 26NMid-At- the in£uence of ultrama¢c rocks and phase separation on lantic Ridge, Earth Planet. Sci. Lett. 127 (1994) 1^10. trace metal content in Mid-Atlantic Ridge hydrothermal [16] J.E. Lupton, E.T. Baker, G.J. Massoth, Variable 3He/heat £uids, Chem. Geol. 184 (2002) 37^48. ratios in submarine hydrothermal systems evidence from [4] J.L. Charlou, J.P. Donval, Y. Fouquet, P. Jean-Baptiste, two plumes over the Juan de Fuca ridge, Nature 337 N. Holm, Geochemistry of high H2 and CH4 vent £uids (1989) 161^164. issuing from ultrama¢c rocks at the Rainbow hydrother- [17] E.T. Baker, J.W. Lavelle, R.A. Feely, G.J. Massoth, S.L. mal ¢eld (36‡14N, MAR), Chem. Geol. 191 (2002) 345^ Walker, Episodic venting of hydrothermal £uids from the 359. Juan de Fuca Ridge, J. Geophys. Res. 94 (1989) 9237^ [5] A.M. Thurnherr, K.J. Richards, C.R. German, G.F. 9250.

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