CH4 hydrothermal plume signatures: TAG site at 26°N and serpentinized ultrabasic diapir at 15°05′N on the Mid-Atlantic Ridge

Geockrmrca et Cosmockimica Copyright 0 1991 Pergamon

0016-7037/91/$3.Otl

Acta Vol. 55, pp. 3209-3222 Press pk. Printed in U.S.A.

+ .@I

Different TDM/CH4 hydrothermal plume signatures: TAG site at 26”N and serpentinized ultrabasic diapir at 15’05’N on the Mid-Atlantic Ridge J. L. CHARLOU,’ H. BOUGAULT,’ P. APPRIOU,’ T. NELSEN,~and P. RONA~ ‘Dtpartement Gtosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 PlouzanCcedex, France 2Universit&de Bretagne Occidentale, Avenue Le Gorgeu, 29287 Brest cedex, France ‘NOAA-AOML-OCD,430 I Rickenbacker Causeway, Miami, FL 33149, USA (Received March 2 1, 1991; accepted in revised form August 30, 199 I) Abstract-As

a part of the 1988 NOAA VENTS Program, CH4 and Mn tracers were used to identify and compare hydrothermal plumes found above the TAG Field (26”N) and in the rift valley at 15”N close to the eastern intersection of the ridge axis with the 15” 20’N Fracture Zone at the Mid-Atlantic Ridge (MAR). Active hydrothermal venting was confirmed at TAG, based on elevated concentrations of total dissolved Mn (TDM up to 30 nmol/kg), high CH4 concentrations (up to 200 nL/L), and elevated nephelometry signals. Plumes of a different composition were identified at 15“N with high CH4 concentrations (up to 400 nL/L), low total dissolved Mn concentrations (TDM < 1 nmol/ kg) and no significant nephelometry signal. The different properties of these tracers and the different tracer ratios can be used to deduce vent fluid characteristics and compare one hydrothermal area to another. TDM / CH4 and Nephel/CH4 ratios at TAG are of the same order of magnitude as those observed at other spreading axis hydrothermal fields. At 15”N, the low TDM /CH, ratio provides evidence of fluid circulation into ultrabasic rocks and offers a potentially useful and single method of exploring for hydrothermal activity associated with serpentinization. Mantle degassing through hydrothermal activity associated with serpentinization is an important process with respect to chemical and thermal exchanges between the upper mantle and the ocean. Different ratios of hydrothermal tracers (i.e., TDM/CHI) provide a useful framework for identifying subseafloor processes along mid-oceanic ridges. INTRODUCTION

(RONA et al., 1987) and the French Ridelente cruise ( BOUGAULT et al., 1989, 1990a,b), in the inner floor of the rift valley, close to the eastern intersection between the rift valley and a major fracture zone ( 15’20’N F.Z.) where indicators of ongoing hydrothermal venting are associated with seafloor exposures of subcrustal rocks (serpentinized peridotite from the lower crust and upper mantle; Fig. 1). Four types of complementary operations were performed on the cruise under acoustic navigation. Bathymetric profiles were taken along selected tracklines at the two study sites to complete the maps established during the previous cruises (RONA et al., 1984b, 1987; BOUGAULTet al., 1989, 1990a,b). Camera-temperature tows were conducted to locate seafloor hydrothermal springs by imaging associated features and by measuring near bottom water temperature anomalies, and to characterize the geological settings of the vent areas ( RONA et al., 199 1) . Physical anomalies obtained with CTD/ Nephelometer profiles and geochemical ( CH4, Mn) anomalies in water samples enabled us to localize hydrothermal activity. Sediment cores were obtained in some areas of interest to determine the distribution of metalliferous components from hot springs and the chronology of hydrothermal events recorded in the sediments. We emphasized recently that the different properties of the tracers and the different tracer ratios were very useful in comparing one hydrothermal area to another. In addition, some discrepancies between tracers have already been recorded around a hydrothermal field or between hydrothermal sites located in different geological contexts ( LUFTON et al., 1989; CHARLOU et al., 1990; 1991a). General results from the MAR/88 cruise are presented in RONA et al., (1988) and RONA et al., ( 199 1). The purpose of this paper is to present

PREVIOUSEVIDENCEfor hydrothermal activity on the MidAtlantic Ridge (MAR) from 10” to 26’N is substantial. It has been documented by hydrothermal manganese concentrations in ocean water ( KLINKHAMMERet al., 1985), temperature anomalies ( LOWELLand RONA, 1976 ) , manganese deposits (SCOTT et al., 1974), heat flow anomalies in the sediment column, photographic evidence (RONA et al., 1984a,b, 1986, 1987; ODP Leg 106, 1986), dredged rocks and enriched metalliferous sediments (CRONANet al., 1979), 3He/4He ratios in the water column (JENKINS et al., 1972, 1980), photographs of vent organisms ( RONA et al., 1984b; KONG et al., 1985 ) , and more recently, from CH4 anomalies in the seawater column between 12” and 26”N (CHARLOU et al., 1987, 1988a; BOUGAULTet al., 1989, 1990a,b). During the 1984 NOAA Mid-Atlantic Ridge cruise and the IFREMER Ridelente 1988 cruise, which was dedicated to surveying and identifying the frequency of hydrothermal plumes, the stations were selected on the basis of various indicators of ongoing or previous venting activity including Mn, 3He, and CH4 in the water column, hydrothermal mineralization on the seafloor, and a magnetic signature characteristic of the hydrothermal alteration of oceanic basalt (RONA et al., 1984b, 1987; BOUGAULTet al., 1990a,b). The first scientific objective of the MAR/88 cruise (Aug. 12 to Sept. 4, 1988; Fig. 1) was to delineate the hydrothermal plume in the water column and venting sources on the seafloor at the TAG Hydrothermal Field in the rift valley of the MAR near latitude 26”N (NELSENand FORDE, 1990; 199 1). The second scientific objective was to study the new target site of 15’OO’N previously investigated during an NOAA cruise 3209

Charlou et al

3210

MORPHOTECT’ONIC

The TAG Hydrothermal Field is located at 26”08’N-44”49’W between the Kane and Atlantis Fracture Zones and occupies 100 km* on the east wall of the MAR rift valley (Fig. 2). At this site, the ridge is spreading asymmetrically at an average of 1.3 cm/y to the east and I. 1 cm/y to the west ( RONA et al., 1986). In cross section, the east wall of the ridge rises up from the rift valley floor at 4000 m to a crest at 2000 m ( RONA et al., 1986); the wall is composed of a series of fault blocks 1OOmhigh and hundreds of meters wide. Each block contains additional minor fault blocks IO m in height and width. The geological setting of the TAG (26”N) area, along with initial discoveries of hydrothermal venting from sources along fault zones at depths from 2500 m to 3700 m. is described by RONA et al., ( 1984b. 1986). Water column surveys by combined nephelometer/CTD tows contributed to the 1985 discovery of the first black smokers on the MAR (NELSEN et al., 1986/87). A preliminary description of the hydrothermal plume observed in 1985 has been presented elsewhere (NELSENand FORDE, 1987), but can be.summarized here as being centered at 3420 m over the study area, averaging 260 m thick and thickening to >400 m near the source discovered in 1985. In 1988. a newly documented plume, south ofthe 1985 study area, was estimated to have a mass nearly an order of magnitude greater than the one observed in 1985 by NELSENet al., ( 1988). who hypothesized that it emanated from a separate source. Detailed examination of the 1988 data provided not only the general plume survey data stated above, but also additional information for illuminating plume behaviour in this region (NELSENand FORDE,1991). Regional water column mapping has helped define the dispersal gra-

FIG. 1. Location of TAG Field and 15”20’N Fracture Zone on the Mid-Atlantic Ridge (MAR). The two studied areas are located respectively at 26”08.19”N-44”49.57’W and at the eastern part of the 15”20’N F.Z.-ridge axis intersection (15”N area). the methane and manganese data obtained in TAG and 15”N plumes in order to compare the geochemical behaviour of the two tracers above the two areas.

44054

45000

-r-

-I---MID ATLANTIC

RIDGE (26

TAG AREA

SETTINGS

44048

44O42

-~- ~-r-

-1-

44;36

Ml “i

26”

26’1 36 .

26°C

-26V.l

FIG. 2. Seabeam bathymetry (200 m isobaths) of TAG area on the MAR at 26”N (from P. RONA, NOAA). The star indicates the site of previously discovered hydrothermal discharges. (26”08.19’N = 26.137”N; 44”49.57’W = 44.826”). The square around the star represents the area where all MAR/88 casts were towed in a 600 m seawater layer above the seafloor.

3211

Hydrothermal plumes above the TAG field dients, the extent, mass, and interactions of the hydrothermal plumes from the known and other nearby sources. The optical signatures of the plumes observed during 1985 and 1988 have shown that a wide variety of thicknesses, concentrations, and structures exist in the 25 30 km2 study area (Fig. 2, 3). Analysis of the gradients and trends allowed not only definition of the size and extent of the hydrothermal plume, but provided insight into the fundamental nature of the plumes (NELSEN and FORDE, 199 I ) . Three distinct plumes were identified: the one associated with the known source ( RONA et al., 1986). a southern plume, and a smaller, less well documented one believed to have an origin on the east wall of the rift valley (Fig. 3A, B. C, D). The l5”N area is part of the MAR rift valley immediately south of the eastern intersection between the 15”20’N F.Z. and the ridge axis (Fig. 4). The axes of the l5”20’N F.Z. and of the inner floor of the rift valley are indicated on Fig. 4. The main ridge axis meets a mount centered at 15”05’N-44”59’W; this mount is about 8 km long and 4 km wide and is elongated north-south; its summit is 1500 m above the inner floor of the valley. It will be designated further on as mount A. The periodicity of this type of structure with time can be observed along the edge of the fracture zone, west of the ridge axis (A, B, C). Another periodicity is also observed along the western wall of the valley where two similar bodies are easily distinguishable (B, D). The map in Fig. 4 was made during the Ridelente cruise of the R/V J. Charcot in 1988 (BOUGAULTet al., 1989, 1990a,b). By analogy with other similar structures on the MAR at 14”43’N-

45”OO’W (2~D-46 dredge, R/VAkademik Boris Petrov, 1985), at 15”36’N_46”35’W, and 16’52’N_46”27’W (respectively, RD-87DR-08 and RD-87-DR-13 dredges, R/V J. Charcot. 1988), which are well documented to be part of the inner floor of the rift valley on which ultrabasic serpentinized rocks were dredged (BOUGAULT et al., 1989, 1990a,b), we thought that the mount A was also a serpentinite structure. This assumption was verified recently during a cruise by the R/V Akademik Boris Petrov in November 1990 (SILANTIEV,pets. comm.), three dredges, two on the top and one half way from the inner floor to the top, brought back serpentinized ultramafics. In addition, one dredge on the top of mount B also recovered serpentinites. We will not enter into a detailed discussion on the mechanisms responsible for the construction of these structures ( BOUGAULTet al.. 199 1)) which are common features along the MAR, and originate in the inner floor of the valley. The combination of slow mantle upwelling (high conductive heat loss and little melt production) and hydrothermal serpentinization (resulting in lower density) allows these mounts to be tectonically emplaced and controlled by the normal fault system. When they are mature, they are part of the rift valley wall system, which is the case of mount A. Further investigations to locate hydrothermal structures were also undertaken in the rift valley at about 14”55’N, close to the eastern wall of the valley where serpentinized ultramafic rocks were previously recovered ( RONAet al., 1987 ) We hope to be able to complete the map for the eastern part of this rift valley-fracture zone intersection in the near future.

C

A

1

26.

26.1

26.

26.

26.1

26.134, &

et0

&

0+

t+

Otb

FIG. 3. Water-column cast locations for 1985 (A, B) and 1988 (C, D). (A) Main study area showing hydrothermal mound (small circle), southern hydrothermal anomaly area (E-E’), southern plume onset (star) from 1988 survey, near-field mound area (dashed box) and I km reference circle; (B) Near-field mound area showing horizontal (H) and upcast (U) through black smoker field; (C) Cast tracklines for northern portion of area shown; (D) Study area southwest of “c” above showing onset of southern plume (star) and latitudes of casts 25D ( 1985) and 7U ( 1988) 1.7 km on east wall of rift valley. (This figure is from NELSEN and FORDE, 199 1)

3212

15-10

Charlou et al.

5”lO’N

1905

15“oc

14%

FIG. 4. Water column stations conducted on the east wall and axial mount close to the 15”20’N F.Z.-Ridge axis intersection located on the simplified Seabeam map established during Ridekntr cruise ( 1988). The solid lines represent the CTD tracks and the open squares are the up-casts at the end of tows. KOI (14”53.8’N-45”01.6’W) and K02 ( 14”55.0’N-44”90,0’W) are lowerings of Klinkhammer at station 9 of&&IR/85 cruise. Hy-04 is a CTD station performed during RidPienre cruise ( 19881. The dashed lines represent the 15’20’N F. Z. and the Ridge axis. A. B. C. D are mounts rising up to 1500 m above the inner floor of the valley. A and B are serpentinite structures.

Hydrocast CTD locations or tracklines conducted at TAG and 15”N areas are indicated respectively on Figs. 3 and 4. The strategy on each lowering was either to trip the bottles at one location and several depths in order to obtain a vertical profile or to collect discrete samples at the depth of the maximum nephel at different locations. Some lowerings involved a combination of these two strategies. Methane in seawater can be anatysed by a multiple phase equilibrium method or a trapping method. The first technique, based on successive gas chromatographic analysis of a headspace repeatedly equilibrated with the solution, was developed by MCAULIFFE( 197 1) and has been used by several investigators ( ELKINS. 1980, BULLISTER et al.. 1982; KIM, 1983). We chose, however, the alternative trapping method ( SWINNERTON et al., 1962; SCRANTONand BREWER,1977: BAROSSet al., 1982) which has previously been used at sea (CHARLOU et al., 1987, 1988a. 199 la,bf and allowed us to work on smaller volumes ( 125-250 mL). The seawater samples, obtained by using General Oceanics Niskin or Go-Fio bottles mounted on a Neil Brown CTD/rosette equipped with a nephelometer, were transferred by gravity flow into glass bulbs with Teflon stopcocks. The bottles were filled from below and allowed to ovefiow vertically about one third volume in order to avoid trapping air bubbles. For long storage of

the sampies.it was necessaryto inhibit microbial activity by poisoning all samples with sodium azide when collected. Methane analyses were performed immediately on receipt in the laboratory f Brest, France). Seawater samples were transferred under methane free helium from the 125 mL glass bulb to a gas stripping system. Dissolved gases were stripped from the seawater by purging with purified helium for ten minutes at a flow of 120 mL/min and were concentrated on two 3/ 16”o.d. stainless steel traps containing, respectively. activated alumina for trapping hydrocarbons heavier than CH, and activated charcoal for trapping CHI and CO. For this work, only CHI was analysed. When stripping was completed, a six-way gas valve was turned to place the activated charcoal trap in line with a 4 ft l/g”o.d. stainless

steel column packed with a 60-80 mesh Porapak Q. The trap was placed in a hot bath at 100°C. By raising the trap temperature. the CH4 was desorbed from activated charcoal and injected into a chromatographic column placed in the oven of a D&i instruments chromatorrraoh eauinued with a flame ionization detector. Peaks were recorded and-i&grated on a ICRl-B Shimadzu integrator. System calibrations were made by injecting known values of two calibration standard gases ( ALFA gas standard 2.1 ppm f 2% and 10 ppm f 2% in ultra pure helium). The limit of detection of the method was 0.5 nL CH, per liter of seawater ( 1 nmol = 2.24. IO-’ mL CH4 STP). The precision was ?3% over a 3-150 nL/L range. ~on~mination by air during storage is assumed to be small: the effect of storage for periods of a few weeks to several months before analysis were studied for Caribbean Sea and Gulf of Maine poisoned samples (SCRANTON and BREWER,1977) and on poisoned Pacific water samples (CHARLOU et al., 1987). The comparison of results obtained at sea with those obtained later in the shore based laboratory showed small variations. All samples with high CH, concentmtions (2 to 8 tmoI/kg) lost CH, (up to 15%). The deep samples with low CH4 concent~tions (0.2 to 1 nmol/kg) gained methane (2 to 5%). Samples which were nearly at equilibrium with the atmosphere (3.4-3.7 nmol/kg) seemed to store well. However. as will be seen, inaccuracies of this order of magnitude do not influence interpretations. CH4 results are given for all CTD/rosette casts in Tables I and 2. Total dissolvable manganese (TDM ) concentrations were measured on the same samples. according to a procedure previously described hy KLINKHAMMER( 1980). TDM is the manganese extracted from acidified and filtered seawater samples by oxime in chloroform after pH adjustment to 9. TDM was measured on shore using a Scintrex AAZ-2 Zeeman corrected atomic absorption spectrophotometer. Blanks were less than 0.02 nmol/kg and the precision was better than 10%. TDM results are given for all C’TD/rosette casts in Tables 1 and 2.

EVIDENCE FOR HYDROTHERMAL ACITVI’IY ALONG THE MID-ATLANTIC RIDGE FROM GEOCHEMICAL TRACERS BETWEEN 15” AND 26“N

En~chment of CH., in surface waters is a common feature. Concentrations of CH4 measured in samples from the mixed layer are commonly higher than concentrations predicted from the solubility of CHI in seawater and known atmospheric concentrations ( LAMONTAGNE et al., 1973; BRINKS and SACKER, 1973). Methane maxima in the upper seawater column indicate the existence of biological methane production at rates much faster than physical removal (i.e., diffusion to the atmosphere) and chemical or biological consumption. CH4 contents decrease regularly with depth, from 50% supersaturated concentrations in surface layers, where the biogenie CH4 production is very important, to S-10 nL/L at 1000 m. In deep waters, typical CH, concentrations are about 4 nLf L in the Pacific and 8 nL/L in the Atlantic ( LAMONTAGNE et al., 1973: CHARLOU et al.. 1987, 1988a, 1991a). Few accurate data exist for manganese in the Atlantic Ocean (STATHAM and BURTON, 1986; BRULAND and FRANKS, 1983) away from the Mid-Atlantic Ridge. The general vertical distribution of dissolved manganese in the North Atlantic Ocean is similar to that reported for the Pacific Ocean, with high surface concentrations rapidly decreasing over the top few hundred meters to generally low and uniform deep water concentrations (
3213

Hydrothermal plumes above the TAG field TABLE 1. Salinity, CH,, and TDM values versus depth at stations

I I I , P0S.I Depth , I---I

I

I

-I I

I I

35.033

I , 35.033

I 13.0

0.20

,

1500

35.131

l 35.131

I 11.5

0.20

I

1501

35.139

I 35.134

, 13.0

0.24

,

1000

35.139

, 35.136

I 11.0

0 22

I

35.151

I ,

I

1000

I

I

I 21

2003

I

31

I

II

I

51

II

I

61

I

71

I

81

I I

91

I

101

501

35.850

I , 35.850

I , 32.0

I

121

501

35.851

, 35.652

I 33.0

CTD # 02

LONG:

44'49.6'W

-1 I

-I I

I

I I

I

I

(ppt) cm

I

21

2998

I l 34.950

I l 34.950

I

31

2996

, 34.960

l 34.950

I 14.6

I

I

41

I

I

I

I

I ,

51 6 I

I

I I

I

I

71

I

81

I

91

2500 2500

,101 I

I

I I

, 34.973

, 10.6

I

I

I

i

I

I

I

I

I

I

I

I

Depth =

3387m

, bl

I I

LAT:

Salinity

bottle

CTD I

_,-I_,_,

I ,

3305

,

2

3301

I

31

I

41

I

51

I

61

I

71

I I

l

81 91

,

10

I

12,

,

3260 2500

I

I

I

I 29.50

,

I

I

I

I

l 34.934

,

I I I

I

I ,

I

, 34.935

I I

I

I

I

I

I

I

, 34.966

, 34.970

,

I

I

I

I

I

I I Depth I , bnl ,

I 1, 31

*I 51

I I

61 7,

I

I

81

,

PI 10 l

I

121

bottle

3715

I , I

3412

I 34.934

21

I I

Salinity

34.929

CTD

I

UT:

26'OB.l'N 44'49.4'W

(ppt) I

I

LONG:

3732m

I I I I CHI I T!JMI I d/l Imd/kg, -1-l

I

I

I 34.930

I 10.0

I I 34.934

I 1149.5

I

I

I I

1.00

I I

22.80

I ,

I

I

3438

34.932

,

,201.o

28

3478

34.938

, 34.932

28.60

34.928

I

1198.0

3464

34.933

1188.5

31

3461

34.929

I l 34.933

I 1173.0

25.70

I I

3418

34.929

I 34.934

1178.0

30.20

I

I

I

-1

34.935

, 34.936

1186.0

, 30.00

I

I

I

I

I

I

I , 34.946

I l 34.939

I 1168.0

I

3318

I I

I l 34.938

I I 87.0

I

3378

I , 34.958

I 22.80 I 12.00

I I

I

3377

, 34.936

l 34.938

1145.0

I 23.90

I

I

3239

, 34.939

l 34.944

, 24.0

I

I

I

I

I

I

I

LAT:

Depth = 3480rn

I

I en,

I

Salinity

I bottle

,pptj cm

I

2.30

I

26'09.4'N 44'48.8'"

I

I

I Depth

I

, CHI I TDHI I Ill/l Inmol/kgl

I

I 3414

, 34.946

34.936

1194.0

3413

I 34.937

34.936

,

68.0

5

3412

34.930

34.934

,210.o

31.60

6

3478

I 34.929

34.932

1190.0

28.00

8

3414

34.930

34.934

1148.0

3380

34.946

34.935

I 16.0

3347

34.931

34.936

,185

0

19.10

,

30.00

,

12

I I

I

Depth =

I-l-l-l-l I

, I I

I I

I I

I

CTD#O6 I , PQS.,

I

/

, 34.932

9

9 10

1185.0

I

I 27.40

3340

12

I

-I

I

I

3327

10

I , Pal.,

Imol/kgl

l 34.934

34.949

I

I

, 34.928 , 34.934

I I

4

I

I I I I I , I

I I

I

I

d/l

I I 1153.0

LONG:

44'49.7'W

-,

I 1 ,

I ,

8

(ppt) I CHI I TDH I

,

I I , 34.938

CTD # 08

26'08.1'~

I

I

I 0.20

I

I

I

LONG:

I

I

34.976

CTD # 03 I I I POS., Depth

I

I

-I

I

71

I I

, 34.978

I

121

0.31

I I 34.971

I I

6

I

I CHI I TDMI

I

31

I

I , 34.946

51

I

I

Salinity

1 I 2 I 41

Inmol/kgl

I

~,_I_,_, I I I II

(rn) I

I d/l

, bottle

,rn)

I

Salinity (pptj I CHI I Tml I bottle I Cl-D , n1/1 Inmcllkgl

_I_‘-I--.--I

-I-

0.25

I

I

LAT. 26'08.3'N LONG: 44'49.8'W I

I

I I

Depth = 3691rn

Depth

POJ.

I...'?:26'08.4'N

Depth = 3652~~

I I I PQ..I Depth

CTD # 07

/ 13.6

/

I

I

,ppt)

-I-I-

I I I

TAG (26”N) area

I CH4 I TDNI CTD , d/l Inmol/kg~

Salinity

, (In, , bottle I

I

in

CTD # 09

IAT:

Depth = 384%

LONG:

I I ,_,-I

I

44'50.8'W

I

I Salinity battle

Depth

Pal.,

',?6'07.8'N

I

,rn)

_I_,-,_

I

,

(pptj I CH4

Cm ,

I

nl,l

I

I

TDH

Imollkg

I

1

I

3263

34.932

34.936

1200.0

26.70

,

2 3

I I

3345

34.929

34.933

1162.0

19.80

I

4

I

3255

34.930

34.935

1169.0

25.30

l

5

I

3443

34.958

34.934

, 92.0

17.00

l

3380

34.936

34.933

,

17.40

,

3320

, 7 I 8I 9 I 6

34.966

, 34.936

, 44.0

3440

34.932

l 34.934

I

3359

34.931

I

1140.0

10

34.937

35.0

6.20

12

I

3314

34.931

I 34.937

1106.0

16.10

,

18.60

80 20

-I-

a sub-kilometer scale and suggests the existence of plumes originating from the rift valley walls ( KLINKHAMMER et al., 1985, 1986; HYDES et al., 1986). Hydrothermal TDM is advetted away from the particle-rich halo surrounding vent fields and persists some time despite the relative instability of Mn2+ in seawater. It can be detected as small anomalies

far from the source. High manganese anomalies shown to have strong positive correlations with particles (NELSEN et al., 1986/87) and methane ( CHARLOU et al., 199 la). Typical background CH4, TDM and nephelometry in the seawater shown in Fig. 5.

have been suspended anomalies profiles of column are

3214

Charlou et al. TABLE2. Salinity, CH4, and TDM values versus depth at stations in the 15”N area. Lmpttl = 3757m

CTDXlO

34.909

i 34.914 i il.6 , 31.918 , 22.5 34.922 , 27.5 I 31.931 I 26.0 I 34.939 I I 31.917 16.0 31.951 60.0

31.926

34.918 , I 31.945 I I , I I I I 31.912 2600 2502 I 31.917

I 81 I 91 , 10 I I 12 I I -I-1-I-I

I

, I ,

2I 3 (

, , I

I

I , 51 6 7 8 I

I

10 I

I I

I I

I

9,

, 12 , I-I_I_I

I

I-I

1

,

2960

I

2

I

2952

I

3 I I , 5 I 61

I I

, I

0.83

I I 0.35 , 0.61

,

=

2964m

I

i

i

I

;

34.935 34.937 31.932 I 36.936 I 34.930 I 34.939 I 34.942 34.946 34.943 34.919 I 34.948 34.953

I

I I I I

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=

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(ml

3295 3219 3119 3018

2850 2800 2700 2603

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-I-_-----I-I

31.942 34.940

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I 2500 , 31.950 2302 34.958 2097 34.972

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, 31.932 I 79.0 I , 34.937 I 60.0 I I I I I I I , 34.941, 20.0, , 34.946 I 72.5 I , 31.962 I 7.0 ,

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I 0.35 , I 0.20

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I 32.0 I 39.0 I I 42.5 I 42.5 I I I 59.0 I 17.0 , 21.5 I 79.0 I

----I-I

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71 6 ,

3190 3140 3090 30.0 2990 2890 2790 2689

i

31.923 34.933 34.933 34.935 34.934 34.939 34.939 34.950 31.943 2191 I 31.951 2389 34.952

i I I I I I

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I I , 9 , 2591I I 10 , I 12 , I

I

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rmpth =

I

I I , I , I I , , , ,

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CTD # 17

I

, C-II

mn

I I I I I

34.925 34.936 34.937 34.939 34.937 34.940 31.940 34.945 34.953 34.954 31.955

i 10.0i 0.33 i I 0.40I 0.39 , I 0.38 I 0.40 ; 0.55 I I I 0 42 I i a.o i 0.58I I 47.0 I 0.59I I 18.0 , 0.51 ; I 11.0, 0.55, I 12.0 I 8.0 I I 7.0 I 7.0 i , 25.0 I 7.6

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,

I

_I. I I I II.0 I 12.0 1170.0 1395.0 13.0

, I 14.5 1152.0 1138.0 1131.0 1129.0

0.80

Depth =

, 0.80I 1.19, 0.40, 0.75

0.46 , 1.01 , 0.97

, 1.05I I 0.92 I I 10.0I 0.10I

LIT: 15'05.7'N LONG: 66'57.0'"

I

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I 0.85 I 0.99 I 0.38

2512 31.915 31.949 1160.0 2398 , 31.954 I 31.956 I 30.0 I 2299 , 34.961 I 31.962 I 8.5.1 2199 , 34.976 34.970 , 7.0 I I I I 1999 I 34.975 34.978 I 7.0 I 1802 I 34.986 34.987 , 7.0 I

0.46 I 0.20 I 0.22 I I 0.25 , 0.43 I

I I I

Along the Mid-Atlantic Ridge, the geochemical anomalies in the water column (Mn, KLINKHAMMERet al., 1985, 1986; C&, CHARLOUet al., 1987,1988a, 1990,1991a,b) associated with nephelometry data ( NELSENet al., 1986/87) have demonstrated the existence of high temperature venting and permitted the location of active sites. CH4 anomalies up to 44 nL/L, located around 3400 m depth, have been previously found between 12” and 15”N (CHARLOUet al., 1988a). Over the TAG hydrothermal field (26’N), CH4 anomalies, reaching up to 105 nL/L in the plume 400m above the seafloor and 2422 nL/L in samples taken 5 m above the black smokers area with a large positive increase of in situ temperature (0.349”C), were measured in MAR/85 samples (CHARLOU et al., 1987). These CH4 anomalies are correlated with temperature anomalies ( RONA et al., 1986 ) , excess of dissolved manganese ( KLINKHAMMER et al., 1985), nephelometry anomalies (NELSEN et al., 1986/87), particulate iron, and excesses of total suspended matter (TREFRY et al., 1985; TROCINEand TREFRY, 1988). In previous work, JENKINSet al. ( 1980) observed significant 3He anomalies in bottom waters over the TAG hydrothermal field. Excess of the primor-

, I I

, I

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I , I I I 0.98I

31.894 31.905

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0.61 , 0.12

,

0.68 0.73 0.97

Lm: 11'57.O'N LONG: 11'53,l'W

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I , , , I I I I I

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-~-~-~-~

dial inert isotope ‘He derived from mantle degassing was measured in all water samples recovered in 1976 and 1977 within 100 m above the seafloor within the hydrothermal zone of the eastern wall of the rift valley. The 3He measurements provided good evidence for subseafloor hydrothermal convection with ongoing hydrothermal discharge at that time. CH4 concentrations along the 27 stations of the Ridelente cruise between 15”N and 23”N ( CHARLOU et al., 1990; CHARLOUand DONVAL,199 1) confirm previously measured anomalies, which were of the same order of magnitude as those found above hydrothermal sites on the EPR ( CHARLOU et al., 199 1a), in the North Fiji Basin near the triple junction centered at 173”30’E and 16”4O’S (AUZENDE et al., 1988; NOJIRI et al., 1989); or in the Lau Basin between 21”25’S and 22’40’s ( FOUQUET et al., 1990, 199 la,b; CHARLOUet al., 199 1b). Black smoker type hydrothermal venting was discovered at 26’N ( RONA et al., 1986) and later at 23”N (ODP Leg 106-1986) in the rift valley of the Mid-Atlantic Ridge. The first fluids emanating from these active sites, the only two known on the MAR at this time, were collected by submersible. The mineral (CAMPBELLet al., 1988; CHARLOU

3215

Hydrothermal plumes above the TAG field 0

, 20

40

(-j

NEPHELS

0

0.5

1.0 (@\ 1

TDM (nmollkg)

0

50

100 (0)

CH4 (WI)

-c

between 3500 and 2500 m depth with a maximum of 80 nL/ L at 2900 m depth, consistent with the previous Mn and 3He data. In the same way, Mn profiles made during this cruise showed small anomalies slightly over 1 nmol/ kg. MAR /88 RESULTS TDM and CH, in Plumes at TAG field

I-

2000

2500

CTD - 0~02

3000 FIG. 5. Typical CH4, TDM, and nephelometry profiles in the Atlantic Ocean without detected hydrothermal anomaly. Reference station 01/02.

et al., 1988ly

DONVAL et al., 1989) and gas composition (JEAN BAPTISTEet al., 1990, 1991) of fluids and associated sulfides (THOMPSON et al., 1988) investigated at these two sites are similar to those collected on the EPR. Two manganese profiles (KOI and K02; Fig. 4) were obtained south of the I5 “N area during MAR/ 84 ( KLINKHAMMERet al., 1985). They are located respectively over the axis ofthe rift valley ( 14”53.8’N-45°01.6’W, water depth = 3798 m) and on an outcrop of ultramafic rocks on the eastern wall (14’55.0’N-44”54.0’W, water depth = 3079 m). Maxima in dissolved (0.6-2.0 nmol/kg) and particulate manganese at a depth of 2875 m, indicate venting of hydrothermal solutions from a source on the east wall of the rift valley at or below the level of the ultramafic outcrop ( RONAet al., 1987 ) . Water samples recovered near this outcrop of ultramafic rocks, at 2915 m water depth (14”56.l’N-44”55.8’W), showed ‘He values of 7.2 and 4.9%, respectively, at 0 and 100 m above the seafloor, in distinct contrast to the background value of 2.0% found in the North Atlantic. In agreement with TDM anomalies, the elevated ‘He values suggest ongoing and well developed hydrothermal discharge emanating from the seafloor at that site ( RONA et al., 1987). During the Ridelente cruise of 1988 (BOUGAULT et al., 1989, 1990a), CH4 anomalies reaching up to 60-80 nL/L were observed in the rift valley close to the eastern and western intersections of the rift valley with the 15’20’N F.Z. (CHARLOU et al., 1990). A CH4 profile (Fig. 5; RD-87-HY-04), positioned at 14”55.66’N-44”55.20’W in the rift axis (water depth = 4005 m; Fig. 4), revealed an anomaly extending

Nine CTD/tows were conducted during the MAR/88 cruise in the TAG Field area (Fig. 2, 3). Very large-scale venting was observed from shipboard nephelometry profiles within the TAG Hydrothermal Field region and south of the 1985 black smoker discovery. Real time data from the nephelometry allowed mapping of light scattering anomalies (NELSENet al., 1988; NELSEN and FORDE, 1990. 1991) and guided water sampling for total suspended matter, trace metal chemistry, methane, and total dissolvable manganese. Cast 01 (26”09.1’N: 44”50,5’W) and Cast 02 (26”08.4’N; 44”49.6’W; Fig. 5) are typical Atlantic reference stations. CH,, and TDM concentrations remain constant ( I2 nL/L; 0.25 nmol/kg) between 1000 and 3000 m depth. On Cast 03 (26”08. I ‘N: 44”49.7’W; Fig. 6), one sample collected at 3305 m depth shows high CH4 ( 185 nL/L) and TDM (29.5 nmol/kg) concentrations. No samples were available from Cast 04 (26”07.7’N; 44”49.4’W) or Cast 05 (26”08.5’N; 46”49.4’W). However, high nephelometry anomalies were observed in real time on board (NELSEN, pers. comm.) . On Cast 06 (26”08. I ‘N; 44”49.4’W; Fig. 6), Cast 07 (26OO8.1‘N; 44”49.8’W; Fig. 6, 9), Cast 08 (26”09.4’N: 44”48.8’W; Fig. 6), and Cast 09 (26”07.8’N; 44”50.8’W; Fig. 6), samples were collected in the plume at depths corresponding to high nephelometry anomalies. In Fig. 6, the plume is clearly identified from nephelometry profiles, between 3200 m and 3500 m depth. All seawater samples collected within this 300 m layer, where the CTD/rosette was yo-towed. show large TDM and CH4 anomalies reaching respectively up to 32 nmol/ kg and 210 nL/L in the densest part of the plume centered around 3400 m depth (Table I ). TDM and CH4 data. correlated with the nephelometry profiles, show a plume substantially larger in size and particle concentration than previously observed in this region south of the 1985 site. This new hydrothermal plume is characterized by elevated TDM and CH4 concentrations (this work); its volume is estimated to be 7.109 m3. compared to 0.8.109 m3 for the plume discovered at TAG in 1985 (NELSENet al., 1988 ) . Transects parallel and normal to the ridge axis did not permit definition of southern or western plume boundaries. However, the high TDM and CH4 concentrations trends suggest a new southern source lying within or west of the ridge axial valley confirming on board nephelometry observations (NELSEN et al., 1988; NELSEN and FORDE, 1990). This southern plume is not a spin-off or eddy of the previously known TAG plume. Concentration gradients to and away from the TAG plume and southern plume, plume doming over source region, plume geometry and symmetry, and plume-induced thermal and salinity gradients within the southern plume are arguments providing compelling evidence that this southern plume is not issued from the known TAG mound (NELSEN and FORDE, 199 I ). In addition the suspended particulate matter mass is much greater in the southern plume than in the TAG plume.

Charlou et al.

3216

MAR188 - TAG (26-N) TDM (nmollkg)

NEPHELS 0

50

103

0

10

al

30

,

.’

: 3800

o

CTD-03

.

CTD-06

v

CTD-07

*

CTD-09

n

4000-

NEPHELOMETRV,

CTD-09

TDM and CH4 PROFILES

FIG. 6. TAG (26”N) area: Nephelometry, TDM, and CH4 chemical data obtained from CTD samples collected 600 m above the seafloor. The plume is 250 m thick, and concentrations are maxima in this layer between 3200 m and 3500 m depth.

Afier the 1985 site survey of TAG Field, strong correlations were shown between nephelometer readings (Nephels ), total suspended matter (TSM), particulate Fe, and total reactive manganese (TRM) throughout a large range (0.2-3 1.O nmol/ kg; NELSEN et al., 1986/87). The relationship between Nephels and TRM was

(Nephels) = 2.095 (TRM) nmol/kg

+ 18.1,

whereas this work shows the following relationship between Nephels and TDM: (Nephels) = 3.300 (TDM) nmol/kg

+ 27.8.

The equations of the representative correlation lines between nephels and manganese are different in slopes and intercepts. If we assume that the nephelometry was responding the same way [Nephels = 1.93 (TSM)rg/L + 4.31 in 1985 and 1988, a given nephel value (i.e., a given TSM value) would lead to a manganese concentration lower in 1988 (TDM) than in 1985 (TRM). Since particulate Mn values have been shown to be low and rather uniform at 0.14 f 0.02 nmol/kg ( TROCINEand TREFRY, 1988), TDM and TRM can be assumed to be almost identical at least for the highest values, and comparison between 1988 and 1985 manganese data seems to indicate a plume less manganese enriched in 1988 than in 1985. These results suggest that the hydrothermal activity discovered in 1985 at TAG Field is not an isolated occurrence and that major venting sites there may be separated by only a few kilometers. Evidence of CH4 and TDM plume south of the previously known TAG plume associated with a smaller plume on the rift valley’s east wall (NELSEN and FORDE, 199 1) confirm that hydrothermal activity at TAG is not restricted to only a mound, but is present along a more extended area. This new discovery offers conclusive evidence that slowspreading (< 1 cm/ year half rate) oceanic ridges support im-

portant hydrothermal activity and are the source of significant mass and heat transfer from the lithosphere to the ocean. TDM and CH., in the 15”N Area We present here TDM and CH., data obtained in the rift valley and on the mount A centered at 15’05’N-44”59’W, where 6 and 3 CTD/ tows were conducted respectively (Fig. 4). Rift Valley Cast 10 (14”55.6’N-44’55.1’W; 3757 m), Cast 11 (14”55.1’N-44’53.8’W; 2964 m), and Cast 12 ( 14”55.0’N44”53.8’W, 3400 m) have very similar TDM and CH, profiles versus depth with pronounced deviations from the background around 2500,2950, and 3350 m depths (Fig. 7). The first anomaly at 2500 m, poorly defined for lack of sampling above 2500 m, is strongly correlated with the observations obtained on the northern dome (see later). CH., and TDM concentrations reach respectively 60-80 nL/L and 0.6-1.0 nmol/kg. A second anomaly is seen around 2950 m depth on the three profiles, 10, 11, 12 (Fig. 7), with positive deviations from the background of the same order of magnitude as that of 2500 m depth. This anomaly confirms the previous CH4 anomaly clearly defined at the RD-87-HY-04 station of Ridelente cruise ( BOUGAULTet al., 1989, 1990a) in the same area. Between 3000 m and the bottom, these profiles (Fig. 7 ) show a gradual decrease of CH4 and TDM concentrations with a small positive undulation in profiles 10 and 12 and a large increase in TDM concentrations (up to 0.8 nmol/kg) on profile 11, around 3300 m depth. These results are consistent with an output of hydrothermal manganese from sources on the east wall at 2950 m and 3300 m depths previously indicated by KLINKHAMMERet al., ( 1985) and RONA et al., (1987). At Cast 13 (14”54.21’N-44”54.6’W), northeast ofstations 10 and 12 (Fig. 4)) the 2400-2500 m maximum is noticeable

3217

Hydrothermal plumes above the TAG field MAR18815”2O’N F. 2. (south)

14ooo,&

lCAST#ll m* RDe7-HY-04

CAST # 10

3

.

z

CAST#

1

influences can create more or less complicated and variable pictures from date to date ( CHARLOU et al., 199 1) . Cast 17 (15”06.7’N-44”56,9’W) is located north of the east wall (Fig. 4). CH4 and TDM concentrations decrease regularly respectively from 110 nL/ L and 1 nmol/ kg at 2500 m depth to the background ( 10 nL/L; 0.25 nmol/kg) which remains constant for CH4 and TDM from 3200 m depth to the bottom (Fig. 7), similarly as pointed out for stations 10, 11, and 12. Cast 18 (14”57.0’N-44”53,4’W), located north of Cast 11 and RD-87 HY-04 close to the ridge axis (Fig. 4)) confirms the geochemical anomalies (CH4: 107 nL/L; TDM: 1.67 nmol/kg) observed in the 2600-3000 m layer on profiles 10, 11, and 12 (Fig. 7).

11

Mount A Centered at 15”05’N-44”59’W

_. .

a

a

a

E3000.

ml&al 26Wm

a.

2 f.&&_ mm

x 1

1 404

CASTtl2

4000

CAST # 16

d

1

CAST i! 13

.

a

CAST # 17

j

1

CAST f 15

CAST # 18

tmms,,som

FIG. 7. Vertical profiles of CH4 and TDM for stations in the 15’N area. The CH4 profile (RD-87-Hy-04) obtained at 14”55’66N44”55’2OW during the Ridelente cruise ( 1988) is superimposed on the CHI profile of Cast 11(these two stations are very near) to show the good agreement between the 1985 and 1988 data with a maximum anomaly around 2950 m depth. CH4 and TDM are well correlated. CH4 concentrations are elevated (up to 150 nL/L). TDM values remain low (up to 1.5 nmol/ kg) relative to other active sites.

(TDM: 0.7 nmol/kg; CH4: 75 nL/L), as shown on casts 10, 11, 12 (Fig. 7). CH4 and TDM concentrations regularly increase from 2000 m down to the bottom. The nephel profile shows small variations over the background between 2950 and 3000 m depth. A TDM profile measured in 1984 (KLINKHAMMER et al., 1985) at 14”55.O’N-44”54.O’W close to station 13, showed a maximum of 2.35 nmol/kg at 2879 m depth, approximately twice the value found in this work. The shape of the two profiles are also quite different. These differences seem to indicate a dramatic change in the characteristics of the emission in this area between the two dates. These variations can probably be interpreted mainly by the characteristics of the emitted sources which can display different compositions, involving discrepancies between the different tracers ( CHARLOU et al., 199 1a). In addition, currents, water masses with different physico-chemical characteristics, eddies produced by hydrothermal buoyancy, and topographic

Cast 14 was a CTD tow ascending along mount A (Fig. 4, 8) from the eastern flank (water depth = 3400 m) to the top (water depth = 2500 m). High CH4 anomalies correlated with small TDM anomalies are clearly seen at two levels during the ascent of this mount: on the east flank around 2950 m depth (sample 4: CH4 = 395 nL/L, TDM = 1.5 nmol/ kg) and on the summit of the mount, at 2500 m depth (sample 8: CHI = 150 nL/L, TDM = 1 nmol/kg) (Fig. 8). Cast I5 (Fig. 4, 8) consisted of two parts: sampling on the east flank of the mount to confirm the temperature anomalies seen in real time on board during the Cast 14 along this portion of ascent of the mount, and sampling along a classical vertical profile at the top of this topographic high. CH4 and TDM concentrations (280-347 nL/L for CH4; 0.85-0.99 nmol/kg for TDM) were found to be similar to those measured in samples from Cast 14, thus confirming a large hydrothermal discharge in this area. Venting on the top of the mount is also confirmed with high CH, and TDM concentrations ( 150 nL/L; 1 nmol/ kg). These data indicate that

.

Sample

location

8. CTD Tow (Cast 14) transect through the mount centered 15”05N-44”59W, showing CH, and TDM concentrations humps at 2800-2900 m (400 nL/L: 1.5 nmol/kg) and 2500 m (150 nL/L: 0.8 nmol/ kg) depth. at

Charlou et al.

3218

this mount emits two distinguishable plumes at 2800-2900 m and 2500 m depths (Fig. 8). Cast 16 ( 15°06.7’N-44056.9’W; 3029 m) was located north of the mount at 15°05’N-44059’W close to the fracture zone (Fig. 4). It shows a typical background profile, except at 2950 m and 2500 m depth, where CH4 concentrations are respectively 25 and 47 nL/L. These depths correspond to discharge depths found on the mount A investigated in casts 14 and 15. These results suggest that the two plumes rising from the mount at 2500 m and 2950 m are drawn to the south in the rift valley, and that no input is coming from the fracture zone. It confirms a similar result obtained at the eastern rift valley-Kane F.Z. intersection (JEAN BAPTISTEet al., 199 1; CHARLOUet al., 199la,b). A CH.,, Mn, 3He hydrothermal signal also exists in the rift valley some miles south of this intersection. An hydrocast located in the Kane fracture zone itself revealed no signal at all ( BOUGAULTet al., 1990a). The CH4 rich waters emitted by the mount, located immediately south of the Eastern 15”20’N F.Z. intersection, follow isodensity layers to the south, and probably mix with the diffuse discharges coming from the eastern wall of the rift valley. These discharges probably contribute to the 2500 and 2950 m anomalies found on the south stations, as previously shown by KLINKHAMMERet al., ( 1985). In addition, a regular decrease of the anomalies is observed from north to south (stations ll, 10, 12) in the axial rift valley, which could be indicative of plume dilution. DISCUSSION Differences Between TAG (26’N) and 15”N Areas from Plume Signatures. Range in TDM and in TDM/CH, Ratio

Immediately south of the previously known TAG Field (Fig. 3), CH4, TDM, and nephelometry data show a new

-II

TAG(Z6.N).aa~olym

_.

hydrothermal area: it is characterized by a classical plume signature (high nephelometry signal correlated to high CH4 and TDM concentrations) extending between 3200 m and 3600 m depth (Fig. 9). Immediately south of the eastern 1S”20’N F.Z. intersection ( 15”N area), mount A located at 15’05’N (Fig. 4) is characterized by a large CH4 outgassing with two distinguishable plumes rising from 2950 m and 2500 m depths, whereas TDM and nephelometry signatures are very weak (Fig. 9 ) . Differences between TDM /CH4 slope in plumes at different sites can be due either to different ratios in original vent fluids at each site or the effects of Mn precipitation or CH., oxidation as plumes age, or the combination of both. TDM / CH., values are 0.30 mol/L at many stations of the EPR at 13”N (CHARLOUet al., 199la,b), at 20”s (KIM, 1983), and on Juan de Fuca ( WINN et al., 1986). This ratio can be found higher [1 mol/L at 2l”N (Pluto-St 13; KIM, 1983)] or lower (0.17 mol/L at 2O”S, 13’N; station HF-19; CHARLOU et al., in prep.) than 0.11 to 0.20 mol/L on the Central Indian Ridge ( PLUGERet al., 1990). On water samples taken in the vicinity of the triple junction in the north Fiji Basin, three possible linear relationships with TDM/CHI ratios of respectively 0.10,0.30, and 0.60 mol/L have been found (AuZENDEet al., 1988; NOJIRI et al., 1989). CH., plumes without noticeable TDM anomalies (low ratio) have been identified in Mariana Trough (CRAIG et al., 1987). For the Mid-Atlantic Ridge, Fig. 10 shows the least square correlations between TDM and CHI concentrations in plumes over TAG and 15”05’N areas. In each case, TDM shows a good linear relationship with CH4, but with quite different slopes corresponding to ratios respectively of 0.15 mol/ L at TAG ( r = 0.97) and 0.005 mol/L over the 15”05’N area (r = 0.43). The regression coefficient r for the 15”05’N area is lower than that of the TAG area. This difference does not necessarily reflect a difference in correlation but is the consequence of

15’OS’N-ast1s

FIG. 9. Comparison of hydrothermal signals at TAG (26”N) and at the 15”N site. Cast 06 (26OO8.1 ‘N-44’49.4’W) and Cast 07 (26OO8.1‘N-44”49.8’W) at TAG shows a good correlation between the 3 tracers with large CH4, TDM and nephelometry anomalies in the 3200-3600 m deep seawater layer. Cast 15 on the mount centered at 15”05’N44’59’W shows two strong CH4 anomalies (up to 400 nL/L) respectively at 2800 and 2500 m with low TDM concentrations ( 1 nmol/kg) and only a background profile for nephelometry. Note the large difference in TDM range between Casts 06 /07 and Cast 15.

Hydrothermal plumes above the TAG field

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MAR/88 TDWCH4

RELATIONSHIP

40

I MAR-TAG

26’N

EPR-13% 0.17

‘I------

mal/l

FIG. 10. Graph showing concentrations of TDM plotted against CH4 for samples collected in the TAG (r = 0.97; IV= 35; Y - intercept = - 1.5)and I5”N (r = 0.43; N = 80; y - intercept = 0.46) plumes on the MAR. Data obtained from the EPR at 13”N (r = 0.93; N = 59; y - intercept = -0.06; CHARLOU et al., 1990) are presented for comparison. The insert shows TDM/CH4 ratios for profiles compared on Fig. 7: Casts 06/07 at TAG (r = 0.96; N = 13; y - intercept = -0.84) and Cast 15 at lS’05’N (r = 0.77; N = 9; y - intercept = 0.32). the combination of low TDM/C& slope and of low accuracy on low TDM values. Accounting for data in hand so far, it has to be noted that the range of TDM/CH4 ratio on the MAR is much larger than elsewhere. Nevertheless, the range of CH4 concentrations (CHARLOU et al., 1987, 1988, this work) on the MAR is similar to that encountered elsewhere. The large difference in TDM/CHI ratio on the MAR is due to the large range of TDM concentrations (KUNKHAMMER et al., 1985, 1986, this work; JONES et al., 1981; JONESand MURRAY, 1985; BWLIXWE and HAMELIN,1983; BOUGAULT et al., 199Oa). The particularly low TDM/CHr ratio (0.005 mol/L), found in plumes over the 15“05’N area on MAR, is probably a characteristic of emitted fluids since sampling over the studied 15”05 ‘N axial mount A was just only some meters above the bottom over probable venting areas. The low TDM/CH, ratio can be explained by interaction between seawater and nonbasaltic rocks, for instance ultrabasics. In the 15”05’N area, intense CH, degassing and low TDM concentrations are related to an axial mount made of serpentinized peridatites recently dredged by the R/V Akademik Boris Petrov (S. SILANTIEV,pers. comm.). A similar result was also obtained at 15” 36’N in the inner floor of the MAR (Ridefentecruise, R/V/J. Charcot; BCWGAULTet al., 1990a,b). Experimental investigation of high temperature interactions (ZOO-500°C and 1000 bars) between seawater and rhyolite, andesite, basalt, and peridotite (HAJASH and CHANDLER, I98 1) shows variations in the Mn concentrations leached from the rock. In rhyolite, andesite, and basalt experiments, proportions larger than 90% of initial Mn in the rock were removed. Only 38% of the Mn in the peridotite is leached at 500°C. No Mn-bearing minerals were identified in the solid alteration products in the peridotite experiment. In addition.

the basic pH of solutions can also explain the low Mn content found in surrounding plumes. Cold water was identified seeping through a serpentinite seamount in Mariana Forearc (ODP-Leg 125) with formation of aragonite, calcite, and amorphous Mg-silicate. Emitted fluids showed a pH of 12.5, very low Mg, Ca, Si contents, very high SO4 (up to 47 mmol/ kg), dissolved sulfide (up to 2 mmol/ kg), and very high CH4 and light hydrocarbons concentrations ( MOTTL and HAGGERTY,1989). If similar (aragonite is also present in 15”36’N and in 15”05’N dredged deposits), basic pH solutions are being emitted from the 15”05’N mount, the major part of TDM involved in the hydrothermal/serpentinization process would remain trapped in the basement. It would explain the low TDM detected in surrounding plumes. The different TDM/CH4 ratios observed along the MAR have a direct consequence on the strategy of hydrothermal exploration along ridges. Tracking of dispersed hydrothermal plumes using nephelometry can be a powerful technique to optimize sampling strategy, to determine the geometry and lateral extent of hydrothermal plumes, and to predict in situ total suspended matter and particulate Fe and Mn concentrations above hot water discharges (NELSENet al., 1986 / 87; BAKERet al., 1987 ). However, this technique, used with success on the Juan de Fuca Ridge and at TAG (26”N-MAR), cannot be considered infallible and universal for plume tracking, since hydrothermal activity without any noticeable suspended matter is found at 15’N area (this work). It demonstrates that tracking hydrothermal emissions by using only one specific tracer is not sufficient unless the discharge characteristics are known, but an unknown area must be documented with as many tracers as possible. In addition, hydrothermal characteristics of different areas can be compared from tracers behaviour. In spite of discrepancies among trac-

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Charlou et al.

ers (CHARLOU et al., 1990, 1991a), low TDM/CH4 ratio may be indicative of hydrothermalism associated with serpentinization. Gases Generated During Serpentinization CH, can be generated by biological production, principally by bacteria at low temperatures, outgassing of juvenile carbon as CH4 from the mantle, and inorganic synthesis involving CO* and Hz and other C-H molecules either through basaltseawater interaction or direct peridotite-seawater interaction. Hydrothermal circulation studied so far along Mid-Oceanic Ridges (EPR, MAR) has been considered to involve only basaltic rocks; sedimentary organic matter can sometimes interfere, such as in Guaymas Basin. The ultramafic rockseawater interaction on the axial serpentinite mount ( 15”OS’N) probably generates solutions of a different composition but still with a large CH4 content, associated with H2 and CO1 gases. According to ABRAJANO( 1984)) Hz, H2S, and hydrocarbons (especially CH4) gases are produced in significant quantity, especially in CO1-rich fluids. CH,-rich gas escapes at low flow rate and ambient temperature from seeps in serpentinized ultramafic rocks in the Zambales Ophiolite, Philippines. Carbon and helium isotopic data are consistent with derivation of the Zambales gas directly from a reduced mantle. Phase equilibria and hydrogen isotope data indicate that the Zambales gas also could have been produced by reduction of water and carbon during low-temperature serpentinization ( 30°C to 350°C) of the ophiolite ( ABRAJANO et al., 1988). Serpentinization in CO*-rich fluids at 300°C. 500 bars results in generation of hydrogen, a major produced component, and in partial conversion of CO2 to hydrocarbons, especially methane, also an important component but at a lower extent than hydrogen (JANECKYand SEYFRIED, 1986; SZATMARI, 1989). The occurrence of free H2 emphasizes hydrocarbon generation by hydrogenation. Hz can be produced by reaction ar high temperature between water and silica radicals ( KITA et al., 1982; SUGISAKIet al., 1983) or by reduction of water by Fe2+ present in rocks at temperature above 800°C ( D’AMOREand NATI, 1977; APPS, 1985 ) and outgasses from the earth’s mantle rocks under stress ( GIARDINI et al., 1976; SUGISAKIet al., 1983). Hydrothermal methane and unsaturated hydrocarbons may be derived either from high temperature ( 150°C-400°C), inorganic chemical reactions (e.g., Fischer-Tropsch synthesis) in the presence of a catalyst, or from reduction of CO2 through the modified Fischer-Tropsch reaction at relatively low temperatures (2O”C-100°C). Hydrogen, carbon oxides, and catalysts (magnetite, hematite) supplies for the Fischer-Tropsch synthesis are potentially present in the 15”05’N serpentinite environment. Hydrocarbon gases ( CH4, CzHs, C3Hs, and C4H10) and reduced bitumens are found in alkalic, mafic, and ultramafic rocks in the Kola Peninsula, the Russian platform, the Urals, and Siberia. By composition, they resemble gases which characterize gas-oil deposits, but differ from them in their hydrocarbon-isotope composition. They formed as a result of inorganic syntheses that took place in cooling intrusive bodies ( PORFIR’EV, 1974). Light hydrocarbons were also abundant (up to 37 mmol of methane, 25 Fmol of ethane,

presence of propane) in the pore-waters sampled during the ODP-Leg 125 drilling on a serpentinite seamount located in the Mariana forearc ( MOTTL and HAGGERTY, 1989). Such volumetrically significant Hz and C02-rich fluid generation during serpentinization processes not only provides a plausible way of forming abiogenic hydrocarbons ( HAWKES,1972, 1980; NEAL and STANGER, 1983) but also could have significance regarding the genesis of certain ore deposits ( COVENEY, 198 1; COVENEYet al., 1987). For example, the occurrence of Hz in serpentinites is thought to have controlled, probably by reduction of their aqueous solutions, the location of elements such as gold veins ( COVENEY,198 1; COVENEY et al., 1987), uranium (LEVINSON, 1977), platinum (BETECHTIN, 196 1) or mercury (BARTON. 1970) ores. CONCLUSIONS

1) The occurrence of serpentinized ultramafics and hydrothermal output on a mount located on the inner floor of the rift valley of the MAR at 15”05’N demonstrates that the upper mantle material is affected by hydrothermal processes. Enhanced permeability close to the rift valley and the fracture zone intersection favors downwelling of seawater to upper mantle ultramafic rocks. The serpentinization process can play a role in tectonically controlled uplifting of ultrabasic bodies, as mounts in the inner floor of the MAR valley. When they are mature, these mounts are part of the wall of the rift valley. 2) The TDM/CH4 ratios in TAG and in 15”05 ‘N plumes in the rift valley of the MAR are quite different. At 15”05’N, close to the eastern 15”20’N F.Z.-ridge axis intersection, the TDM/CH4 ratio is very low due to low TDM concentrations. These 15”05’N output plume characteristics are most likely the consequence of the interaction of seawater with ultrabasic rocks. CH4 is generated by reactions between rocks, H20, COz, and Hz according to inorganic reactions similar to FischerTropsch synthesis. Low TDM concentrations may reflect high pH fluids associated with ultrabasic structures, as already found on a serpentinite seamount in the Mariana forearc. Low/TDM ratio in plumes offers a potentially useful and simple method of exploring for mafic and ultramafic associated vent sources. 3) The different TDM/CH4 ratios due to low TDM concentrations in plumes close to venting structures enhance the need of several tracers to be used to locate and characterize hydrothermal sites. Acknowledgments-This work is part of a French-US cooperation (IFREMER-Universitk de Bretatme Occidentale and NOAA/AOML GENTS Program) and is a contribution to the FARA (French-American Ridge Atlantic) Program. We thank Commanding Officer J. K. Callahan, Operations Officers, and the crew of the NOAA ship “Oceanographer” for their splendid support and their ship-handling capabilities during CTD tows and survey. Acknowledgments are due to the team from Florida Institute of Technology of J. Trefry, S. Metz, and R. Trocine for sampling qualification and for stimulating conversations, to D. Sweeney, who provided valuable assistance in data acquisition during the cruise, and to J. P. Donval for help in data display. We gratefully acknowledge the reviewers R. J. Poreda, M. K. Tivey, and an anonymous referee whose contributions greatly improved the manuscript. Editorial handling: T. S. Bowers

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