Journal of Conference Abstracts

Methane Degassing, Hydrothermal Activity and Serpentinization Between the Fifteen-Twenty Fracture Zone Area and the Azores Triple Junction Area (Mid-Atlantic Ridge)

J.-L. Charlou Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

charlou@ifremer.fr

H. Bougault Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

Y. Fouquet Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

J. P. Donval Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

E. Douville Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

J. Radford-Knoery Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

M. Aballéa Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

H. D. Needham Département Géosciences Marines, IFREMER Centre de Brest, B. P. 70, 29280 Plouzané Cédex, France

P. Jean-Baptiste CEA-CNRS, LMCE-CFR, CEN-Saclay, 91191, Gif-sur-Yvette, France

P. A. Rona Institute of Marine and Coastal Sciences, Rutgers University, P. O. Box 231, New Brunswick, NJ, USA

C. H. Langmuir Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, Palisades, NY, USA

C. R. German Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Introduction

Many surface or diving cruises have been conducted in the last few years, as part of the French-American Ridge Atlantic (FARA) project, to study geochemical ridge crest processes along the Mid-Atlantic Ridge (MAR), between the 15°20'N Fracture Zone and the Azores Triple Junction (ATJ). These cruises were guided by bathymetric maps obtained during cruise SIGMA (Needham et al., 1992). During these cruises, CH₄ was used, in connexion with other physical (temperature, nephelometry) or chemical (Mn, helium) tracers, to determine the hydrothermally active segments, and to survey in detail the hydrothermal plume within an active segment or within focused areas. This paper reviews recent findings of hydrothermal activity and hot fluid chemistry in contrasting types of hydrothermal systems along the 15°-41°N section of the MAR. Methane degassing is related to the nature of seawater-rock interaction, including interactions in hydrothermal systems hosted primarily in basaltic rocks and interactions of seawater with ultramafic rocks.

Fifteen Twenty Fracture Zone area

During the FARANAUT (1992) diving cruise (Charlou et al., 1992), high CH₄ concentrations (400nl/l) and low Mn concentrations (<1nmol/kg) were observed at the top and on the east flank of the diapir centered around 15°05'N-44°59'W, where serpentinized rocks were sampled, confirming the results of Ridelente (1988) (Charlou et al., 1993; Bougault et al., 1993) and NOAA vents (1990) (Charlou et al., 1991; Rona et al., 1992) cruises in this area. The low TDM/CH₄ ratio provides evidence of fluid circulation in ultramafic rocks. The Atlantic CH₄ background is around 10 nl/l far from ridge axis. On the western part of the 15°20'N Fracture zone, seven hydrocasts and Nautile sampling in the seawater column show a large CH₄ enriched (up to 800 nl/l) plume located at 3200 m depth, coinciding with major fault scarps in ultramafic rocks exposed in the walls of the rift valley. CH₄ concentrations of 150-200 nl/l are persistent on the walls along the 3200 m isobath. Mantle degassing, hydrothermal activity and serpentinization processes are involved to explain the intense CH₄ input in the two areas.

Recently, during the MICROSMOKE cruise (Nov. 1995), high temperature fluids (up to 354°C) were collected with Nautile near 14°45'N in a field of active and inactive sulfide mounds hosted in serpentinized ultramafic rocks (gabbros, hazburgites) in the lower east wall. From preliminary analyses of major elements, these fluids show a uniform composition. Na, K, Cl, Br end members (extrapolated to Mg=0) are close to Snakepit values. Ca, Sr, Rb and Ba are enriched by a factor of 2, while Li is depleted. Metal (Fe, Mn, Cu, Zn) concentrations are in the same order of magnitude as concentrations previously found in hydrothermal fluids at Snakepit (23°N) (Campbell et al., 1988; Jean-Baptiste et al., 1991; Edmond et al., 1995), TAG (26°N) (Campbell et al., 1988; Edmond et al., 1995) or Broken Spur (29°N) (James et al., 1995).

Segments between 33°N and the ATJ

During the FAZAR cruise (1992), each of the 14 segments of the MAR between 32°N and 41°N was sampled for CH₄ anomalies research. Whereas Mn/CH₄ ratios of about 0.3 mol/l characterize hydrothermal sites like Snakepit (23°N) and TAG (26°N), much lower Mn/CH₄ ratios (0.005 mol/l) are indicative of active serpentinization processes, as observed at 15°20'N. CH₄ anomalies (up to 100 nl/l, between 2800 and 3200 m) are detected on the Hayes segment near 33°59'N (4064m), at North Oceanographer segment (50 nl/l, 2000 m) close to 35°14'N (3000 m), on the AMAR segment (200 nl/l, 2000 m) at 36°16'N in good agreement with optical sensors observations by German et al. (1996) in the Famous segment (250 nl/l, 2000-2800 m) close to 36°37'N, at 37°17'N (Lucky Strike) (300 nl/l,1570 m), 37°50'N (Menez Gwen) (150 nl/l, 700-900 m), and close to 40°10'N, south of Kurchatov (200 nl/l at 2200 m). All these CH₄ anomalies were confirmed and new ones found during the HEAT (1994) and ESCAPE (1995) cruises, two cruises of the European MARFLUX project, giving new potentially active hydrothermal sites.

Hydrothermal active sites along the MAR: CH₄ enrichment

Six hydrothermal fields are now known and have already been sampled along the MAR: 14°45'N, 23°N (Snakepit), 26°N (TAG), 29°N (Broken Spur), 37°17'N (Lucky Strike), 37°50'N (Menez Gwen). Other new potentially active areas have been located, particularly in the AMAR segment, Famous segment, and South Kurchatov Fracture zone. Snakepit, TAG and Broken Spur fluids show similarity in exit temperature and chemical compositions, including mineral elements and gases. However, the results of the Lucky Strike cruise (1993) (Colodner et al., 1993; Langmuir et al., submitted) and DIVA 1 cruise (1994) (Fouquet et al., 1995; Charlou et al., 1995) demonstrate that variability in fluid composition occurs at Lucky Strike and at the new Menez Gwen, the last hydrothermal site recently found on the MAR near the Azores. Chlorinity, metals and gas content are not uniform in the Lucky Strike fluids located at 1740 m depth at the edge of a lava lake in a volcanic environment. The Menez Gwen hydrothermal vent, located at a shallow depth (900 m), emits fluids which are clearly different and are among the highest CH₄ enriched fluids (40-50 ml/kg of fluid) collected on unsedimented ridges, demonstrating an increase in mantle outgassing as towards the ATJ (Charlou et al., submitted). Cl, H₂S, and metal depletion associated with gas enrichment (especially CO₂, CH₄) show that phase separated effluents are delivered by Lucky Srike and Menez Gwen fluids to deep ocean. Pressure variation and control of hydrothermal discharge near a lava lake, first observed on a slow spreading ridge (Fouquet et al., 1995), may explain the different fluid chemistries.

Conclusion

The CH₄ background in the axial valley is found to be relatively high (20nl/l) everywhere along this section of the MAR between 15°N and the Azores Triple Junction. This observation, associated with the discovery of numerous CH₄ anomalies on the segments studied, demonstrates an important mantle degassing along the ridge axis. CH₄ is degassed from the mantle through hydrothermal activity in the ridge valley and serpentinization processes occurring on the walls of the rift mountains. The flux of CH₄ increases when approaching the ATJ. This may be related to the enrichment of carbon observed in basalts (Kingsley and Schilling, 1994). So, the CH₄ enrichment is produced by outgassing of juvenile carbon as CH₄ from the mantle via high-temperature hydrothermal systems, but a large part of the abiogenic CH₄ flux along the MAR is probably also generated in the crust at relatively shallow depths, at intermediate temperatures (100-500°C), under the strongly reducing conditions resulting from hydrolysis by mafic and ultramafic rocks. The existence of abiogenic CO₂, CH₄, H₂ is consistent with an origin from reactions involving Fe²⁺ oxidation occurring during serpentinization.

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