Journal of Conference Abstracts

Mafic- and Ultramafic-hosted Hydrothermal Systems at the TAG (26°N) and Fifteen-Twenty Fracture Zone Areas of the Mid-Atlantic Ridge Rift Valley

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

NJ 08903-0231, USA

rona@ahab.rutgers.edu

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

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

Y. Fouquet 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

This paper reviews recent findings of geologic setting (tectonic and magmatic regimes), solution chemistry, hydrothermal precipitates and heat transfer in contrasting types of hydrothermal systems hosted primarily in mafic rocks at the TAG hydrothermal field and in ultramafic rocks in the rift valley north and south of the Fifteen-Twenty Fracture Zone. Studies to date have concentrated on mafic-hosted high-temperature hydrothermal systems which tend to occur near the center of spreading segments. This study shows that geologic conditions favoring low- and high-temperature systems hosted in ultramafic rocks tend to occur in the rift valley of slow-to intermediate rate spreading axes near intersections with transform faults and may consititute a distinct, significant component of global seafloor hydrothermal fluxes.

Tectonic and magmatic regimes

TAG. The TAG field occupies a 5 x 5 km area of the eastern floor and wall of the rift valley near the center of a 40 km-long spreading segment between short non-tranform offsets. The field comprises an active high-temperature sulfide mound, two extensive inactive sulfide zones, and an active low-temperature zone (Rona et al., 1993) with radiometrically determined ages extending to 140,000 years (Lalou et al., 1995). Oceanic crust is inferred to be thin from the presence of a relatively weak mantle Bouguer 'bull's eye' anomaly (Lin et al., 1990) coinciding with the segment center and exposures of sheeted dyke complex in the lower east wall (Zonenshain et al., 1989). The occurrence of pillow lava domes, inferred to be volcanic centers, adjacent to the active and two inactive sulfide zones and the distribution of heat flow values suggest that intrusions at the centers have supplied heat to drive hydrothermal activity in the adjacent zones (Rona et al., 1993). Chronology of the hydrothermal precipitates (Lalou et al., 1995) and of the lava flow sequences (Zonenshain et al., 1989) is consistent with episodic, incremental intrusions on a time scale of thousands of years.

Fifteen-Twenty Fracture Zone. The rift valley to some tens of kilometers north and south of the Fifteen-Twenty Fracture Zone encompasses the largest known oceanic area of exposed ultramafic and gabbroic rocks (Rona et al., 1992; Bougault et al., 1993; Silantiev et al., in press). The distribution of the ultramafics which crop out in both walls of the rift valley favor an hypothesis of emplacement by symmetrical lithosphere necking under magma-starved conditions (Casey et al., 1992; Cannat, 1993).

Solutions, precipitates and heat transfer

TAG. The composition, temperature and equilibration depth (2 km beneath the seafloor) of end member solutions that discharge from a black-smoker complex at the top of the active sulfide mound are similar to those of black smokers in fields on the intermediate spreading rate East Pacific Rise at 21°N (Campbell et al., 1988; Edmond et al., 1995). A "largely solid but still hot" layer centered at 3 km was delineated by seismic methods beneath the axis and active sulfide mound (Kong et al., 1992). Copper sulfides (chalcopyrite) predominently precipitate from the end member solutions and zinc sulfides from those solutions that have partially mixed with seawater which also precipitate a significant volume of calcium sulfate (anhydrite) within the mound (Tivey et al., 1995; Humphris et al., 1995). Heat at the active sulfide mound is being transferred convectively by high-temperature venting (Rona et al., 1993), diffuse flow and conduction (Becker and Von Herzen, 1995), and by conduction in the inactive zones (Rona et al., 1993).

Fifteen-Twenty Fracture Zone. Solutions characterized by high methane contents and low contents of particulate and dissolved metals discharge diffusely at ambient seawater temperature from fractures and faults in serpentinized ultramafic rocks exposed on the floor and in the walls of the north and south rift valley (Charlou et al., 1991; Charlou and Donval, 1993). Extensive methane-rich plumes (<4 nmol/kg; background is 0.4 nmol/kg) were detected by hydrocasts in the north and south rift valley (Charlou et al., 1992). The plumes exhibit maxima in methane and ³He at a depth of about 3200 m which coincide with major fault scarps in ultramafic rocks exposed in walls of the rift valley (Charlou et al., 1992; Rona et al., 1992). The methane is inferred to be generated abiotically by the serpentinization process. Low-temperature hydrothermal aragonite/chrysotile deposits including spherulitic concretions and blue clay were dredged at a fault scarp exposing harzburgites and dunite at the inner corner high of the south rift valley (Silantiev et al., in press). Disseminated sulfides recovered from a fault zone between ultramafic rocks of that inner corner high and a basaltic neovolcanic ridge comprise a suite including pentlandite, nickel sulfide and galena reflecting an ultramafic influence (Fouquet et al., 1992). High-temperature venting was observed about 35 km to the south at 14°45'N in a field containing 12 active and inactive sulfide mounds up to 125 m wide x 200 m long x 20 m high hosted in serpentinized ultramafic rocks in the lower east wall (Batuyev et al., 1994; Bogdanov et al., 1995; Krasnov et al., 1995). One chimney vented a vertical buoyant black smoker plume, while horizontal and downward turning plumes were also observed suggesting variations in solution density (Bogdanov et al., 1995). Massive sulfides sampled from the mounds included chalcopyrite, chalcocite and pyrite with high bulk Cu contents (<40 weight %), high As contents (< 389 ppm), and low Zn contents relative to other occurrences.

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