Serpentinization and hydrothermal veining in peridotites at Site 920 in the MARK area

Abstract

Cores from Site 920 in the Mid-Atlantic Ridge at the Kane Transform (MARK) area record polyphase deformation in two holes reaching 126 and 200 m depth in serpentinized peridotites with minor metagabbros, amphibolite gneiss, and diabase units. The majority of the recovered mantle-derived rocks are composed of serpentinized harzburgites, which display a variably elongated, medium-to coarse-grained Porphyroclastic texture produced by high-temperature crystal-plastic deformation. The Porphyroclastic texture is overprinted by an anastomosing foliation defined by dark green to black serpentine and magnetite bands that wrap around orthopyroxene porphyroclasts. These bands are composed of a variant of mesh-textured serpentine, and contain mainly lizardite ± chrysotile, whereas mesh centers consist of fine-grained lizardite + brucite. Serpentine fibers in the bands are generally wall perpendicular, suggesting that the foliation is mainly a dilational fabric. The anastomosing foliation is cut by a set of veins (VI), which contain serpentine + actinolite + chlorite ± talc and oblique-slip fibers. They display a wide range of dips with no preferred dip orientation, and indicate shearing synchronous with dilation. Compared to veins of later generations and to the pseudomorphic serpentine, serpentine in the VI veins is relatively rich in iron, with FeO (total Fe as FeO) content ranging from 4.55 to 5.76 wt%. A network of thin and discontinuous veins (V2) composed of chrysotile and mag-netite postdates the VI veins and is either parallel to the anastomosing foliation and to the mesoscopic Porphyroclastic fabric or, less commonly, is at a high angle to the foliation. Both the intensity of the anastomosing foliation and the spacing and distribu-tion of V2 veins are strongly dependent on the porphyroclast content of the peridotite; a reduction in the pyroxene content com-monly coincides with a drop in the intensity of the anastomosing foliation and the near absence of V2 veins. The V2 veins have a mean attitude of 336729°NE for both holes that overlaps with the corrected attitudes of the anastomosing foliation and the mesoscopic Porphyroclastic fabric. More discrete and commonly branching veins (V3) crosscut the V2 veins and contain pale green-white chalky serpentine identified as lizardite, with minor amounts of carbonate and clay minerals, pyrite, and magnetite. They display wall-parallel bands with a pseudofibrous internal fabric and wallrock inclusions characteristic of antitaxial crack-seal veins. They have consistently steep dips in both holes. The latest generation (V4) of veins contains carbonate, pyrite, and/ or clay minerals, and locally reactivates the earlier veins. The dominant fabric of the anastomosing foliation and the serpentine mineral phases in serpentinized peridotites indicates that much of the hydration and static serpentinization occurred at temperatures around 350°-400°C. Gabbroic veins and vein-lets represent a phase of magmatic veining in the serpentinized peridotites before formation of the hydrothermal veins, and they are altered by Ca-enriched fluids under greenschist facies conditions at temperatures of 300°-400°C. The VI veins with oblique slip-fibers indicate dilation and contemporaneous shearing that enhanced fluid circulation into the serpentinized peri-dotites under greenschist facies conditions. The V2 veins are mainly extensional, as evidenced by the wall-perpendicular ser-pentine fibers. They were produced by elevated pore fluid pressures contemporaneous with stress release during exhumation of the peridotites. Reactivation of these veins during later extensional events and the generation of V3 crack-seal veins were related to further unroofing of the serpentinites and their emplacement along the rift valley walls because of extensional tecton-ics. Composite V4 veins containing carbonate + pyrite ± clay minerals indicate the sealing of extensional fractures in the ser-pentinized peridotites and the cessation of fluid flow following the emplacement of the ultramafic suite on the western wall of the median valley. Structural evidence from the core samples studied and the regional geological data suggest that the exposure of serpentinized peridotite on the seafloor in the MARK area is a result of exhumation by tectonic extension in the absence of an active magma chamber, rather than a result of diapiric ascent of serpentinite.