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

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(...)