Data report: oxides, sulfides, and associated phases in veins and hydrothermally altered peridotitic rocks

Abstract

The ductile to brittle deformation and the associated hydrothermal mineralogical record in Holes 920B and 920D peridotites are complex. Remnants of a high-temperature phase (brown hornblende, talc) are preserved in the seΦentinized harzburgites. Primary chromites in the harzburgite are altered to ferritchromite. Magnetite, which originated from olivine breakdown during seΦβntinization, (1) recrystallizes, mimicking mesh and ribbon textures, and (2) reorganizes in veins and trails. Primary sulfide minerals rarely survive the extensive seΦentinization, and are affected by significant mobilization.An ilvaite + chlorite + fibrous salitic pyroxene ± carbonate ± grandite garnet + magnetite ± sulfides (pyrite > pyrrhotite > chalcopyrite) assemblage that developed in pyroxenite veins is cut by later seΦentine veins. Late hydrothermal veins, pyrite (+ silicates) precipitate in veins, which are cut in turn by pyrrhotite + chalcopyrite (+ phyllosilicate) veins. PETROGRAPHIC FEATURESPetrographic features and intersection criteria allow us to discriminate three main events in Holes 920B and 920D harzburgites: (1) hydrous alteration preceding serpentinization, (2) the main serpentinization, and (3) late processes that overprint the serpentinite texture. A polyphase set of composite macroscopic veins is developed through the peridotite and is less developed in gabbro and diabase intervals. Pyroxenitic and gabbroic veins of magmatic origin are discordant with the high-temperature plastic fabric, and predate the development of serpentine-bearing veins (Shipboard Scientific Party, 1995, fig. 22).Vein systems are the most characteristic features associated with fluid introduction during serpentinization, and control metamorphic recrystallization during uplift. Different sets of hydrothermal veins, ascribed to the main serpentinization and to late processes overprinting the serpentinite texture, were distinguished in chronological order on the basis of visual core descriptions (Shipboard Scientific Party, 1995):1. Serpentine + magnetite veins. An early net of fine serpentine veins (0.03-0.05 mm wide) postdates the intrusion of igneous veins. The serpentine veining is associated with the pervasive background alteration of olivine and orthopyroxene. 2. Serpentine + amphibole ± chlorite ± talc. This set of veins is poorly developed, and consists of dark-green to black veins, 2-3 mm wide. An alteration halo of chlorite ± serpentine ± amphibole is commonly present along the vein-host rock boundary. 3. Serpentine + magnetite. These veins (up to 1 mm wide) occur as a widespread anastomosing network of white serpentine and black magnetite around orthopyroxene porphyroclasts. 4. Serpentine ± carbonate ± pyrite ± clay minerals. ing chrysotile and aragonite and are coated by sulfides and minor phyllosilicates. 5. Carbonate minerals ± sulfides ± smectite. This set is represented by a thin (up to 1 mm wide) network with a random orientation that has sometimes reactivated earlier vein arrays. The serpentinization of harzburgite is wides...

Ocean‐floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

“…These compositions are similar to those observed in serpentinized peridotites (e.g. ODP leg 920B; Gaggero & Cortesogno, 1997b).…”

Unravelling the root zone of ultramafic‐hosted black smokers‐like hydrothermalism from an Alpine analog

“…On the deepest parts, the mineralogical zonation from barren serpentinite to Fe‐Ca‐silicates to semi‐massive sulphides and then to Cu‐rich MS (Figures e and b) has never been reported before this study. The ilvaite‐andradite‐diopside assemblage has been described in low‐grade metamorphic serpentinites (Agata & Agachi, ) and in oceanic ultramafic settings (Gaggero, Cortesogno, & Gazzoti, ). Gaggero et al () suggested that this assemblage precipitated from early serpentinization‐derived fluids.…”

“…The ilvaite‐andradite‐diopside assemblage has been described in low‐grade metamorphic serpentinites (Agata & Agachi, ) and in oceanic ultramafic settings (Gaggero, Cortesogno, & Gazzoti, ). Gaggero et al () suggested that this assemblage precipitated from early serpentinization‐derived fluids. Gustafson () showed that the ilvaite‐hedenbergite‐andradite‐magnetite assemblage is stable at low f O2 (10 –25 to 10 –32 ), high‐T (425 ± 75°C) conditions without strong changes if considering diopside instead of hedenbergite.…”

Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites

“…All serpentine analyses, except those of V4 veins, also contain significant amounts of sulfur, probably located in sulfide micrograins disseminated both in serpentine and among magnetite grains in veins [ Gaggero et al , 1997; Alt and Shanks , 2003]. The high sulfur content of MARK serpentinites has already been noted by Alt and Shanks [2003], which they explain by invoking an earlier phase of fluid circulation through grabbros before percolating through ultramafic units.…”