DATA REPORT : OXIDES , SULFIDES , AND ASSOCIATED PHASES IN VEINS AND HYDROTHERMALLY ALTERED PERIDOTITIC ROCKS

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 FEATURES Petrographic 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. Wide (about 5 mm), randomly oriented, open fractures are filled by radiat1 Karson, J.A. , Cannat, M. , Miller, D.J., and Elthon, D. , (Eds.), 1997. Proc. ODP, Sci. Results, 153: College Station, TX (Ocean Drilling Program). Dipartimento di Scienze della Terra, Universita di Genova, Corso Europa 26, I— 16132 Genova, Italy, gaggero@dister.unige. it Dipartimento di Scienze della Terra, Universita di Modena, Piazzale S. Eufemia 36,1-41100 Modena, Italy. 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 widespread and, in part, obscures earlier alteration phases (Shipboard Scientific Party, 1995). In the present study, minerals were identified and phase relationships determined by optical examination of 36 polished thin sections in transmitted and reflected light. Quantitative electron microprobe analyses of mineral phases were determined using a scanning electron microscope (SEM-EDS) equipped with an X-ray dispersive analyzer (EDAX PV 9100) microprobe, installed at the Dipartimento di Scienze della Terra, Universita di Genova. Operating conditions were 15-kV accelerating voltage and 2.20-nA beam current. Natural standards were used. The cation sum of Ca amphiboles was normalized to 13 (Ca + Na + K), as suggested by Laird and Albee (1981); Fe = 46 total cation charge, and Fe = Fe t o t Fe ; A1 = 8 Si, and A1 = Altot A1. The nomenclature of Leake (1978) was adopted. H2O was allocated on the basis of stoichiometric OH". Plagioclase analyses were recalculated to total cations = 5, on the basis of 8 oxygens. Garnets were recalculated to total cations = 8, on the basis of 12 oxygens; end members were recast according to Rickwood (1968). Chromites were recast on the basis of 4 oxygens. Ilvaite was recast to 6 cations and 8.5 oxygens.