Mineralogy and geochemistry of altered ash, pelagic clay, and a clay vein in basalt from DSDP Leg 62 Holes

Volcanic ash was recovered from lower Aptian to Albian deposits from DSDP Sites 463, 465, and 466; pelagic clay of the upper Pleistocene to Upper Cretaceous was recovered mainly from Site 464, with minor amounts at Sites 465 and 466. We present X-ray-mineralogy data on pelagic clay and altered volcanic ash recovered from the four Leg 62 sites. In addition, two ash samples from Sites 463 and 465, a pelagic clay from Site 464, and a clay vein from the basaltic basement at Site 464 each were analyzed for major, minor, and trace elements. Our purpose is to describe the mineralogy and chemistry of altered ash and pelagic clays, to determine the sources of their parent material, and to delineate the diagenetic history of these clay-rich deposits. Correlation of chemistry and mineralogy of ash and pelagic clay with volcanic rocks suspected to be their parent material is not always straightforward, because weathering and diagenetic alteration caused depletion or enrichment of many elements.

Oxygen and sulfur isotopes of peridotite and gabbro from the Mid-Atlantic Ridge

Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20'N Fracture Zone on the Mid-Atlantic Ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement. Three processes are identified. (1) High-temperature hydrothermal alteration (~250-350°C) at Sites 1268 and 1271 is characterized by 18O depletion (2.6-4.4 per mil), elevated sulfide-S, and high delta34S (up to ~2 wt% and 4.4-10.8 per mil). Fluids were derived from high-temperature (>350°C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low-temperature (<150°C) serpentinization at Sites 1272 and 1274 is characterized by elevated delta18O (up to 8.1 per mil), high sulfide-S (up to ~3000 ppm), and negative delta34S (to -32.1 per mil) that reflect microbial reduction of seawater sulfate. These holes penetrate faults at depth, suggesting links between faulting and temperatures of serpentinization. (3) Late low-temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor. Sulfate at all sites contains a component of oxidized sulfide minerals. Low delta34S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low-delta34S sulfide derived from microbial sulfate reduction. Results show that peridotite alteration may be commonly affected by fluids +/- heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor.

Sulphate and d34S in pore waters and d13C, d18O and mineral composition of carbonate nodules from ODP Leg 188 sites

Carbon and oxygen isotopic compositions of authigenic carbonate nodules or layers reflect the diagenetic conditions at the time of nodule growth. The shallowest samples of carbonate nodules and dissolved inorganic carbon of pore water samples beneath the sulfate reduction zone (0-160 meters below seafloor [mbsf]) at Site 1165 have extremely negative d13C values (-50 per mil and -62 per mil, respectively). These negative d13C values indicate nodule formation in association with anaerobic methane oxidation coupled with sulfate reduction. The 34S of residual sulfate at Site 1165 shows only minor 34S enrichment (+6 per mil), even with complete sulfate reduction. This small degree of apparent 34S enrichment is due to extreme "open-system" sulfate reduction, with sulfate abundantly resupplied by diffusion from overlying seawater. Ten calcite nodules from Site 1165 contain minor quartz and feldspar and have d13C values ranging from -49.7 per mil to -8.2 per mil. The nodules with the most negative d13C values currently are at depths of 273 to 350 mbsf and must have precipitated from carbonate largely derived from subsurface anaerobic methane oxidation. The processes of sulfate reduction coupled with methane oxidation in sediments of Hole 1165B are indicated by characteristic concentration and isotopic (d34S and d13C) profiles of dissolved sulfate and bicarbonate. Three siderite nodules from Site 1166 contain feldspar and mica and one has significant carbonate-apatite. The siderite has d13C values ranging from -15.3 per mil to -7.6 per mil. These siderite nodules probably represent early diagenetic carbonate precipitation during microbial methanogenesis.