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

• Main Authors: Alt, Jeffrey C, Shanks, Wayne C, Paulick, Holger, Garrido, Carlos J, Beaudoin, Georges
• Format: Other/Unknown Material
• Language: English
• Published: PANGAEA 2007
• Subjects: 209-1268A; 209-1270A; 209-1270B; 209-1270C; 209-1270D; 209-1271A; 209-1271B; 209-1272A; 209-1274A; 209-1275B; 209-1275D; DRILL; Drilling/drill rig; Joides Resolution; Leg209; North Atlantic Ocean; Ocean Drilling Program; ODP; Mid-Atlantic Ridge; North Atlantic
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.734345; https://doi.org/10.1594/PANGAEA.734345

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.