Whereas serpentinized peridotites recovered during the previous two ODP expeditions targeting exposures of mantle rock were from closely spaced drilling targets, during Leg 209 we sampled four sites (1268, 1271, 1272, and 1274) from the western wall of the Mid-Atlantic Ridge separated by nearly a degree of latitude and from both north and south of the 15°20´N Fracture Zone. The cores from these sites represent 600 m of penetration and show an extensive range in degree and style of hydrothermal alteration and sulfide mineralization. Owing to the division of labor among shipboard scientists, this study focuses only on recovery from Site 1268.
Twenty-nine cores were recovered to a depth of nearly 148 mbsf. Two general types of lithologies were recovered at Site 1268, ultramafic (harzburgite and dunite) and gabbroic rocks. The upper 100 m of the section is nearly exclusively ultramafic except for a 15-m interval between 63 and 77 mbsf that includes a gabbroic intrusion breccia. Most of the sulfide minerals described in this paper were found in veins and in halos around veins in serpentinized peridotite. The only sulfide phases noted that occurred in association with mafic rocks are those from cores within or in close proximity to the mafic intrusion breccia (Cores 209-1268A-12R through 15R). Below 100 mbsf, the recovery was predominantly gabbroic rock. Details of the lithologic variability are available in the Leg 209 Initial Reports volume (Shipboard Scientific Party, 2004).
Cores from this site are intensely veined with fairly abundant but highly localized sulfide and sulfide/oxide-bearing veins. Sulfide mineral bearing veins compose as much as 10% of the total vein volume and at least 1 vol% of the bulk core. Primary sulfides were identified on board as pyrrhotite and pentlandite and the presence of millerite was identified as an alteration product, suggesting reducing conditions during mobilization of sulfur.