Leg 209 was the first Ocean Drilling Program (ODP) expedition designed to investigate spatial (and potentially temporal) heterogeneities in Earth's mantle. The location (north and south of the 15°20´N Fracture Zone on the Mid-Atlantic Ridge; Fig. F1) was selected because precruise dredging and submersible surveys had revealed extensive exposures of mantle peridotite along the walls of the rift valley and on off-axis topographic domes. Four of the eight sites occupied during Leg 209 recovered substantial volumes of rock exhumed from Earth's mantle and emplaced on the western wall of the rift valley. These four sites (1268, 1271, 1272, and 1274) sampled mantle peridotite emplaced in different sections of the ridge-transform system from the inside corner high to near the ridge segment center, as well as a site between the segment center and end.
One of the many striking discoveries from Leg 209 was the degree of variability in hydrothermal alteration of serpentinized peridotites. In general these ranged from alteration of olivine to serpentine accompanied by alteration of orthopyroxene to tremolite and talc to complete replacement of serpentinized peridotite with talc. A distinct manifestation of the variability in alteration of the serpentinized peridotites recovered during Leg 209 is the abundance and distribution of sulfide mineralization. This study forms part of a collaborative effort among several shipboard scientists to document and interpret serpentinization reactions in and hydrothermal alteration of Earth's mantle and to test theoretical and experimental models of fluid circulation in the oceanic crust. This report characterizes the chemical variability of sulfide mineralization in samples from Hole 1268A.