Five sites were drilled during Ocean Drilling Program (ODP) Leg 149 at the ocean/continent transition west of Portugal (Fig. 1). Of these sites, Holes 897D and 899B of Sites 897 and 899, respectively, reached acoustic basement, and a sequence of serpentinized peridotite and associated mantle rocks was recovered from the basement. At Site 897, which is situated in the ocean/continent transition over a north-south basement ridge that has been linked to a peridotite ridge drilled west of Galicia Bank 140 km to the north (see Fig. 1), cores were obtained from three holes, which penetrated up to 694 m of Pleistocene to Lower Cretaceous sediments and up to 153 m of basement. Site 899 lies within the same part of the ocean/continent transition and is about 20 km east of Site 897. At Site 899, 563 m of upper Pliocene to Upper Cretaceous sediment overlying the serpentinized peridotites were penetrated in two holes (Fig. 2).
The peridotite recovered during Leg 149 has suffered extensive low-temperature and late-stage alteration, which includes serpentinization, serpentine and calcite veining, and replacement by calcite. The two most obvious visible features resulting from this alteration are color variation and calcite veining. At both sites, the upper part of the peridotite section immediately underlying the Pleistocene to Lower Cretaceous sedimentary cover is pervasively veined and altered to a brown-colored serpentinized peridotite and breccia. The finer grained matrix within the upper part is dominantly dark yellowish brown (10YR 4/2) to moderate yellowish brown (10YR 5/4). The lower part of the peridotite section consists of "fresher" (compared with that of the upper part of the section) serpentinized peridotites, which are generally dark green (5GY 4/1) or greenish black (5G 2/1). At both holes, Lower Cretaceous (upper Hauterivian) claystone and siltstone are found at the bottom of the basement section (see Fig. 2), although these sediments in Hole 897D are relatively minor compared with those of Hole 899B. Preliminary shipboard geochemical studies indicate that originally the peridotite section was relatively uniform in composition, but that now the "altered" rocks in the upper part are significantly enriched in CaCO3, Sr, Ba, P, K, and V, but depleted in Mg, Fe, and Si relative to the underlying fresher greenish peridotites (Sawyer, Whitmarsh, Klaus, et al., 1994). Conversely, elements such as Ni and Cr, abundant in the unaltered fresher lower part of the section, are depleted in the altered upper part. Shear textures of the altered peridotites within the upper part of the section are confined to basal margins of these rocks, suggesting they formed at shallow depth during the addition of seawater (Seifert et al., 1993). Relict mineral and X-ray fluorescence data indicate that the peridotites were dominantly lherzolite with lesser amounts of harzburgite and dunite.