Hole 1271B was drilled to 103.8 meters below seafloor (mbsf) on the inner corner high along the south wall of the 15°20´ Fracture Zone (Fig. F1). The total length of core collected was 15.9 m (recovery = ~15%). Recovered cores are so severely altered so that primary lithology was estimated from assemblages of relict and alteration minerals. Spinel remains as a relict mineral in peridotite, although olivine and orthopyroxene are replaced by serpentine. In Hole 1271B, dunite is dominant (56%) followed by brown amphibole gabbro (21%) and olivine gabbro and troctolite (14%). Harzburgite occupies <1% in Hole 1271B. The large amount of dunite in Hole 1271B differs from the other holes drilled near the 15°20´ Fracture Zone, where harzburgite and gabbroic plutonic rocks are more dominant.
The cores in Hole 1271B are divided into four units on the basis of rock type variation (Fig. F2) (Kelemen, Kikawa, Miller, et al., 2004). Units I, II, and IV mainly consist of dunite and brown amphibole gabbro with minor amounts of harzburgite and chromitite, whereas Unit III consists of olivine gabbro, troctolite, dunite, and brown amphibole gabbro. Olivine gabbro and troctolite dominate in the upper and lower horizon of Unit III. In contrast, brown amphibole gabbro is present through all Hole 1271B units. A small chromitite pod enclosed in spinel-rich dunite was recovered from Unit III (interval 209-1271B-13R-1, 32–38 cm).