Hole 735B is located on an uplifted block of 11.5-Ma lower oceanic crust east of the Atlantis II Fracture Zone on the Southwest Indian Ridge. During Ocean Drilling Program Leg 118, 504 m of gabbroic rock was penetrated at this site, and the hole was extended 1004 m deeper during Leg 176, reaching a total penetration of 1508 m into lower crustal rock (Dick et al., 1991; Shipboard Scientific Party, 1999). The upper 0.3-0.5 km of the core exhibits abundant effects of high-temperature axial hydrothermal alteration (Dick et al., 1991; Stakes et al., 1991; Vanko and Stakes, 1991), but in the deeper 1 km penetrated during Leg 176, these hydrothermal effects are significantly diminished and are essentially restricted to minor amphibole veins and microveinlets (Shipboard Scientific Party, 1999). Zones of lower-temperature alteration are common locally, as documented by the presence of carbonate and smectite in veins and rocks. As part of a general study of the mineralogy and chemistry of alteration of the drill core, we undertook a study focusing on these lower-temperature hydrothermal effects. This report documents the secondary mineralogy of lower-temperature phases and presents stable and strontium isotopic data for vein minerals in order to understand the conditions of late fluid penetration and alteration. Complementary data on the chemical effects of this alteration on the crust are presented elsewhere (Bach et al., 2001).