Ocean Drilling Program (ODP) Hole 735B is situated on top of Atlantis Bank, an uplifted and unroofed section of lower ocean crust (Fig. F1). Massive and foliated gabbros are exposed on the bank in 650-700 m of water. Hole 735B, drilled during Legs 118 and 176, penetrated 1508 m into the section with an average recovery of 87% (Robinson, Von Herzen, et al., 1989; Shipboard Scientific Party, 1999; Dick et al., 2000). Most of the unrecovered core was in the upper 100 m of the hole or in fault zones located at various levels in the section. Thus, for much of the drilled sequence core recovery was close to 100%, providing a nearly complete sample of much of oceanic Layer 3. Based on variations in texture and mineralogy, 12 major lithologic units are recognized in the section, ranging from 39.5 to 354 m thick (Robinson, Von Herzen, et al., 1989; Shipboard Scientific Party, 1989, 1999). The principal lithologies include troctolite, troctolitic gabbro, olivine gabbro and microgabbro, gabbro, gabbronorite and Fe-Ti oxide gabbro, gabbronorite, and microgabbro. Highly deformed mylonites, cataclasites, and amphibole gneisses are locally present, as are small quantities of pyroxenite, anorthositic gabbro, and trondhjemite.
Igneous and hydrothermal veins are abundant and widely distributed throughout the drilled section, but most are present in the upper 800 m. Individual veins were not logged during Leg 118, but the area percentages of the different vein types were estimated later (Dick et al., 1991). During Leg 176, each vein in the core was logged and its composition and dimensions were recorded. Nearly 3000 veins were logged in the core from the lower 1053 m of the hole (Table T1). Smectite veins are the most numerous, making up nearly half the total number, followed in order of abundance by amphibole, amphibole + plagioclase, carbonate, plagioclase, and felsic veins. These six varieties make up 92% of the total veins logged during Leg 176, and only three other varieties comprise >1% each.
Using data from both legs, we estimated the area percentage of veins for the entire section (Fig. F2). Although smectite veins are the most numerous, felsic and amphibole veins are far more voluminous. Felsic veins alone make up ~38% by area, and amphibole and amphibole + plagioclase veins combined make up ~41%. Diopside and plagioclase + diopside veins combined make up nearly 15% by area, and all the rest account for <6%.
The veins listed in Table T1 were identified macroscopically on the basis of their dominant mineralogy. Detailed petrographic examination revealed considerable overlap in mineralogy among the different veins; for example, most diopside and plagioclase veins contain at least small amounts of amphibole, many plagioclase veins contain significant amounts of quartz, and many smectite veins have small amounts of carbonate and prehnite.
Compositional zoning in some veins further complicates the classification. Typically, though not always, relatively high-temperature minerals are present along the vein walls and lower temperature minerals near the center. In a few cases, veins with relatively high-temperature mineral assemblages have reopened and the new crack is filled with low-temperature minerals such as smectite, carbonate, or zeolite.
Based on our thin section descriptions, we recognize seven major types of veins: felsic and plagioclase rich, plagioclase + amphibole, amphibole, diopside and diopside + plagioclase, smectite ± prehnite ± carbonate, zeolite ± prehnite ± carbonate, and carbonate. A few epidote and chlorite veins are also present but are volumetrically insignificant. Although the groups are named for their dominant mineralogy, most veins contain at least a small percentage of several other minerals. Most of the veins show no obvious correlation with primary lithology, although felsic varieties are most abundant in the oxide gabbros of lithologic Unit IV.
The vein assemblage described here is quite different from that recovered from Hess Deep gabbros in ODP Hole 894. The three main types recovered from that section are amphibole veins, composite veins containing mixtures of chlorite ± prehnite ± epidote ± titanite ± clay minerals, and smectite ± chlorite ± zeolite ± calcite veins (Mével, Gillis, Allen, and Meyer, 1993; Manning and MacLeod, 1996; Früh-Green et al., 1996). The abundant felsic, plagioclase + amphibole, and plagioclase + diopside veins in Hole 735B appear to be entirely absent from the Hess Deep section.