It is clear that the veins in Hole 735B span the entire range of temperatures from magmatic to ambient seawater and thus provide a unique record of the cooling of this crustal segment and the fluids circulating through it. Alteration of the Hole 735B rocks reflects penetration of aqueous fluids into the section and the veins provided pathways by which fluids were able to migrate through the crust. It is clear, however, that in most cases, the fluids passing through the veins did not interact extensively with the immediately adjacent host rock. The vein walls are sharp and the geochemical maps of the veins clearly indicate an abrupt chemical boundary. The only exception to this rule seems to be where olivine was present in the vein wall. In such cases, the olivine is typically replaced by talc and colorless amphibole when adjacent to medium- or high-temperature veins and by carbonate and Fe oxyhydroxides when adjacent to carbonate veins.
Most of the groundmass alteration in the plutonic rocks occurs in areas that have undergone significant brittle or ductile deformation. Cataclastic zones associated with faults are always highly altered, and zones of brittle-ductile deformation show weak to strong metamorphism and alteration. Where brittle-ductile deformation is sparse or absent, for example, in the lower 500 m of the hole, background alteration is weak to nonexistent.
Although crosscutting vein relationships are rarely observed in the Hole 735B core (except for low-temperature, late-stage veins), we infer a vein sequence based on texture, mineralogy, and estimated temperature of formation. The felsic and plagioclase-rich veins are inferred to be the earliest in the sequence. Most of these are 5-10 mm wide and have sharp contacts with the host rock, indicating that they formed by brittle fracture. Some are present as anastomosing networks that splay outward from irregular felsic patches. The felsic and plagioclase-rich veins are most abundant between ~250 and 850 mbsf, and many are associated with the Fe-Ti oxide gabbros in lithologic Unit IV. Their compositions and temperatures of formation strongly indicate that most, if not all, of these veins were originally magmatic. All of these veins are plagioclase rich with relatively high Al2O3 and, hence, are not likely to be hydrothermal. Some have well-developed myrmekitic textures and many contain small quantities of zircon and apatite, suggesting a late-magmatic origin. Their temperatures of formation, according to the edenite-richterite geothermometer of Holland and Blundy (1994), range from 834° to 525°C with a mean of 674°C (standard deviation = ±77°C). The highest temperatures are believed to approximate the original temperatures of formation, whereas the lower temperatures reflect the strong hydrothermal overprint observed in most of these veins.
It is clear that these veins continued to act as pathways for migration of high-temperature hydrothermal fluids long after their igneous formation. The original, relatively calcic plagioclase was corroded and resorbed, producing a porous texture in many samples. Oligoclase and albite were deposited on the rims of the original plagioclase, producing strongly zoned, euhedral crystals. As temperatures decreased further, quartz was deposited between the plagioclase crystals and some of the plagioclase was altered to epidote. In some veins, new cracks developed that were filled with diopside, actinolite, and even lower-temperature zeolites or carbonate. Thus, the felsic and plagioclase-rich veins span a wide temperature range from late magmatic (>700°C) to near ambient seawater. Although they formed originally during the late stages of igneous crystallization, they remained as fluid pathways long after the crustal section had migrated out of the axial zone.
The next to form were presumably the monomineralic amphibole veins, although these may have overlapped with the felsic and plagioclase-rich veins. The amphibole veins are clearly associated with the intense deformation and metamorphism of the gabbros, particularly in the upper 70 m of the hole. Because the well-developed foliation in these rocks both cuts and is offset by the amphibole veins, brittle and ductile deformation were clearly penecontemporaneous. The ductile deformation and metamorphism of the gabbros began at high temperatures (849°-908°C) shortly after their formation. The amphibole gneisses mark the start of hydrous metamorphism and occurred at temperatures between ~590° and 720°C (Stakes et al., 1991), roughly the same temperature range calculated for sparse amphibole + plagioclase veins in this part of the section.
The amphibole veins in Hole 735B are presumed to have formed from the same fluids as those involved in metamorphism of the gabbros. 18O depletion in the plagioclase and amphibole from the amphibole gneisses suggest that these were seawater-dominated fluids (Stakes et al., 1991) that gained access to the gabbro as it was being deformed by a detachment fault at the ridge axis.
Diopside and diopside + plagioclase veins formed at somewhat lower temperatures than either the felsic or amphibole veins and appear to be entirely hydrothermal in origin. Diopside is present either in nearly monomineralic veins or as a minor phase in felsic and plagioclase-rich veins. The estimated temperatures of formation (310°-420°C) are clearly within the hydrothermal range.
The diopside veins are very similar to those reported from the Skaergaard Intrusion by Bird et al. (1986), which have been attributed to brittle deformation associated with subsolidus cooling. Bird et al. (1986) found that formation of diopside was favored where the vein fluids had high Ca/Mg ratios, whereas amphibole formed when the ratios were low. Early precipitation of amphibole would quickly increase the Ca/Mg ratio of the fluid, leading to the formation of diopside.
The other veins in Hole 735B have relatively low temperatures of formation, and most are clearly hydrothermal in origin. A few greenschist facies minerals, such as chlorite, actinolite, epidote, and albite, are present in the groundmass of some felsic veins but rarely form separate veins. They appear to have formed by interaction between cooling fluids and the common silicate minerals in the veins, particularly plagioclase, amphibole, and diopside. Prehnite, smectite, and zeolite veins are common only in the lower part of the section where they are hosted in relatively fresh olivine gabbro. In some cases, these minerals fill new cracks in felsic or plagioclase-rich veins; in others they are present as separate veins. Carbonate veins are irregularly distributed throughout the core and are associated with oxidation of iron in pyroxenes and olivine. They are believed to represent circulation of seawater through late cracks.