The entire sequence of basaltic basement cored from Site 1185 has undergone pervasive low-temperature interactions with seawater-derived fluids. We divide the basement section into two superposed zones that display the effects of low-temperature submarine alteration under different conditions and correspond to a difference in primary permeability of the basalt. The upper alteration zone consists of all units in Hole 1185A and Units 1-9 of Hole 1185B, all of which are dominated by pillow-lava flows. The lower alteration zone comprises Units 10-12 of Hole 1185B, which are massive lava flows.
Alteration in all the basalt cored in Hole 1185A and in the upper part (Units 1-9) of Hole 1185B occurred under highly oxidizing conditions and with high water-rock ratios. These characteristics differ significantly from those at Site 1183. The oxidative alteration led to the development of light and dark yellow-brown colors through the complete replacement of olivine and the pervasive alteration of groundmass near the outer zones of cooling units (e.g., pillow margins; Fig. F48). Away from pillow margins, the color grades into dark brown and dark gray in the coarser grained pillow interiors (Fig. F49). However, the aphanitic basalt adjacent to glassy pillow rims is generally almost black, with the exception of brown spherulites (Fig. F20). The spherulites range in diameter from 3 to 6 mm. They formed around olivine phenocrysts as the result of rapid cooling of the basaltic magma near pillow margins. Replacement of olivine by smectite and Fe oxyhydroxide enhances the fibroradial textures by staining the acicular plagioclase and clinopyroxene crystals radiating from the olivine nuclei (Fig. F50). Where abundant, spherulites give the basalt a striking mottled appearance, which is characteristic of Cores 192-1185B-3R (Unit 1), 4R (Unit 2), 9R (Units 7-9), and 10R (Unit 9) from the upper part of Hole 1185B (Fig. F21). Spherulites were not observed in the massive lava flows from the lower part of Hole 1185B (below Core 192-1185B-16R). In the upper alteration zone, basalts commonly have partially altered glassy pillow margins that grade into black aphanitic pillow interiors. A few spectacularly colorful hyaloclastites cemented by calcite are also present in this zone (Fig. F51).
Secondary minerals are predominantly smectite (saponite and nontronite) and Fe oxyhydroxide. The same secondary minerals fill miarolitic cavities and have partly to completely replaced glassy mesostasis. Olivine phenocrysts are commonly completely replaced by smectite, Fe oxyhydroxide, and, more rarely, calcite (Figs. F52, F53). Unaltered olivine is seen only in aphanitic dark gray to black areas close to pillow rims. Rare replacement of the wall rock proximal to veins by zeolite is observed in thin section (Fig. F54).
Basalts below Core 192-1185B-16R show a dramatic change in alteration character to a type similar to that observed at Site 1183. The top of Core 192-1185B-17R (Unit 10) contains breccia made up of angular basalt fragments cemented by sediment; these fragments are the most pervasively altered basaltic material observed during Leg 192 (Fig. F55). Such severe alteration is probably the result of exposure of a highly permeable basaltic seafloor to bottom seawater over an extended period, possibly several million years. This means that Units 9 and 10 probably belong to totally separate episodes of plateau construction. For the basement to remain exposed to seawater during a long period, the sedimentation rate had to be small and/or the basement cropped out along fault scarps.
The groundmass in Core 192-1185B-17R and Sections 192-1185B-20R-1 and 21R-1 is characterized by a dusky green color caused by pervasive replacement by celadonite. Olivine phenocrysts in these basalts are also replaced by celadonite (Fig. F56). Dark yellowish brown to olive and gray are the most common colors, respectively, in halos and in pillow interiors. The least altered basalt below Core 192-1185B-16R is dark gray (Fig. F57). Secondary minerals replacing groundmass and olivine phenocrysts in the dark yellowish brown areas are predominantly smectite with Fe oxyhydroxide (Fig. F58). The gray basalts do not contain Fe oxyhydroxide and have celadonite as a minor component. Calcite is present as a minor phase filling miarolitic cavities and vesicles, together with smectite and celadonite in gray and dusky green basalt and together with smectite and Fe oxyhydroxide in dark yellowish brown basalts (Figs. F59, F60). Unaltered glass is much rarer below the top of Core 192-1185B-17R, largely as a result of the abundance of thicker, massive igneous units (and a scarcity of pillow rims) in the lower part of the hole rather than because of a higher degree of glass alteration.
Veins throughout Holes 1185A and 1185B predominantly contain calcite, zeolites (common phillipsite and rare gmelinite/chabazite), smectite, Fe oxyhydroxide, and rare celadonite and pyrite (Figs. F61, F62, F63, F64, F65). Some veins are filled with micritic pink carbonate that contains Fe oxyhydroxide pellets and foraminifer ghosts, indicating that the veins are sediment-filled open fissures (Fig. F66). These veins range in width from a few centimeters to <1 mm and are located both near the upper boundaries of units and within units, raising the question of the mechanism by which sediments can migrate down into basaltic crust.
Broad (centimeter scale), dusky green halos are developed near veins of Sections 192-1185B-17R-4, 20R-3, and 21R-1 through 21R-3. Reduction fronts that extend a few millimeters to centimeters beyond these halos and consist of scattered pyrite grains are common in the groundmass of the dark gray rock interiors. The dusky green halos may be equivalent to the black halos we described from Site 1183; the greenish shade results from the higher abundance of celadonite. Smaller (millimeter scale) brown halos are developed close to veins and occur both as discrete features and in association with dusky green halos. Where dusky green halos are associated with brown halos, the latter are commonly located proximal to the vein and the former extend toward the dark gray basalt interiors (Fig. F67).
We noticed a clear relationship between vein density and host-rock alteration color. The lighter yellow-brown colors in the basalts are generally associated with portions of cores displaying more horizontal and subhorizontal veins; that is, from 3 to 6 veins per 10 cm of core length (Fig. F49). The degree of alteration is highest in the rocks with the highest permeability (e.g., fractured pillow lavas, hyaloclastites, and breccias) in which the rock color is also the lightest. The reason why horizontal and subhorizontal open fractures are most abundant in some upper portions of cooling units remains to be explained.