Basalt recovered from Hole 1155A is slightly to moderately altered by low-temperature alteration. However, a few pieces are highly altered (e.g., Section 187-1155A-5R-1 [Piece 2]). The degree of alteration varies from piece to piece and within pieces. In hand specimen, basalt from Unit 1 (Sections 187-1155A-2R-1 through 4R-1) appears fresher than basalt from Unit 2 (Sections 187-1155A-5R-1 through 7R-1). Some basalt fragments are entirely bounded by buff weathered, roughly planar fracture surfaces (e.g., Section 187-1155A-7R-1 [Piece 6]), suggesting that they are blocky fragments from a talus deposit.
Fractures occur predominantly as exterior faces of pieces and are partially (40%-60%) coated with patchy encrustations of Mn oxide and, in some places, calcite. Fractures cutting across pieces are rare. Alteration halos extend from a few millimeters to 2 cm into pieces. In the most altered pieces, halos are subparallel to the piece margins, but in less altered pieces halo widths can be variable due to the irregular nature of the alteration front. As described for Sites 1153 and 1154, phenocrysts within the alteration halos are not visible on the cut surface in hand specimen but can be seen under the binocular microscope. Within the alteration halos, olivine is replaced by Fe oxyhydroxide where strongly altered and/or by a yellow-green clay (smectite?) where less altered (20%-60%). Olivine is mostly unaffected by alteration in the fresher piece interiors. Plagioclase phenocrysts are fresh throughout. Thin-section inspection of the groundmass confirms that replacement of olivine and clinopyroxene by Fe oxyhydroxide and smectite is lowest (~1%) in the fresher interiors of pieces and highest (~10%-15%) within the alteration halos. Vesicles are variably filled by cryptocrystalline silica, smectite, and Fe oxyhydroxide or by smectite and calcite; in the latter case, calcite forms the center of the vesicle filling, suggesting that calcite precipitation occurred after smectite (Fig. F10A, F10B).
Low-temperature alteration is pervasive throughout Hole 1155B, with recovered basalt pieces ranging from highly to slightly altered. The overall degree of alteration decreases downhole from highly to moderately altered in Sections 187-1155B-2R-1 through 6R-4 to moderately to slightly altered in Sections 187-1155B-7R-1 through 9R-4. Alteration is strongest around fractures and glassy pillow margins and is clearly visible as oxidation halos as wide as 3 cm (Fig. F11). Altered fractures form two roughly orthogonal sets that are oriented perpendicular (1 mm to 1 cm wide) and parallel (<1-3 mm) to the chilled margins (Figs. F11, F12). Fracture densities are highest within several centimeters of the chilled margins, because in this area the fractures commonly form anastomosing networks. They are mainly filled with micritic calcite or sparry calcite; the gradation from micritic to sparry calcite can be seen in thin section. Additionally, sharp contacts between micritic calcite and sparry calcite growing perpendicular to the fracture walls are present in some specimens (Fig. F13).
Mn oxide is commonly associated with calcite, covering the inner walls of fractures (Fig. F12), or occurring as small (<0.2 mm) spots or clusters (0.5 mm) within the calcite veins (Fig. F14). Minute Mn oxide veins <0.1 mm wide commonly dissect even apparently fresh groundmass. In some places, several layers of milky white calcite are separated by thin dark layers of Mn oxide, indicating that the carbonate built up over several episodes, possibly within an opening fracture. Iron-stained silica veins occur exclusively within or near palagonitized pillow margins, reaching up to 4 cm into the pillows; beyond this point, the vein filling abruptly changes to calcite. Thin section inspection of calcite veins, however, reveals a more complex picture with linings of aragonite and silica ± Mn oxides at fracture edges (Fig. F15). One such vein taken near a glassy margin, analyzed by shipboard X-ray diffraction, consists of smectite and quartz; this underlines the complex interplay of fluid migration at the interface of crystallized basaltic rock and altered basaltic glass, and the rapid transition from silica to calcite-rich veins.
Within the glassy margins, symmetric rims of yellowish brown to orange altered basaltic glass (palagonite) are developed along fractures and cracks. Within individual margins, the thicknesses of altered glass layers vary from 300 µm to hardly visible, suggesting several generations of cracks and/or different alteration rates (Fig. F16). The thickness of altered glass may also vary along individual cracks. Generally, the altered glass rims show weak parallel zoning; however, at the alteration front, dendritic features are commonly observed extending into fresh glass, and these are believed to be related to microbial degradation of the glass (Fig. F17). The outer parts of the alteration rims consist of crystalline smectite, whereas the inner parts are amorphous and darker in color (Fig. F18). Within veins, the outer surface of the altered glass is often lined with thin layers (2-10 µm) of Fe oxyhydroxide and/or silica.
In the highly and moderately altered sections, as much as 80% of olivine phenocrysts are replaced by Fe oxyhydroxide and clay; rarely, as much as 20% of plagioclase is altered cloudy white. In slightly altered pieces, olivine is partially to completely altered to Fe oxyhydroxide and clay only within the oxidation halos of fractures, whereas plagioclase remains mostly fresh throughout, even when crosscut by calcite veins (Fig. F19). The groundmass is variably altered through replacement of olivine, clinopyroxene, and mesostasis to smectite and Fe oxyhydroxide, and visible as intense or patchy change to grayish green. Groundmass plagioclase usually remains fresh. Patchy groundmass alteration forms a crude network along which alteration appears to have progressed and split the rock into smaller alteration domains. In summary, alteration in Hole 1155B is mainly controlled by high fracture densities (see "Structural Geology"). Micritic limestone associated with the basalt is a likely source for the calcite vein fillings.