ALTERATION

Basalts recovered from Holes 1152A and 1152B are generally slightly altered but show variable degrees of alteration within individual pieces. Macroscopically, alteration seems to be confined to coatings, vesicle fillings, veins, fractures, and distinctive concentric zones a few millimeters to several centimeters in width. Clay minerals (smectite), Fe oxyhydroxides, and cryptocrystalline silica are the predominant secondary minerals. These minerals occur as fillings of vesicles, fractures, and veins. Alteration minerals of the groundmass include smectite and Fe oxyhydroxide, which commonly replace clinopyroxene and olivine. Small (<1 mm) spots of Mn oxide are present on the weathered surfaces of basalt fragments from Hole 1152B. A crust consisting of dendritic textured Mn oxide interfingered with pelagic sediment is present near the top of Unit 1 in Hole 1152B (Fig. F7). In the same section, XRD analysis of a vein (Sample 187-1152B-2R-1 [Piece 1]) reveals the presence of smectite and minor quartz.

Distinctive concentric alteration halos follow the shape of the outer surfaces of individual basalt pieces from Hole 1152B. These zones usually have sharp smooth contacts with the less altered interior, although in some pieces they are more gradational and irregular (e.g., Section 187-1152B-6R-1 [Piece 5]; Fig. F8). Based on color, texture, abundance of secondary minerals, filling of vesicles, and apparent differences in crystallinity, two distinct alteration zones can be distinguished both macroscopically and microscopically:

1. The outermost zone is light brown, 1-3 mm thick, and dominated by smectite and Fe oxyhydroxide. This type of alteration usually affects the original (predrilling) surfaces of basalt clasts, especially in the lower half of Unit 2 of Hole 1152B, and also occurs along fractures.
2. Adjacent to the outer zone is a light gray zone, which is usually 5-10 mm wide. In cut surfaces of hand specimens this zone appears to be aphyric and unaltered. However, under the microscope, plagioclase and olivine phenocrysts are visible and small vesicles (<0.5 mm) are seen to be completely filled by smectite. In some areas, the groundmass has been replaced by the same clay mineral; groundmass alteration decreases toward the interiors of pieces. Plagioclase phenocrysts appear unaffected by this alteration, but olivine phenocrysts are partially iddingsitized (Fig. F9).

The macroscopically sharp boundary separating the outer and inner zones has a more irregular character in thin section, manifested by a distinct decrease in smectite alteration of the groundmass. The interior portions of each piece are medium gray and slightly vesicular; both groundmass and phenocryst phases are free of alteration. The distinct alteration halos are therefore interpreted to reflect low-temperature alteration fronts, progressing from the exterior and fracture surfaces to the center of each fracture-bounded basalt fragment. The degree of alteration varies with clast size, type of quench zone texture (such as spherulitic, sheaf, or plumose), and fracture density. The degree of alteration does not change with depth.

Rare glassy pillow rinds of 0.5-10 mm thickness were recovered from both Hole 1152A and 1152B. Within the rinds, glass ranges from fresh to completely altered. Fresh glasses are dark brown in thin section and shiny black in hand specimen. Altered glass (palagonite) is yellow to orange and has dull surfaces. Fresh glass and palagonite commonly occur in thin (0.5-2 mm), irregularly interlayered sheets, with palagonite being most abundant (>90%) in the outermost sections. Macroscopically, the boundary between fresh glass and palagonite is sharp; thus, alteration appears to have preferentially progressed along (cooling?) cracks.