Aphyric basalt from Hole 1164A has been slightly to moderately altered at low temperature. In some places, small patches of clay or Fe oxyhydroxide and spots (<0.5 mm) or dendritic patches of Mn oxide cover the outer surfaces of pieces. The fracture plus vein density is comparatively low, 4.5/m. The few fractures present are <0.3 mm wide, free of alteration halos, and either lined or filled with whitish blue cryptocrystalline silica ± Mn oxide. The highest degree (15%-30%) of alteration occurs in 1- to 20-mm-wide halos that are aligned subparallel to piece or chilled margins (e.g., Section 187-1164-2R-1 [Piece 1]). Broader alteration halos are subdivided into outer and inner zones. The outer zones are as wide as 15 mm and altered to grayish brown; they are characterized by pervasive replacement of groundmass and total replacement of olivine microphenocrysts by Fe oxyhydroxide and clay. The inner zones are narrower (3-5 mm) and less altered to dark gray; these are characterized by patchy groundmass replacement and partial (20%-50%) replacement of olivine microphenocrysts by Fe oxyhydroxide and clay. Lining or filling of vesicles with cryptocrystalline silica, calcite, Fe oxyhydroxide, green clays, and Mn oxide occurs mainly in the alteration halos. A sharp boundary separates the alteration halos from the light gray fresher interiors. Calcite (<1%) replaces groundmass, fills miarolitic cavities, (Fig. F21) and replaces olivine microphenocrysts in the fresher interiors of pieces (Fig. F22). Quenched pillow margins are present throughout the core. These typically consist of 1- to 3-mm-wide zones of fresh glass covered by 0.5 mm layers of orange-brown palagonite.
Subrounded basalt pieces with outer surfaces weathered to yellowish brown are present throughout the core. Together with an unsystematic intermixture of aphyric and slightly phyric basalts (Sections 187-1164-1W-2 through 4R-1), this suggests that a rubble deposit was sampled (see "Igneous Petrology"). The sparsely plagioclase-olivine phyric basalt, aphyric basalt, and basalt breccia from Hole 1164B were subject to pervasive low-temperature alteration. About 85% of the rock from this hole is highly (~55%) or moderately (~30%) altered (Table T2). The least-altered rocks (i.e., where >50% of each piece is classified as slightly altered) are restricted to Sections 187-1164B-1W-1, 1W-2, and 10R-1. Six veins were recorded, giving a fracture plus vein density of 10/m of core. Fractures are rare; where present, they are usually lined with Mn oxide and, in places, with Fe oxyhydroxide or clay. Very minor small veins (<0.2 mm) are mostly filled with Mn oxide. Veins and fractures do not have substantial alteration halos. Vesicles are rare throughout (<1%); where present, they are 0.3-0.5 mm and spherical, and most are unfilled.
The highly (40%-80%) altered basalts are characterized by an intense brown coloration. These are either pervasively altered (Fig. F23) or have alteration halos that extend 10-30 mm into piece interiors and make up >60% of individual pieces. Within these halos, alteration is progressively less intense toward the interior of individual pieces. The alteration halos are characterized by 30%-60% replacement of the groundmass by Fe oxyhydroxide and clay. Pieces with more pervasive groundmass alteration (>50% of the groundmass is replaced by Fe oxyhydroxide and clay) have more intense brown coloration. Groundmass olivine (Fig. F23), clinopyroxene, and mesostasis (Fig. F24) are commonly altered, whereas plagioclase is fresh throughout except for Mn oxide staining. In addition, 2- to 4-mm-sized irregularly shaped patches of Mn oxide replace groundmass phases (Fig. F25). Olivine phenocrysts are totally replaced by Fe oxyhydroxide and clay; plagioclase pheno-crysts are occasionally stained black by Mn oxide or yellowish by Fe oxide but are otherwise fresh. Within the alteration halos or in pieces with pervasive groundmass replacement, some vesicles are lined with cryptocrystalline silica and/or Fe oxyhydroxide or filled with clay.
Moderately altered basalts have grayish brown interiors rimmed by comparatively thin alteration halos (1.5-5 cm) making up <60% of each piece. Within the alteration halos, the groundmass is mostly replaced (30%-60%) by Fe oxyhydroxide and clay, as it is in the highly altered pieces. The most clearly visible expressions of groundmass alteration are olivine microphenocrysts, altered completely to reddish brown Fe oxyhydroxide and clay. Within the fresher interiors of pieces, patchy areas of groundmass (1-8 mm across) are replaced by Fe oxyhydroxide and clay. Olivine phenocrysts are 100% replaced by Fe oxyhydroxide and clay, whereas plagioclase phenocrysts are mostly fresh. Some vesicles within the alteration halos are lined with cryptocrystalline silica and/or Fe oxyhydroxide or filled with clay.
The few slightly altered rocks (Table T2) typically have alteration halos 1 to 10 mm wide aligned subparallel to the piece margins; these halos do not exceed 30% of the total volume of individual pieces. Within alteration halos, the groundmass is 20%-50% replaced by Fe oxyhydroxide and clay. In many cases, the alteration of olivine microphenocrysts to Fe oxyhydroxide and clay within the alteration halos makes it difficult to distinguish phyric from aphyric basalt. This is because fresh, transparent olivine microphenocrysts are not readily visible relative to the dark groundmass of slightly altered pieces (Fig. F9).
In the fresher, light to medium gray interior of some pieces, miarolitic cavities are commonly lined by Mn oxide and filled with calcite (Fig. F14). Calcite is the most abundant secondary mineral in the interiors of slightly altered pieces. In contrast, calcite is almost entirely absent from the moderately and highly altered pieces. This suggests that calcite deposition takes place at the initial stages of low-temperature alteration. Chilled margins are present in most sections. Fresh glass rinds range from <1 to 10 mm in thickness (Fig. F25), usually with associated orange-brown layers or small veins of palagonite.
Glass shards make up 10%-60% in the 2- to 3-mm range of the basalt breccia in Sections 187-1164B-3R-1 and 4R-2 (see "Igneous Petrology"). Both fresh and palagonitized shards are common, suggesting that palagonitization occurred before breccia formation (Fig. F26). Palagonitized glass is commonly rimmed by dark brown palagonite that, in contrast to earlier sites, does not display dendritic textures along the palagonite/glass interface (Fig. F27).