Basalt from Hole 1158A is slightly to moderately altered. Most of the basalt pieces are subangular and 2-5 cm in diameter, and these have weathered, uncut surfaces, indicating that this is rubble. Uncut surfaces of all pieces have spots or coatings of Mn oxide (as thick as 1 mm in Sample 187-1158A-1W-2 [Piece 5]) and/or thin (<0.2 mm) patches of white cryptocrystalline silica, which is commonly associated with spots of Mn oxide. Alteration halos extend 6-8 mm from the edges into the pieces. Within these halos, olivine is mostly (70%) replaced by Fe oxyhydroxides, and groundmass is replaced by smectite. The intensity of alteration does not change systematically downhole. Vesicles (1%; ~0.5 mm) are lined with bluish cryptocrystalline silica or yellowish green smectite. The vein density averages 1.1/m of core. Rare veins are ~1 mm wide and filled with silica that is in places Fe stained. The veins are surrounded by up to 2-mm-wide oxidation halos in which the groundmass is replaced by Fe oxyhydroxide and smectite. Within the fresher parts of the basalt, groundmass olivine ranges from fresh (e.g., Section 187-1558-3R-1) to 60% replaced by Fe oxyhydroxide. Replacement of groundmass by smectite is also common (Fig. F7).
The outermost layer of some glassy margins (e.g., Section 187-1158A-1W-2 [Piece 1]) consists of brownish orange, up to 2-mm-thick, dense palagonite that gives the altered glass a cherty appearance not seen in the previous sites. Bleached and partly silicified spherulitic quench zones are also common. In places, Fe oxyhydroxide and clay partially replace the glassy groundmass of the spherulitic quench zone and thereby highlight spherulite textures (spherulitic vs. coalesced spherulites).
Basalt in Sections 187-1158B-2R-1 and 3R-1 shows only slight low-temperature alteration effects, whereas Section 187-1158B-4R-1 is moderately altered. Uncut surfaces are generally weathered to a buff color, with oxidation halos reaching 0.5-10 mm into the basalt, suggesting again that this is rubble. Surficial coatings such as Mn oxide spots, silica, or sediment are absent. Vesicles (<1%, 0.5 mm) are variably filled or lined with Mn oxide, smectite, or bluish cryptocrystalline silica. The average vein density is 0.6/m, but the combined fracture + vein density averages 21.2/m, which is higher than in Hole 1158A (8.9/m). The rare veins are <1 mm wide, filled with silica (in places Fe stained), and usually surrounded by 1- to 8-mm-wide oxidation halos. Fractures are lined with Mn oxide or cryptocrystalline silica. Minute fractures commonly extend from phenocrysts into the groundmass or appear to migrate around phenocrysts. Both observations may reflect the rheological differences between phenocrysts and groundmass.
Olivine phenocrysts are most severely altered (100%) within the oxidation halos of the weathered margins and are partially (65%-80%) replaced by Fe oxyhydroxides and smectite in the fresher interiors. Plagioclase phenocrysts are fresh throughout, except within oxidation halos, where ~3% are replaced by a cream-white clay. Within the slightly altered sections <10% of the groundmass is replaced by Fe oxyhydroxide and smectite, whereas moderately altered sections display 10%-20% alteration of the groundmass to Fe oxyhydroxide and smectite. Glass rinds were only recovered in Section 187-1158B-4R-1 and, generally, are partially altered to palagonite that is dissected by quartz filled veins (<0.3 mm), the majority of which are aligned subparallel to the chilled margin. In places, palagonite is coated with bluish silica. The groundmass of chilled margins (spherulitic through coalesced spherulite zones) is partly replaced by a white-gray mixture of clay/silica and rarely by brown Fe oxyhydroxide, which highlights spherulites up to 1.5 mm in diameter.
Aphyric basalt of igneous Unit 1 (see "Igneous Petrology") is slightly to moderately altered with <10%-20% of groundmass replaced by Fe oxyhydroxides and smectite. Again, buff weathered surfaces on the uncut surfaces of pieces in Section 187-1158C-1W-1 suggest that this is rubble. As noted for Hole 1158B, surface coatings such as Mn oxide spots, silica, or sediment are not present. Vesicles (<1%) are filled with smectite and/or Fe oxyhydroxide. Rare open fractures are <1 mm wide; some are lined with smectite, and a few are surrounded by up to 2-mm-wide oxidation halos. Groundmass olivine is mostly (~60%) replaced by Fe oxyhydroxide; replacement of groundmass by smectite and Fe oxyhydroxide ranges from ~10% to 20% overall.
Diabase of igneous Unit 2 (see "Igneous Petrology") appears moderately to highly altered in hand specimen because of pervasive Fe staining and the subophitic texture that gives the rock a friable texture and grainy appearance. The uncut surfaces are weathered brown but are free of Mn oxide, silica, or sediment coatings. In some cases (e.g., Section 187-1158C-2R-1 [Pieces 9 and 14]), oxidation halos extend as far as 15 mm from weathered fracture surfaces into the diabase and are marked by higher abundances of Fe oxyhydroxide. Throughout the section, veins are absent, and the fracture density is comparatively low, with 3.4 fractures per meter of core. Vesicles (5%-7%; ~1 mm) are filled with yellowish smectite, Fe-stained silica, and, rarely, Mn oxide. In Pieces 19-27 of Section 187-1158C-2R-1, 10% of plagioclase is replaced by a pale clay. Inspection under the binocular microscope or in thin section reveals that alteration of groundmass clinopyroxene to Fe oxyhydroxide/smectite has caused pervasive Fe staining of plagioclase (Fig. F8), giving the rock a more altered appearance than its actual slightly to moderately altered status.