Basalts recovered from Hole 1159A represent a single lithologic unit that is slightly to moderately altered by low-temperature reactions. The alteration is strongest within oxidation halos along fractures, veins, and outer edges of pieces, in areas with vugs, and within the inner part of chilled margins that have large coalesced spherulites. The alteration elsewhere is slight but commonly pervasive.
Throughout the hole, alteration halos (1-7 mm wide) are present along thin (<1 mm) fractures that range from unfilled to silica and/or clay lined. Except for Sections 187-1159A-7R-1 and 8G-1, thin (as thick as 1 mm), branching veins that are lined with Fe oxyhydroxide ± Mn oxide and filled with orange-red to yellow and green clay and/or silica are also common. In veins where both silica and clay are present, silica cuts through the clay, indicating that silica infilling was the last stage in vein formation (Figs. F4, F5). Around uncut outer surfaces of pieces, oxidation halos vary in width from 1 to 15 mm, with an average of 5 mm. These surfaces are commonly coated with silica and clay and/or Mn oxide and probably represent fracture and/or vein surfaces.
Fracture plus vein density averages 19.8/m (ranging from 5.0 to 30.8/m), and vein density averages 3.9/m (ranging from 0 to 7.7/m). The calculated volume percent of veins averages 0.11 vol% (ranging from 0 to 0.23 vol%).
In Sections 187-1159A-2R-1 (Piece 8) and 5R-1 (Piece 2), vugs (as long as 1.7 cm and as wide as 5 mm) are lined with Fe oxyhydroxide and Mn oxide and are partially to completely filled with silica or clay of various colors (red, brown, or yellowish white to green). In Section 2R-1 (Piece 8), the vugs are surrounded by 3-4 mm oxidation rims (Fig. F6). In Section 187-1159A-5R-1 (Piece 2), however, no distinct oxidation rims are developed around the vugs, but a larger area surrounding the vugs is altered to a higher degree than outside halos in other pieces (Fig. F7). In most pieces throughout the core, vesicles are unfilled or lined to completely filled with Fe oxyhydroxide ± Mn oxide ± orange-brown or yellowish white to green clay ± silica (Figs. F8, F9).
Symmetrical rims of yellow-brown altered glass (palagonite) are developed along a network of silica-filled veins within one piece of shiny black glass (Section 187-1159A-4R-1 [Piece 6]). A 3- to 5-mm-wide fracture cutting through the piece is partially filled with quartz-cemented, small altered glass fragments (Fig. F10). Within the inner part of chilled margins, a 1- to 2-cm layer of large coalesced spherulites is commonly highlighted because the surrounding groundmass has been replaced by a light brown clay. The alteration is usually connected to fractures that are parallel to or crosscut the spherulitic layer (e.g., Fig. F11). The outer part of the partially crystallized quenched margin shows, however, little sign of alteration along the fractures. This may indicate that the alteration is controlled by different quench textures. Glass fragments in pieces of clay-cemented hyaloclastite (Sections 187-1159A-6R-2 and 7R-1) are partially to completely replaced by 0.5- to 5-mm-thick palagonite rims around the edges (Fig. F12).
Within the oxidation halos and within areas with vugs, 20%-100% of the groundmass is generally replaced by Fe oxyhydroxide and/or red-brown to yellowish green clay. Elsewhere the groundmass shows a patchy replacement from 1% to 20% by Fe oxyhydroxide and/or yellowish to green clay. In many cases the patches are forming thin, arcuate, wavy patterns, suggesting that these may be outlining quench crystallization textures. When present, phenocrysts of olivine are partially to completely replaced by Fe oxyhydroxide, but plagioclase is unaltered.