Mixed basaltic rubble and basalt breccia were recovered from Holes 1161A and 1161B (see "Igneous Petrology"). Basaltic rubble and basaltic clasts in the breccia from both holes are moderately to highly altered by low-temperature reactions.
In the breccia from Hole 1161A, most of the pebble-sized (~2-4 cm) basalt clasts have altered margins from 5 to 10 mm wide, whereas smaller clasts (<0.1-1 cm) are altered throughout (Fig. F20). The alteration of the basalt clasts is characterized by partial to complete replacement of olivine and pyroxene by Fe oxyhydroxides and brown clay. A thin layer of Mn oxide commonly encircles the clasts, and Mn oxide dendrites occasionally penetrate several millimeters into the interior. Clasts of glassy pillow margin material are partially to completely altered to palagonite. The palagonite typically forms concentric layers of varying colors, mimicking the shape of the clasts. As all clasts have unbroken altered margins, alteration most likely occurred after deposition.
Plagioclase clasts are usually fresh, but some have a birefringent material along internal microcracks. The cream to white clayey silt matrix between the clasts consists mainly of completely altered, disaggregated material from chilled margins. The clasts are surrounded by thin (~10 µm) selvages of cryptocrystalline silica and/or clay (Fig. F21A). Patches of Fe oxyhydroxide and Mn oxide are distributed randomly throughout the matrix, occasionally lining or filling spaces between clasts (Fig. F21B). Locally, quartz and calcite are present in the matrix. Because of the fine-grained nature of the matrix, it is unclear whether the calcite is a sediment infill or has precipitated within the deposit.
In the breccia from Hole 1161B, most of the pebble-sized (<1-4 cm) clasts have altered margins from 1 to 5 mm wide (Fig. F22). The altered margins as well as smaller clasts (<0.1-1.5 cm) of both basalt and chilled margins are frequently completely replaced by light brown to gray clay (Figs. F18, F23, F24). In some glassy clasts, the outer margins consist of reddish palagonite. Clasts with both types of alteration margins are also present. Quartz is more abundant than in cores from Hole 1161B than cores from Hole 1161A.
In basalt from Hole 1161A, the fracture plus vein density ranges from 3.2/m to 10.7/m and averages 8.3/m. Rare veins are present in Sections 187-1161A-3R-1 and 3R-2. The vein density from Hole 1161A averages 0.6/m, and the calculated volume percent of veins averages 0.02. In basalts from Hole 1161B, the fracture plus vein density ranges from 6.0/m to 11.8/m and averages 8.1/m. Veins were only noted in Core 187-1161B-3R. The vein density from Hole 1161B averages 2.3/m, and the calculated volume percent of veins averages 0.03. The fractures and veins are usually coated or filled with Mn oxide or silica in cores from both holes. In pervasively altered pieces, there are no distinct alteration halos along fractures and veins. In moderately altered pieces, however, alteration halos up to 1 cm wide are occasionally present. Alteration along fractures through chilled margins highlights a 1- to 2-cm zone of large coalesced spherulites in several pieces (Fig. F25). Rare cavities (<3 mm wide) are unfilled to partially filled with clay and Mn oxide or lined with blue silica, quartz, Mn oxide, and occasionally with unidentified gray to orange platy crystals (Section 187-1161A-4R-1 [Piece 3]). Vesicles are unfilled or lined to filled with yellow to green clay ± blue silica ± calcite.
In moderately altered pieces the alteration is strongest within 5- to 25-mm-wide halos along outer edges (Fig. F26) and within the inner parts of chilled margins with large coalesced spherulites (Fig. F25). Pieces with pervasive alteration are common in both holes. The surfaces of pieces are commonly coated with cream-colored silty sediment with Mn oxide concretions and yellow fragments of palagonite (e.g., Section 187-1161A-3R-1) ± clay ± calcite ± quartz. Some pieces have dendritic growth of Mn oxide penetrating ~3 mm into the interior. Glassy margins are usually heavily altered to yellow-brown palagonite. In Section 187-1161A-3R-1 (Piece 18), however, the glass is partially replaced by bluish clay and possibly silica (Fig. F27).
The alteration halos are characterized by partial to complete replacement of olivine phenocrysts and groundmass olivine and pyroxene by Fe oxyhydroxides and brown clay. Occasionally pieces with some unaltered olivine are present (e.g., Section 187-1161A-3R-1 [Pieces 12 and 14]). Plagioclase phenocrysts are unaltered to partially Fe stained throughout. Small primary sulfide globules and larger single composite grains of pyrite, chalcopyrite, magnetite, and hematite are occasionally present (e.g., Section 187-1161A-2R-1) (Fig. F28). Due to the interstitial occurrence within other phases and an irregular morphology, the larger composite grains are probably secondary.