Basalts recovered from Hole 1163A are divided into two lithologic units. Both units have undergone slight to moderate low-temperature alteration. In slightly altered pieces, alteration is generally confined to narrow halos along fractures, veins, and exterior surfaces; many pieces have fresh interiors. In moderately altered pieces, alteration is more pervasive, which is mostly due to increased vein density and slightly wider alteration halos (e.g., Sections 187-1163A-6R-1 and 2).
Calcareous sediment with Mn oxide spots and lithic clasts of basalt and/or partially to completely altered glass is present as single pieces (Fig. F9) or attached to basalt pieces (Fig. F10) in several sections. The palagonite around glass fragments is concentric, suggesting that alteration took place within the sediment.
Fractures, unfilled or lined with Fe oxyhydroxide or/and Mn oxide, are present in basalt pieces throughout the core (Fig. F11). In some places, thin (<0.2-0.25 mm wide) veins filled with white clay and cryptocrystalline silica (e.g., Section 187-1163A-3R-2, [Piece 1]), quartz, and Mn oxide (Section 187-1163A-5R-2 [Piece 12]), or red silica and clay (Section 187-1163A-10R-2 [Piece 5]) are present. Veins (0.2-10 mm wide) filled with cream-colored carbonate and/or clay ± Mn oxide are common in most sections (Figs. F12, F13). The carbonate includes both micritic and sparry calcite (Fig. F14). Larger micritic veins commonly also contain 5%-20% lithic fragments of basalt and palagonite (Fig. F13) probably derived from the host pillow basalt, implying transport of sediment (similar to sediment attached to outer surfaces of basalt pieces) into the veins. Sparry calcite varies from predominantly pinkish brown to white. White sparry calcite crosscuts both the micrite and the pinkish brown sparry calcite, whereas the pinkish brown sparry calcite only cuts through the micrite. The sparry calcite may, to some extent, have been derived from the micrite by recrystallization. Mn oxide is present in all types of calcite but is most common in the micrite. Locally, vein boundaries are lined with Mn oxide and/or Fe oxyhydroxide.
Alteration is mostly restricted to oxidation halos, paralleling fractures, veins, and outer margins of pieces (Figs. F12, F13, F15). In slightly altered pieces, the alteration halos are narrow (3-5 mm) (Fig. F13). In moderately altered pieces, the vein density is higher, and the halos are generally wider (up to 10 mm). The halo width may vary along a single vein, as in Section 187-1163A-6R-1 (Piece 7E), where it changes from 0 to 7 mm over a distance of a few centimeters. Rarely, 2- to 4-cm-wide halos are developed along outer margins of pieces (e.g., Section 187-1163A-2R-2 [Piece 10] and 8R-1 [Piece 1]). Some pieces have concentrically zoned alteration halos that change from dark orange-brown to brown in the outermost zone to a lighter brown zone and, finally, to a darker gray inner zone adjacent to the lighter gray fresh interior (e.g., Section 187-1163A-3R-1 [Piece 1] and 9R-1 [Piece 9]).
Within the alteration halos, 50%-100% of olivine is replaced by Fe oxyhydroxide and clay. Plagioclase is usually unaltered. In moderately but pervasively altered pieces, the alteration involves replacement of olivine and groundmass by Fe oxyhydroxide + clay ± calcite. Vesicles are unfilled to variably filled with clay or calcite or lined with blue cryptocrystalline silica and clay ± Fe oxyhydroxide ± Mn oxide. In some places, cavities are lined with blue cryptocrystalline silica and clay or calcite. Glassy pillow margins were recovered in most cores, and these are altered on the outer surfaces to yellowish brown to orange palagonite.