Basalt recovered from Hole 1157A has undergone slight to moderate low-temperature alteration. Along unfilled or calcite-lined fractures, minor alteration has formed halos as wide as 2 mm. Alteration halos within pieces of the basaltic rubble and larger clasts of the basalt-carbonate breccia (Sections 187-1157A-2R-1 and 3R-1) (see "Hole 1157A" in "Igneous Petrology") extend from 2 to 10 mm inward from outer surfaces and constitute between 5% and 50% of single pieces and clasts (Fig. F6). Smaller clasts of basalt and glass, suspended within the matrix of the breccia, are partially to completely altered (Fig. F25). Outer surfaces of clasts (Section 187-1157A-3R-1) generally have attached Mn oxide concretions, and these surfaces are paralleled by the alteration halos. Some clasts have, however, fractures through the alteration halos, indicating that the alteration occurred before the final phase of brecciation. The majority of pieces in the lower part of the unit (Sections 187-1157A-3R-2 and 4R-1) have highly oxidized, rounded outer margins with coalescing patches of Mn oxide. Pink micrite is occasionally attached to the surfaces of pieces (e.g., Sections 187-1157A-3R-1 [Piece 7] and 4R-1 [Piece 2]).
Within the alteration halos, phenocrysts of olivine are usually partially to completely replaced by Fe oxyhydroxide, but plagioclase is unaltered. The groundmass is partly replaced by a mixture of Fe oxyhydroxide, clay, or calcite. Equant to skeletal hematite is occasionally present within the groundmass calcite. Rare vesicles are empty to partly filled with clay and/or calcite or lined with blue cryptocrystalline silica.
Basalt recovered from Hole 1157B represents a single lithologic unit that has been slightly to moderately altered at low temperature. The alteration is generally confined to oxidation halos along fractures, veins, and exterior surfaces (Fig. F26). Many pieces have fresh interiors (e.g., Section 187-1157B-9R-1), but pieces with more pervasive alteration are also present (e.g., Section 8R-1).
Most fractures are unfilled, but some are lined with Mn oxide and/or blue silica and/or clay. Veins (0.1-25 mm wide) filled with carbonate ± Mn oxide ± clay are present in most cores (Figs. F20, F26). Larger veins are filled by micrite sediment and/or sparry calcite. Evidence for transport of micrite sediment into veins comes from micritic sediment pockets attached to the basalt (Fig. F20) with similar micrite in the adjacent veins. Calcite precipitation has also played an important role in the alteration of this core. The sparry calcite may to some extent have been derived from the micrite by recrystallization (Figs. F22, F27). The boundaries between walls and vein fillings are commonly irregular and diffuse. This texture is caused by (1) minute veins/fractures extending into the basalt leading to complete or partial incorporation of small wall fragments into the carbonate (Figs. F28, F29), (2) propagation of fractures around plagioclase microphenocrysts (Fig. F22), and (3) calcite replacement of the wall-rock basalt (Fig. F30).
As much as 40% of vein infill may consist of lithic fragments, including (1) olivine replaced by clay, (2) palagonite (Fig. F31), (3) unaltered to altered plagioclase (Fig. F32), and (4) basalt fragments. All of these clast types are consistent with derivation from the host pillow basalt. Vein boundaries are lined with Fe oxyhydroxide ± clay ± calcite (Fig. F22). Mn oxide dendrites growing into the carbonate from the altered basalt wall, as well as minute red spots of Fe oxides, are commonly present.
Calcite veins are usually surrounded by symmetric alteration halos as wide as ~25 mm, generally ~10 mm (Fig. F26). Within the alteration halos in the upper part of the unit (Cores 187-1157B-2R through 4R), 100% of olivine phenocrysts are completely replaced by Fe oxyhydroxide; elsewhere, 0%-40% of olivine is altered to Fe oxyhydroxide or white clay. Within alteration halos in the lower part of the unit (Sections 1157B-5R-1 through 9R-1), 30%-100% of olivine phenocrysts are partly to completely replaced by Fe oxyhydroxide ± green-yellow clay or calcite (Fig. F33). Plagioclase phenocrysts are usually unaltered throughout. The groundmass usually shows patchy replacement of olivine and mesostasis by yellow-brown and/or olive-green clay ± Fe oxyhydroxide and rarely calcite within the halos. In Section 187-1157B-8R-2, however, the alteration tends to be pervasive. Vesicles are empty to variably filled with clay or lined with Fe oxyhydroxide or blue silica.
The outer parts of glassy pillow margins are altered in variable degree to yellowish brown to orange palagonite. No alteration halos are observed adjacent to the glassy margins.