The two basalt units recovered from Hole 1156A are generally slightly to moderately affected by low-temperature alteration. A few pieces and discrete areas within pieces are highly altered.
Unit 1 is a carbonate-cemented basalt breccia. A fundamental observation is the occurrence of at least three types of calcite: (1) the first-generation calcite is a pink micritic calcite that adheres to clasts, forming composite fragments and 0.1- to 1.5-cm angular clasts; (2) the second-generation calcite is a light gray micritic calcite (Fig. F5) forming the matrix of the breccia and surrounding the first-generation calcite; and (3) the third-generation is sparry calcite, which occurs as veins and irregular patches that may be replacement features and/or vug fillings. Throughout this chapter, we refer to these calcite varieties as first-, second-, and third-generation calcite, respectively.
First-generation calcite is frequently separated from the adjacent basalt clast by a thin Mn oxide layer from which dendrites grow into the clast. Occasional fractures cutting basalt pieces within the breccia are filled with calcite, have Mn oxide-covered fracture walls, and are surrounded by 0.5- to 1-cm-wide oxidation halos. Vesicles are <1%, usually <<1 mm in diameter, and lined with cryptocrystalline silica. The margins of basalt clasts often have 1- to 2-cm yellowish brown oxidation halos in which olivine phenocrysts are replaced by Fe oxyhydroxide; the groundmass is mostly altered to smectite and Fe oxyhydroxide, very similar to Sites 1152 and 1154. The interiors of basalt clasts are fresher with minor smectite replacement of the groundmass (Fig. F6) and partially iddingsitized olivine (Fig. F7). The alteration halos are cut by and predate second-generation calcite veins; these veins have caused only minor (1-3 mm wide) alteration within the newly exposed basalt margins. Highly angular basalt splinters preserved within a matrix of second-generation calcite (Fig. F8) also display only minor alteration.
Crescent-shaped glass/palagonite clasts with attached first-generation calcite are enclosed by 2-to 4-mm-thick layers of yellowish to orange-brown palagonite within the second-generation calcite (Fig. F9); the centers of the larger clasts are usually fresh glass. Smaller glass clasts (0.3-1 cm) are completely altered to palagonite (Fig. F9). The thickness of palagonite encrustation is constant within each glass clast, suggesting equally intense glass alteration. This is in contrast to palagonitization of glassy pillow margins observed at the earlier sites, where palagonite dominates the outermost quench zones in layers of different thickness. Distinctive bleached whitish palagonite is occasionally present and is enclosed by a thin (<0.5 mm wide) layer of Mn oxide (Figs. F8, F10). This type of palagonite was not observed in earlier sites. The crescent-shaped glass/palagonite clasts probably represent reworked hyaloclastite, possibly filling intrapillow spaces before brecciation. The overall texture of the breccia indicates at least two brecciation events, which were accompanied by carbonate deposition and transport of smaller clasts along the opening veins.
Basalts of Unit 2 from Hole 1156A are slightly to moderately altered with the highest degrees of alteration within (1) veins as thick as 4 mm; (2) halos as wide as 1 cm, subparallel to the veins; (3) yellowish brown oxidation halos within the spherulitic zones of chilled pillow margins; and (4) palagonite layers within the glassy pillow rims. Veins and fractures are predominantly orthogonal to the glass rinds with smaller (<<1 mm) veins or open fractures branching off from the main veins in the vicinity of the chilled margins. Veins are generally filled with calcite and as wide as 4 mm (Fig. F11); fractures are covered with Mn oxide. Olivine replacement by Fe oxyhydroxide is strongest within the buff-colored halos, whereas, in the fresher medium gray areas, olivine is variably altered to a waxy yellow-green clay (smectite) along crystal outer surfaces and cracks. Although plagioclase phenocrysts appear fresh in hand specimen throughout the unit, thin-section inspection of the alteration halo in Sample 187-1156A-3R-1, 5-9 cm, reveals that plagioclase phenocrysts are in places substantially affected by iron staining along cracks and cleavage planes (Fig. F12). Groundmass plagioclase and clinopyroxene of the same section are mostly (30%-60%) replaced by smectite and Fe oxyhydroxide (Fig. F13), as is interstitial, devitrified glass. Thin-section inspection of the boundary between the alteration halo and fresh basalt in Sample 187-1156A-2R-3, 129-133 cm, reveals that the groundmass alteration grades within <0.5 mm from partially smectite/Fe oxyhydroxide replacement to virtually fresh (Fig. F14).
Basalt from Hole 1156B comprises a single lithologic unit that was slightly to moderately altered at low temperature. Alteration is most intense around fractures and veins and is readily apparent in hand specimen as buff-colored oxidation halos. Pieces with pervasive alteration are present in several sections (e.g., 187-1156B-2R-1, 3R-1, 3R-2, 4R-1, and 5R-1 and 187-1156A-6R-1 and 6R-2). The extent of alteration appears to be related to the high density of veins/fractures (see "Structural Geology"). Several basalt fragments have smooth, weathering rinds a few millimeters thick (Section 187-1156B-4R-1); others have weathered fracture surfaces (Section 187-1156B-5R-2), suggesting that these were recovered from a talus deposit.
Veins <0.5-7 mm wide filled with calcite ± Mn oxide ± Fe-stained clay are present throughout the core. Some veins are anastomosing and/or branching and have diffuse, irregular boundaries with the host basalt (e.g., Sections 187-1156B-5R-1, 6R-1, and 6R-2). In Piece 1 of Section 187-1156B-5R-1 vein width increases downhole with a corresponding increase in width of the oxidation halos from 2-7 mm to 5-30 mm. Commonly, alteration halos are ~10 mm wide, but some are as narrow as 2 mm (e.g., Section 187-1156B-4R-3). Halo-free veins are also present (Section 187-1156B-2R-1). Open fractures are lined with Mn oxide (Sections 187-1156B-3R-2 and 6R-1) or, rarely, Fe stained with a thin coating of grayish blue silica ± spotty Mn oxide (Section 187-1156B-4R-2). Some calcite veins and fractures occur as exterior faces of pieces in Section 187-1156B-4R-3 (Pieces 2A and 2B) and in Section 5R-1. In Section 187-1156B-3R-2 (Piece 3B), a 5- to 10-mm-wide micritic calcite vein is attached with an irregular contact to basalt, and highly altered basalt pieces lined with Mn oxide are enclosed in the vein material.
In the glassy pillow margins in Sections 187-1156B-3R-1, 4R-1, 4R-2, 6R-1, and 6R-2, yellowish brown to orange palagonite has developed along fractures and cracks in the glass. Buff alteration halos up to 2 cm wide, adjacent to the glassy margins, highlight the spherulitic and coalesced spherulitic quench zones and, in Section 187-1156B-4R-2, appear to be related to cracks/veins running parallel to the glass rind. Within these halos, 50%-100% of olivine phenocrysts are partially to completely replaced by Fe oxyhydroxide and clay ± calcite, whereas plagioclase is unaltered.
In pillow interiors, pervasive olivine alteration is also common. In Sections 187-1156B-4R-2 and 4R-3, however, olivine is unaltered or only partially replaced by Fe oxyhydroxide. The groundmass is variably altered, with complete to patchy replacement by a mixture of Fe oxyhydroxide and brown to greenish clay with sporadic calcite. Some pieces also show a concentric arrangement of alteration zones in which the oxidized zone forms the outer edge (e.g., Section 187-1156B-6R-2). Inside the oxidized edge is a zone in which the groundmass is replaced by clay. The interiors of these pieces are less altered but still contain some groundmass clay. The boundaries between the different clay-bearing zones are relatively sharp. Rare vesicles are empty to variably filled with clay and/or calcite; some are lined with blue to white cryptocrystalline silica. Vugs are filled with drusy calcite, and Mn oxide is present in some pieces.