Volcanic rocks were encountered in Holes 1152A and 1152B at <1 and 22.6 mbsf, respectively. The drill holes penetrated what is interpreted as a talus pile composed of pillow basalt clasts. This interpretation is based on the absence of lithologic contacts, subangular shapes of clasts, and the predominance of weathered rinds or planar fracture surfaces on the majority of pieces (see "Alteration"). Thus, no primary magmatic stratigraphic significance can be associated with the position of individual pieces in the holes. Nonetheless, similarities in rock type justified grouping the rocks into distinct lithologic units. In Hole 1152A, 0.6 m (5% recovery) of light gray, aphyric basalt was recovered and designated as Unit 1. Total recovery for Hole 1152B was 3.6 m (15% recovery), and the material was divided into a unit of aphyric basalt and a unit of sparsely to moderately plagioclase-clinopyroxene phyric basalt. No lithologic contact was recovered, and the unit boundary in Hole 1152B was defined between Pieces 5 and 6 in Section 187-1152B-4R-1 on the basis of a marked change in phenocryst content. The lithologic units identified are summarized in Table T2 and described below.
All of the core recovered in Hole 1152A has been assigned to a single lithologic unit. This unit is a light gray aphyric basalt and is slightly altered throughout (see "Alteration"). Small vesicles, typically <1 mm in size, are uniformly distributed and make up ~3% of the rock. Rare plagioclase phenocrysts (<<1%) as large as 2.5 mm are present in clusters (glomerocrysts); the crystals have rounded (partially resorbed?) shapes, are twinned, and display a fine-scale oscillatory zoning. Skeletal to subhedral olivine microphenocrysts as large as 0.8 mm are present as discrete crystals and in association with lath-shaped plagioclase microphenocrysts. Phenocrysts and microphenocrysts are unaltered.
Thin sections show that the microcystalline groundmass consists of quench textures, ranging from sheaf to plumose, though sheaf textures predominate (Fig. F1). The sheafs consist mainly of acicular to skeletal plagioclase intergrown with plumose quench clinopyroxene. Proportions of groundmass phases are difficult to estimate because of the predominance of cryptocrystalline quench morphologies, but plagioclase content is probably greater than clinopyroxene, both of which are significantly more abundant than olivine. Dark brown interstitial material (altered glass + quench phases?) makes up ~10% of the rock, and minute (<20 µm) equant opaque minerals make up 1%-2% of the groundmass; ~1% of the opaque minerals are 2- to 3-µm pyrite globules. Alteration is usually restricted to localized replacement of groundmass by Fe oxyhydroxides and a more pervasive, although still minor, replacement of groundmass by smectite. Vesicles are also filled with smectite.
Glassy rinds were recovered on four pieces of the core (Table T3). The most complete rind (Sample 187-1152A-1R-1 [Piece 8]) consists of a thin layer (1-2 mm thick) of clear, dark brown glass containing plagioclase microlites and a few dispersed spherulites, followed inward by a wider zone of light gray, coalesced spherulites (~5 mm wide). This is followed in turn by a light brown spherulitic zone that is at least 1 cm wide and texturally grades into the more fully crystallized interior of the rock. The widths of the coalesced spherulite and spherulitic zones in Sample 187-1152A-1R-1 (Piece 8) are typical of those observed elsewhere at Site 1152 (e.g., see Sample 187-1152B-5R-1 [Piece 25] in Fig. F2). For most samples from Hole 1152A, clear glass was not recovered, and the glassy rinds begin with the zones of coalesced spherulites (Table T3).
The presence of high-angle intersecting fractures in Sample 187-1152A-1R-1 (Piece 3) probably explains the distinct V shapes of many fragments at Site 1152. These are radial fractures typical of pillow lavas.
Unit 1 of Hole 1152B is an aphyric basalt macroscopically similar to the aphyric basalt of Hole 1152A, although petrographic observations and geochemistry indicate that these are distinct units. Unit 1 of Hole 1152B contains <1% plagioclase microphenocrysts, most of which are <1 mm. In contrast, olivine microphenocrysts have not been observed, and clinopyroxene (as large as 0.2 mm) constitutes a larger proportion of the groundmass. In thin section, the groundmass texture is quite variable, ranging from sheaf and plumose quench textures (Fig. F3) to intersertal (Fig. F4) and even subophitic in more coarse-grained areas. Smaller groundmass plagioclase crystals (<0.5 mm) are acicular to skeletal, but larger microphenocrysts are prismatic to tabular and contain pockets of devitrified glass (Fig. F5). In most cases, these glass pockets have irregular shapes, which suggests they have formed by partial resorption of the plagioclase. Some larger microphenocrysts display oscillatory zoning, whereas most groundmass plagioclase is either unzoned or normally zoned.
Proportions of groundmass phases are difficult to estimate because of the predominance of cryptocrystalline quench morphologies, but clinopyroxene is probably equal to or greater than plagioclase in abundance. Dark brown interstitial material (altered glass + quench phases?) makes up ~15% of the rock. Minute (<30 µm) equant to acicular opaque minerals make up ~3% of the groundmass. Pyrite globules (<2 µm) constitute only a trace proportion (<<1%) of the opaque minerals. Small vesicles, typically <1 mm, are variably distributed throughout and make up between 2% and 5% of the rock.
The rocks are slightly altered, with alteration concentrated toward outer surfaces of pieces or along fractures (see "Alteration"). In thin section, alteration appears as patchy replacement of groundmass by Fe oxyhydroxides and smectite. Some smectite patches have a distinctive diamond shape, which may indicate replacement of groundmass olivine, but these apparent pseudomorphs are uncommon, and fresh olivine has not been observed. Most vesicles are also filled with smectite.
Eight pieces of core from this unit include glassy rinds or fragments (Table T3). For most pieces, the glass consists of coalesced spherulites (i.e., clear glass is not present). The most complete glass rind for this unit (Sample 187-1152B-3R-1 [Piece 1]) displays the same sequence of quench zones as Sample 187-1152A-1R-1 (Piece 8), described above, although the amount of clear glass recovered with Sample 187-1152A-3R-1 (Piece 1) is significantly larger (as thick as 1 cm). Sample 187-1152B-2R-1 (Piece 4) is a unique occurrence in which the glass (~2 mm thick) is incorporated as a relatively unaltered tabular fragment within layers of palagonite and a sedimentary crust composed of Mn oxides and pelagic sediment.
Unit 2 in Hole 1152B is a sparsely to moderately plagioclase-clinopyroxene phyric basalt that is slightly altered. Prismatic to tabular, subhedral plagioclase phenocrysts range from 1 to 4 mm and make up 2%-5% of the rock. Subhedral to anhedral clinopyroxene phenocrysts (1-6 mm) constitute 1%-3% of the rock. Subhedral olivine phenocrysts (0.5-4 mm) are present throughout, although their abundance is usually <1%.
Olivine phenocrysts are partially replaced by iddingsite, but other phenocryst phases tend to be free of alteration. Clinopyroxene phenocrysts are present as discrete crystals and in association with plagioclase as glomerocrysts. Plagioclase + olivine glomerocrysts are also present. Plagioclase in glomerocrysts tend to display complex oscillatory zoning and to contain pockets of devitrified glass, probably because of partial resorption. In addition, both clinopyroxene and plagioclase phenocrysts commonly have quench overgrowths.
In thin section, the groundmass texture is dominated by immature plumose quench textures (Fig. F6), with as much as 5% acicular to skeletal plagioclase crystals (<0.2 mm). As with the other lithologic units at Site 1152, the proportions of groundmass phases are difficult to estimate because of the predominance of cryptocrystalline quench morphologies. However, based on the predominance of plumose quench textures, there is probably more clinopyroxene than plagioclase. Minute (<20 µm) equant to acicular opaque minerals make up ~1%-2% of the groundmass; ~1% of the opaque minerals are tiny (~2 µm) pyrite globules. Vesicles constitute 2%-3% of the rock and are typically <1 mm in diameter, although some as large as 3 mm were observed. Vesicles tend to be filled with smectite only in distinct zones of alteration (see "Alteration") toward the rims of pieces.
Glassy rinds or fragments were recovered on nine pieces of core from this unit (Table T3). In contrast to the glassy rinds from the two aphyric units, all of the rinds recovered in Unit 2 contain at least some clear glass. Sample 187-1152B-5R-1 (Piece 25) is typical (Fig. F2).