Holes 1160A and 1160B were rotary cored into igneous basement from 166.0 to 171.1 mbsf and from 160.1 to 205.2 mbsf, respectively.
Two cores were recovered from Hole 1160A (Cores 187-1160A-2R and 3R), penetrating 5.1 m into basement, with 0.43 m (8.4%) recovered. One lithologic unit of buff to medium gray aphyric pillow-basalt rubble was defined (Table T2).
Nine cores were recovered from Hole 1160B (Cores 187-1160B-1W to 9R), which penetrated an alternating sequence of pillow lavas and massive flows (Table T2) recovering 13.0 m (28.8%) of core. Seven units were defined based on morphology and lithology. Massive flows (Units 2, 4, and 6) are interlayered with pillow flows (Units 1, 3, 5, and 7). Aphyric basalt (Units 1, 2, and 7) alternates with plagioclase ± olivine phyric basalt (Units 3-6).
All the basement recovered in Hole 1160A has been assigned to a single lithologic unit, a buff to medium gray aphyric pillow basalt. Less than 1% fresh phenocrysts of plagioclase and olivine as large as 1 mm and 1.5 mm, respectively, are present throughout the unit (e.g., Section 187-1160A-2R-1 [Piece 4]). About 40% of the prismatic plagioclase phenocrysts form glomerocrysts as large as 3 mm in size. In thin section, the microcrystalline groundmass texture ranges from intergranular to intersertal. It consists of 46% prismatic plagioclase (as large as 0.2 mm), 42% granular clinopyroxene (as large as 0.1 mm), and 12% mesostasis (e.g., Section 187-1160A-2R-1 [Piece 5]). Rare, small (<1 mm) vesicles make up <1% of the rock. Three pieces out of 14 include glassy rinds ranging from <1 mm to 2 mm in thickness. These have been partially or entirely replaced by palagonite.
Overall, the unit is slightly to moderately altered (see "Hole 1160A" in "Alteration"). The more altered areas of the pieces are generally located on the rims and along fractures (e.g., Section 187-1160A-3R-1 [Piece 1]). In the alteration halos, groundmass replacement by Fe oxyhydroxide and smectite varies between 5% and 15%.
The first unit of Hole 1160B is a buff (where altered) to medium gray (where fresh) aphyric pillow basalt. In Section 187-1160B-3R-1, two relatively flat outer surfaces on Piece 5 intersect at an angle of ~130° and can be interpreted as cooling joints. In Section 187-1160B-3R-1, Piece 3 displays radial fractures and an arcuate shape, indicating that it is a pillow-lava fragment. In several pieces, rare olivine microphenocrysts <1 mm in size are totally replaced by Fe oxyhydroxide (e.g., Section 3R-1 [Piece 9]). The microcrystalline groundmass texture varies from intergranular to subophitic on the scale of a single piece (e.g., Section 187-1160B-2R-1 [Piece 3] and Section 3R-1 [Piece 13]) (Fig. F1), with 4% olivine (as large as 0.1 mm), 50% prismatic to tabular plagioclase (as large as 0.4-0.8 mm), 45% clinopyroxene (as large as 0.4-0.6 mm), and 1% opaque minerals (as large as 20-50 µm). Clinopyroxene is anhedral and intergrown with plagioclase.
Nine pieces out of 39 (23%) retain glassy pillow rinds or chilled margins. Glass thickness ranges from 0.3 to 1 cm. Generally, chilled margins display a millimeter-thick palagonite layer (e.g., Section 187-1160B-3R-1 [Pieces 1 and 3]). In some cases, they also contain a spherulitic zone that is commonly altered to clay and palagonite (e.g., Section 187-1160B-3R-1 [Pieces 2 and 6]). The remaining areas of the rock are usually fresh.
Overall, the unit is moderately (e.g., Section 187-1160B-1W-1 [Pieces 4 to 12]) to highly altered (e.g., Section 3R-1 [Pieces 1 to 9]; see "Hole 1160B" in "Alteration"). Spherical vesicles as large as 1 mm constitute <1% of the basalt. For most pieces, alteration is pervasive and is the result of replacement of groundmass olivine by Fe oxyhydroxide ± clays (Fig. F2).
The second unit of Hole 1160B is a medium gray aphyric massive basalt. This unit is interpreted as a massive flow because of the presence of numerous long (as large as 110 cm) continuous core pieces with homogeneous textures (e.g., Section 187-1160B-4R-2 [Piece 1]; Fig. F3). Plagioclase and olivine phenocrysts represent <1% of the rock. Plagioclase as large as 2 mm is prismatic to tabular and subhedral to anhedral. Olivine as large as 1.5 mm is equant and anhedral. About 30% of olivine and plagioclase phenocrysts form glomerocrysts (e.g., Section 187-1160B-4R-1 [Piece 8C]). The groundmass consists of ~2% equant olivine (as large as 0.1 mm), 50% prismatic to tabular plagioclase (as large as 0.8 mm), 46% anhedral clinopyroxene (as large as 2.5 mm), and 2% equant anhedral opaque minerals (as large as 75 µm) (Fig. F4). The groundmass texture is fine grained and ranges from intergranular to subophitic. Where the texture is subophitic, clinopyroxene (as long as 1.5 mm) partially encloses plagioclase. In areas where the groundmass is intergranular, granular crystals of pyroxene ranging from 0.1 to 0.3 mm are present in the interstices between plagioclase laths. The rocks are fresh to slightly altered (see "Hole 1160B" in "Alteration"). Numerous fractures are lined with chlorite (e.g., Section 187-1160B-4R-1 [Pieces 7 and 8]). However, throughout the unit, alteration halos (1-3 cm wide) are most commonly associated with Fe oxyhydroxide and calcite veins (e.g., Section 187-1160B-4R-2 [Piece 1A]).
The third unit of Hole 1160B is a reddish brown to gray moderately plagioclase phyric pillow basalt. Most of the pieces of this unit are pillow lava debris ranging from 1.5 to 5 cm in size; most surfaces have not been cut by the drill. This unit contains, on average, 4% prismatic to tabular to rounded plagioclase phenocrysts as large as 7 mm that are commonly twinned. Approximately 30% of the plagioclase phenocrysts contain spherical melt inclusions (e.g., Section 187-1160B-4R-2 [Piece 7C]; Fig. F5). Plagioclase phenocrysts from Sections 187-1160B-4R-2 to 5R-1 are light yellow, either throughout the crystal or along crystal edges and faces; this coloration is the result of Fe staining, probably related to alteration of the groundmass (see "Hole 1160B" in "Alteration"). Equant olivine as large as 1 mm is only present in Sections 187-1160B-6R-1 to 6R-3, where it represents <1% of the rock. The original grains have been totally replaced by Fe oxyhydroxide. In some pieces (e.g., Section 187-1160B-6R-2 [Piece 1]), as much as 40% of phenocrysts are included in glomerocrysts of plagioclase ± olivine that are as large as 7 mm. The microcrystalline groundmass ranges from intergranular to subophitic. It consists of 9% equant olivine (as large as 0.1 mm), 51% acicular to lath-like plagioclase (as large as 0.4 mm), 38% quench-textured and granular clinopyroxene (as large as 0.3 mm), and 2% equant opaque minerals (as large as 50 µm). Small vesicles <0.65 mm in diameter are present in some sections (e.g., Sections 187-1160B-6R-1 and 4R-2 [Pieces 4-9]).
About 8% of the pieces in this unit have glassy chilled margins that range from 0.5 to 1.4 cm in thickness and contain ~2% prismatic to tabular plagioclase phenocrysts. In general, chilled margins consist of three layers: (1) palagonite with a thickness of <1 mm, (2) glass + phenocrysts ranging in thickness from <1 to 10 mm, and, in some cases, (3) a spherulitic zone ranging in thickness from 2 to 3 mm. Some of the glasses have curved margins (e.g., Section 187-1160B-7R-1 [Piece 3]).
Overall, the rocks are moderately (e.g., Section 187-1160B-4R-2 [Pieces 4 to 9]) to highly (e.g., Section 187-1160B-5R-1) altered (see "Hole 1160B" in "Alteration"). Throughout the unit, alteration is dominated by pervasive groundmass replacement by Fe oxyhydroxide and clays.
The fourth unit is a buff (where altered) to gray (where fresh), moderately plagioclase phyric basalt except for Section 187-1160B-8R-1 (Piece 1), which is a moderately plagioclase-olivine phyric basalt. Long pieces with uniform texture (e.g., Section 187-1160B-7R-1 [Piece 10]) indicate that this unit is a massive flow.
Unit 4 basalt contains 3% plagioclase phenocrysts and <1% olivine phenocrysts. Plagioclase, as large as 7 mm, is rounded to prismatic and is commonly twinned. A heterogeneous distribution of plagioclase phenocrysts in Pieces 2A and 2C, with more abundant plagioclase at the tops of each piece, may reflect flow banding. Rare equant olivine phenocrysts as large as 0.6 mm are present throughout the unit (e.g., Section 187-1160B-7R-3 [Piece 2E] and Section 8R-1 [Piece 1]). The lower two sections of Unit 4 (Sections 187-1160B-7R-3 and 8R-1) have higher olivine contents (~1%) than the higher sections, suggesting accumulation of olivine at the bottom of the flow.
The groundmass of the massive flow is fine grained and displays an intergranular to subophitic texture (e.g., Section 187-1160B-7R-1 [Piece 10A]). The groundmass consists of 2% equant olivine, 44% plagioclase laths, 51% elongate to granular clinopyroxene, and 3% equant to subspherical opaque minerals. Overall, the unit is fresh to slightly altered (see "Hole 1160B" in "Alteration"). Numerous fractures are lined with chlorite (e.g., Section 187-1160B-7R-3 and Section 7R-1 [Piece 10]), like those in Unit 2. Alteration is concentrated along the margins of pieces and in halos around some veins; it consists, predominantly, of replacement of groundmass by Fe oxyhydroxide ± clays (e.g., Section 187-1160B-8R-1 [Piece 1]).
Unit 5 is a grayish brown to light brown, moderately plagioclase-olivine phyric basalt. Some pieces display radial fractures (e.g., Section 187-1160B-8R-1 [Piece 5] and Section 9R-1 [Pieces 1, 2, 3, and 15]), indicating that this unit consists of pillow lavas.
The basalts contain, on average, 3% phenocrysts of plagioclase and olivine, with plagioclase being the more abundant phase. Prismatic to tabular plagioclase is light yellow, either throughout the crystal or along the edges or faces, due to Fe staining (see "Hole 1160B" in "Alteration"). Larger rounded plagioclase phenocrysts as large as 4 mm display sieve textures. Equant olivine as large as 0.5 mm is totally replaced by Fe oxyhydroxide. Glomerocrysts of plagioclase and olivine as large as 6 mm are present throughout the unit.
Around 17% of the pieces have chilled margins. Generally, chilled margins consist of three layers: a layer of palagonite with a thickness of 1 mm, a layer of glass and phenocrysts ranging in thickness from 4 to 6 mm, and a layer of spherulites ranging in thickness from 2 to 3 mm.
Overall, the unit is moderately to highly altered (see "Hole 1160B" in "Alteration").
Alteration is characterized by pervasive and homogeneous replacement of groundmass by Fe oxyhydroxide and brown clays (e.g., Section 187-1160B-9R-1 [Pieces 4, 9, and 10]).
Unit 6 is a massive flow of light gray, moderately plagioclase phyric basalt. Because of the common occurrence of long pieces with no chilled margin, this unit is interpreted to be a massive flow. Plagioclase phenocrysts represent, on average, 2% of the rock. Prismatic to tabular plagioclase as large as 5 mm is commonly twinned and zoned. Approximately 25% of these plagioclase crystals have melt inclusions in their cores (e.g., Section 187-1160B-9R-1 [Piece 21]). In thin section, rare olivine phenocrysts as large 0.8 mm are equant and subhedral. Small prismatic plagioclase crystals (1 mm) usually form single-phase glomerocrysts or mixed-phase glomerocrysts with olivine (Fig. F6).
In thin section, the fine-grained groundmass texture is intergranular to subophitic. The groundmass consists of 3% equant olivine (as large as 0.1 mm), 47% lath-shaped plagioclase (as large as 0.4 mm), 47% elongate pyroxene (as large as 1.25 mm), and 3% equant opaque mineral (as large as 0.1 mm) (e.g., Section 187-1160B-9R-1 [Piece 21]). Large clinopyroxenes partly or completely enclose plagioclase. Overall, the unit is slightly to moderately altered (see "Hole 1160B" in "Alteration"). As in Units 2 and 4, fractures across some pieces are lined with chlorite (e.g., Section 187-1160B-9R-2 [Pieces 1 and 8]). Alteration is limited to halos as wide as a centimeter that are located along the edges of pieces or adjacent to veins.
Unit 7 is a light gray aphyric pillow basalt, although no glassy margins were recovered. In thin section, rare prismatic to tabular plagioclase as large as 1.2 mm makes up <1% of the rock and is commonly twinned. About 50% of the plagioclase phenocrysts form glomerocrysts as large as 3 mm (e.g., Section 187-1160B-9R-3 [Piece 3]). In thin section, the microcrystalline groundmass texture is intersertal and consists of 47% lath-shaped plagioclase (as large as 0.2 mm), 41% granular pyroxene (as large as 0.1 mm), 6% opaque (as large as 0.1 mm), and 5% mesostasis (e.g., Section 187-1160B-9R-3 [Piece 3]).
Overall, the unit is slightly altered (see "Hole 1160B" in "Alteration"). Alteration tends to be concentrated in alteration halos (as wide as 1 cm) where groundmass is partially replaced by smectite. Small (<0.2 mm) spherical vesicles represent <1% of the rock. Where located in the alteration halos, they are filled with dark green smectite or yellow clays.