Hole 1159A was rotary cored into igneous basement from 145.6 to 173.3 mbsf. We recovered 7.96 m of core, equal to 28.7% recovery (Sections 187-1159A-2R-1 through 7R-2). Following hole collapse after Core 187-1159A-7R, we attempted to resume coring and recovered 0.24 m of core (Section 8G-1) with no further penetration.
From this hole we recovered a single unit of medium to dark gray aphyric basalt interpreted as intact pillow lava, based on uniform lithology and the high percentage of pieces with arcuate chilled margins and/or glass rinds. Two pieces of hyaloclastite breccia—consisting of angular to subrounded glass/palagonite clasts (1-3 cm in size) in a pinkish tan clay matrix (Fig. F1)—were recovered, one in Section 187-1159A-6R-2 and one in Section 7R-1. We interpret these pieces as accumulations of glass that spalled off pillows and collected with clay-rich sediment in interpillow spaces.
Overall, the basalt is slightly to moderately altered (see "Alteration"). Phenocrysts include <1% equant to euhedral olivine (up to 1 mm in size) and <1% prismatic plagioclase (up to 2 mm in size). Many pieces from this unit display chilled margins, with 12% of the pieces having an outer glassy/palagonite rind (as thick as 2 cm but usually ~1 mm thick) and 33% of the pieces having a spherulitic margin that grades from a dark gray to black (in hand specimen) outer band of small, 3- to 4-µm cryptocrystalline quench crystals to a tan-brown band of larger (~1 to 2 mm in diameter) coalesced plagioclase quench spherulites (Fig. F2).
As seen in thin section, the microcrystalline groundmass of pillow interiors is dominated by plagioclase sheaf quench textures (~35%-40%), with 10%-15% equant to elongated quenched olivines <0.2 mm in size (Fig. F3). The remaining 45%-55% is black mesostasis. The basalt contains 1%-2% spherical vesicles up to 1.5 mm in diameter that are variably lined, partially or completely filled with green clay (smectite?), yellow-tan clay, cryptocrystalline quartz, Fe oxyhydroxide, or Mn oxide.