The igneous rocks from Holes 1149B, 1149C, and 1149D are described together in this chapter. Descriptions are based on hand specimen examination of all cores, and on four thin sections from Holes 1149B and 1149C (Table T7). Most basement rocks are aphyric pillow basalts containing <1% phenocrysts. Pillow rims are glassy to hypocrystalline, whereas pillow interiors are microcrystalline (Fig. F41).
The first piece from Hole 1149D (Piece 13 from Section 185-1149D-5R-1) differs in primary mineralogy and alteration from the other rocks and is given a separate unit designation (Unit 1, see "Lithologic Units"). This piece contains ~2% olivine glomerocrysts and lacks alteration halos. The matrix and abundance of plagioclase phenocrysts are otherwise similar to the other rocks. Olivine phenocryst abundance increases at the bottom of Hole 1149D, exceeding 1% in the last 30 m of the hole (below 417 mbsf) in Unit 6 (see the Igneous Mineralogy Description Logs in the "Supplementary Materials" contents list). The maximum phenocryst sizes are observed toward the bottom of the hole (Fig. F42).
Throughout the basement sections phenocrysts are generally rare (<1%), but euhedral plagioclase is the most common and ranges in size from 0.1 to 1.2 mm. Olivine, the second most common phenocryst (always much less than 1%), is euhedral to subhedral in shape and 0.1-1 mm in size. Pyroxene phenocrysts were observed only in Section 185-1149B-29R-2 (Piece 13). These occur as glomerocrysts with plagioclase and olivine, have euhedral crystal shapes, and range in size from 0.2 to 0.4 mm. One thin section of a pillow rim was investigated. It consists of 94% altered glass and includes 2% plagioclase phenocrysts and 1% olivine phenocrysts (Fig. F43).
Plagioclase and olivine phenocrysts are altered to smectite and Fe oxide (Figs. F44A, F44B, F45). The cores of plagioclase are altered but the rims are unaltered, whereas olivine is completely altered. Interstitial material in the groundmass is intensely altered (see "Basement Alteration"). Vesicles are filled with saponite, Fe oxides, celadonite, and calcite.
Two samples of basalt from Hole 1149B were analyzed with shipboard XRF. These samples and a further 15 samples (four from Hole 1149B, four from Hole 1149C, and seven from Hole 1149D) were analyzed in a shore-based laboratory by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) for major elements and trace elements (Table T8). The samples analyzed represent the least-altered basalts recovered from the three drill holes, although alteration in this uppermost sequence is intense and pervasive (see "Basement Alteration").
The Site 1149 basalts are distinctly lower in Fe2O3 and higher in MgO than tholeiites recovered from Hole 801C (Fig. F46). If these values are primary, they would indicate that Site 1149 lavas are more primitive than those from Hole 801C. Although MgO for Site 1149 basalts is within the range of the modern East Pacific Rise (EPR) (Langmuir et al., 1986), Fe2O3 is anomalously low, and alteration may have affected the Fe and/or the Mg contents of these basalts. Sample 185-1149B-30R-1, 91-94 cm, with the lowest Fe2O3 and highest MgO, contains ~100 ppm Ba, which is surprisingly high for a mid-ocean-ridge basalt (MORB), and may be the result of alteration. With the exception of the basalts from the base of Hole 1149D (Unit 6), the immobile trace elements in basalts from Site 1149 are comparable to those in normal MORBs from both Site 801 and the modern EPR but displaced to lower trace element abundances, consistent with their higher MgO contents (Fig. F46). The basalts encountered at the base of Hole 1149D (Unit 6) are enriched MORBs (E-MORBs) with higher Zr/Y and Nb abundances. These are nonetheless comparable to E-MORBs from the EPR. Thus, Unit 6, which lies below a major breccia unit, is notable because of a major change in both its chemical composition and phenocryst content.