Volcanic basement was encountered at Hole 1173B at 737.1 mbsf. No contact with the overlying sediment was recovered. We recognized two lava units within the recovered portion (5.19 m) of the 19.5 m of igneous basement drilled at Hole 1173B (27% recovery). In this section we describe the lithology, volcanic architecture, petrography, alteration features, and geochemistry of these units.
The volcanic basement section recovered at Site 1173 consists of two basalt lava units distinguished on the basis of differences in their phenocryst abundance.
Unit 1 (737.1-747.49 mbsf) is a sparsely vesicular plagioclase-phyric basalt with rare olivine phenocrysts. Equant plagioclase is the dominant phenocryst phase, typically 1-3 mm in size (range = 0.3-5 mm), comprising 3%-7% of the rock. The olivine content is typically <1%, and the phenocrysts range from 0.3 to 2 mm in size. The olivine phenocrysts are partly or completely pseudomorphed and are identified by their distinctive crystal habits and "ghost" fracture patterns.
Unit 2 (751.7-754.14 mbsf) is characterized by the same phenocryst assemblage as Unit 1, but the phenocryst abundance is markedly higher, the plagioclase content being typically ~10% and olivine content 2%-4%. As in Unit 1, olivine phenocrysts are partially to completely replaced by secondary minerals.
Both lava units contain small (<2 mm in diameter) spherical vesicles typically comprising <2% of the rock but rarely up to 6%. A few of the vesicles contain secondary sulfide (pyrite), and in one example (Sample 196-1173B-1R-1 [Piece 1]) spherical to elongate segregation vesicles are lined with a black cryptocrystalline segregation material with calcite filling the central void.
The groundmass is typically microcrystalline to fine grained with crystallinity ranging from holocrystalline through hypocrystalline to hypohyaline. The fine-grained holocrystalline lava is characterized by an intergranular texture, where dendritic feathery clinopyroxene and minute iron oxides occupy the void spaces, in a framework of plagioclase laths. The microcrystalline lava is typically hypocrystalline and exhibits a distinctive variolitic texture characterized by spherical domains of fanning intergrowths of skeletal plagioclase and dendritic clinopyroxene. In several places the variolitic portion of the lava grades into a 3- to 5-mm-thick horizon of silicified glass containing isolated submillimeter domains of ghost varioles. These relict glassy rinds are found only at the margins of rock pieces.
Hole 1173B lavas are characterized by cyclic variations in grain size and crystallinity, alternating between domains of fine-grained and microcrystalline lava (Table T10). The observed variations in groundmass textures, grain size, and crystallinity, along with the presence of relict glassy selvages/rinds, indicate that the lavas of Units 1 and 2 consist of multiple lobes, as indicated on the visual core descriptions (see "Site 1173 Visual Core Descriptions"). Furthermore, high cooling rates are implied by the skeletal and dendritic habits of crystals in the groundmass, along with the occurrence of variolitic texture (e.g., Lofgren et al., 1974). This evidence is consistent with the notion that each lava unit is made up of several thin, water-cooled lobes, and thus we interpret the sequence as pillow lavas.
Plagioclase phenocrysts are readily distinguished from quenched groundmass plagioclase by their more equant, as opposed to lathlike, morphology and by slight resorption along the margins. Although plagioclase morphology is similar in the two units, plagioclase phenocrysts in Unit 2 exhibit optical zonation and occur either as single crystals or glomerocrysts. The olivine phenocrysts are partly or completely pseudomorphed by serpentine or talc (after serpentine), and/or red brown amorphous material (iron oxyhydroxide), but are readily identified by their distinctive crystal habits and ghost fracture patterns (Fig. F37).
In most thin sections groundmass consists of a framework of skeletal plagioclase (40%-45%) with interstitial dendritic, sometimes branching, clinopyroxene (~40%) and subordinate amounts of interstitial titanomagnetite (2%-7%) (Fig. F38). Relict glass, indicated by interstitial patches of green-brown clay, is present in variable abundance. In one thin section (Sample 196-1173B-1R-2, 18-20 cm), the groundmass is characterized by variolitic texture with fanning domains of skeletal plagioclase needles intergrown with dendritic clinopyroxene (Fig. F39).
Fluid/rock interaction after emplacement has slightly to moderately altered all volcanic rocks encountered at Hole 1173B. Secondary minerals fill veins and vesicles and partly or completely replace mesostasis and primary phases, such as olivine. Alteration consists mostly of red-brown (iron oxyhydroxide) alteration halos, 3 to 5 mm wide. Small (<2 mm wide) fractures and veins are randomly oriented and are filled with several successive stages of alteration minerals, usually calcite and green and brown clay minerals. In Section 196-1173-2R-1, veins are further lined by secondary sulfides, which are identified as pyrite in the thin section of Sample 196-1173-3R-2, 83-85 cm.
Two samples of the igneous basement were analyzed using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Analytical techniques are described in "Igneous Petrology and Volcanology" in the "Explanatory Notes" chapter. To compare the compositions of the units, samples were taken from near the top (Unit 1, next to thin section Sample 196-1173B-1R-1, 99-101 cm) and bottom of the core (Unit 2, next to thin section Sample 196-1173B-3R-2, 83-85 cm). Each sample was analyzed twice (Table T11).
On the basis of SiO2, Na2O, and K2O contents, both samples are tholeiitic basalts. They are evolved basalts with moderate MgO (5.95 and 6.19 wt%) and Ni contents (76 and 88 ppm). There are significant compositional differences between the two samples. Compared to the Unit 2 sample, the Unit 1 sample has higher TiO2, P2O5, Zr, and Y contents. These elements are relatively immobile during seafloor alteration; thus, the differences between the samples are unlikely to be the result of variable alteration in different parts of the core. These samples have compositions similar to average N-type mid-ocean-ridge basalt but have higher Sr and Ba concentrations.
3This section was written during Leg 197. Leg 197 contributor addresses can be found under "Leg 197 Contributors" in the preliminary pages of the volume.