IGNEOUS PETROLOGY

Holes 1164A and 1164B were rotary cored into igneous basement from 138.5 to 147.0 mbsf and 150.4 to 215.9 mbsf, respectively. Hole 1164A (Sections 187-1164B-2R-1 through 4R-1) was drilled 8.5 m into basement, resulting in 0.97 m of recovered core (equal to 11.41% recovery). All recovered material from the basement core has been assigned to one lithologic unit: aphyric basalt. Based on the high proportion of glassy chilled margins recovered (i.e., 33% of the pieces), the rocks are interpreted to be pillow lavas.

Hole 1164B (Sections 187-1164B-1W-1 through 10R-1) was drilled 65.7 m into basement, resulting in 10.65 m of recovered core (equal to 16.21% recovery). This hole yielded basaltic rubble, consisting predominantly of highly altered aphyric basalt, with two intervals of basaltic breccia. The material was assigned to a single unit: basaltic rubble. Based on the abundance of chilled margins recovered, the presence of V-shaped pieces, radial fractures, and chilled margins with arcuate shapes, the material is interpreted as being derived from a pillow basalt flow.

Unit 1

All of the basement rocks recovered in Hole 1164A have been assigned to a single lithologic unit consisting of a slightly altered, light gray, aphyric basalt (Fig. F1). The rocks contain rare (<0.5%) phenocrysts of plagioclase (as long as 3 mm) and euhedral phenocrysts/microphenocrysts of olivine (as large as 1.5 mm); the latter is commonly replaced by Fe oxyhydroxides and clay in alteration halos and by calcite (Fig. F2) in less altered interiors of pieces (see "Hole 1164A" in "Alteration"). Prismatic plagioclase is seriate, with crystals ranging from ~2 mm long to microlites ~0.1 mm, although most crystals are <~1 mm in length.

Groundmass phases fall into two textural categories. Approximately 50% of the rock is made up of microcrystalline plumose quench crystals (Fig. F3) that are predominantly intergrowths of clinopyroxene and plagioclase ± olivine. The rest consists of microphenocrysts and/or prismatic groundmass plagioclase intimately intergrown with anhedral clinopyroxene in a subophitic texture (Fig. F4). These clusters are not xenocrysts but equilibrium intergrowths, as clinopyroxene is also observed as a euhedral microphenocryst phase (Fig. F3) and is the only mafic groundmass phase identified in the available thin section for this unit (Sample 187-1164A-4R-1, 20-22 cm). This indicates that these basalts are multiply saturated with olivine, plagioclase, and clinopyroxene. Clinopyroxene ranges from anhedral crystals (as large as 0.6 mm) in the crystal clusters to equant groundmass crystals (<25 µm); plagioclase ranges from prismatic to tabular crystals ~1 mm long to acicular and skeletal microlites ~30 µm long (Figs. F3, F4). The prismatic crystals typically have narrow embayments in the cores that are elongated parallel to cleavage (Fig. F4), which is consistent with growth from skeletal crystals during cooling. Proportions of groundmass phases are difficult to estimate because of the predominance of microcrystalline quench morphologies, but plagioclase and clinopyroxene are probably present in roughly equal amounts. Aside from microphenocrysts, olivine was not positively identified in the groundmass. Dark brown interstitial material (altered glass + cryptocrystalline quench phases) makes up ~10% of the rock. Minute (<20 µm) equant opaque minerals form <2% of the groundmass.

Small vesicles as large as 2 mm in diameter represent ~1% of the rock. In hand specimen, most of these appear to be either unfilled or lined with blue cryptocrystalline silica/clay, calcite, Fe oxyhydroxides, Mn oxide, or light green clays. In thin section, however, many are observed to have been infilled with quench crystals that are finer grained than the surrounding groundmass (Fig. F5). Some of these "refilled" vesicles also have secondary calcite in the center (Fig. F5; see "Hole 1164A" in "Alteration").

Chilled margins were recovered on three pieces (33%) from this unit. In all cases the thickness of clear glass is small (<2 mm), and most of what is retained is spherulitic quench crystals + glass, partially altered to palagonite. The zone of coalesced spherulites is relatively thin (2-3 mm), and the spherulites are small (~100-200 µm in diameter).

Unit 1

Unit 1 of Hole 1164B consists of basaltic rubble with two intervals of basaltic breccia: Section 187-1164B-R-1 (Pieces 7-12) and Section 187-1164B-4R-2 (Pieces 13-15). Three igneous lithologies are present in the rubble: (1) moderately plagioclase-olivine phyric basalt, (2) sparsely plagioclase-olivine phyric basalt, and (3) aphyric basalt with plagioclase and olivine microphenocrysts. The moderately plagioclase-olivine phyric basalts are present only at the top of the basement section (Sections 187-1164B-1W-2 through 3R-1) and represent 2% of the core. These are among the least-altered rocks recovered in Hole 1164B, being slightly to moderately altered (see "Hole 1164B" in "Alteration"). The sparsely plagioclase-olivine phyric basalts, which make up 8% of the core, are most common from Sections 187-1164B-3R-1 through 4R-1; below this, only aphyric basalts were recovered (i.e., 87% of the core). Furthermore, the sparsely plagioclase-olivine phyric basalts and the aphyric basalts appear to be part of a lithologic continuum that straddles the boundary between our applied definition of aphyric and phyric (see "Igneous Petrology" in "Explanatory Notes" and "Aphyric [to Sparsely Plagioclase-Olivine Phyric] Basalts"). Therefore, these two lithologies will be discussed together and, for brevity, will be referred to simply as aphyric basalt.

In contrast to the moderately plagioclase-olivine phyric basalts, ~55% of the aphyric basalts are highly altered and 25% are moderately altered (see "Hole 1164B" in "Alteration"). This high degree of alteration, along with the small size of most pieces and the recovery of basaltic breccia in Sections 187-1164B-3R-1 and 4R-2, is the basis for the interpretation of the unit as a rubble pile. However, the aphyric basalts in Section 187-1164B-10R-1, which are lithologically indistinguishable from the aphyric basalts elsewhere in Hole 1164B, are significantly less altered than the overlying rocks and there are fewer small (i.e., <~4-5 cm) round pieces. This suggests that the rubble intervals may be part of a relatively intact sequence of pillow lavas.

Petrography of Basaltic Rubble

Moderately Plagioclase-Olivine Phyric Basalts. These basalts (Fig. F6) consist of 1%-2% equant olivine and 1%-4% prismatic to tabular plagioclase phenocrysts. Plagioclase ranges from 1 to 5 mm and olivine from 1 to 4 mm. Approximately 10% of the phenocrysts are included in glomerocrysts. These range from loose clusters of prismatic plagioclase (2-3 crystals; <~1 mm long) + small equant to skeletal olivine (1-3 crystals; <~0.2 mm) to more tightly intergrown glomerocrysts of prismatic to tabular plagioclase crystals (10-20 crystals; ~0.5-1 mm long). Larger plagioclase phenocrysts (3-4 mm) display discontinuous zoning and are sieve textured, some extensively (Fig. F7). These textures clearly indicate partial resorption and disequilibrium with the host basalt. Smaller prismatic crystals have narrow embayments parallel to cleavage or twin planes and are unzoned, consistent with development of these features during crystal growth. All plagioclase phenocrysts show albite twinning and are generally unaltered. Equant olivine phenocrysts range from subhedral to euhedral; they are altered to Fe oxyhydroxides in alteration halos but elsewhere are either unaltered or partially replaced by a yellow to white clay. Cr spinel is present in trace amounts as small (~30µm) euhedral inclusions in olivine and as discrete subhedral crystals as large as 0.4 mm.

Groundmass textures are microcrystalline, ranging from intersertal to sheaf quench crystal morphologies. Acicular to skeletal plagioclase forms ~40% of the groundmass and equant olivines (<~0.5 mm) form 2%-3% of the groundmass. Clinopyroxene is present predominantly as plumose quench crystals intergrown with plagioclase, but anhedral, granular crystals as large as ~50 µm are present adjacent to and within miarolitic cavities; these cavities are commonly filled with secondary clay or calcite (see "Hole 1164B" in "Alteration"). Fe-Ti oxides are present as minute (<10 µm) equant crystals and constitute ~1%-2% of the groundmass. Dark brown mesostasis—which here includes glass + indistinguishable quench crystals of plagioclase, clinopyroxene, and olivine—constitutes >50% of the rock. Vesicles vary in size and abundance, ranging from as large as 1 mm in diameter, forming ~1% of the rock, to rare (<<1%) and small (<~0.5 mm). They are spherical and tend to be lined or filled with blue to white cryptocrystalline silica/clay or Fe oxyhydroxides (see "Hole 1164B" in "Alteration").

Aphyric (to Sparsely Plagioclase-Olivine Phyric) Basalts. These basalts (Fig. F8) contain 1% prismatic to tabular plagioclase phenocrysts and 1%-5% olivine microphenocrysts. In most samples, prismatic plagioclase is seriate, with crystals ranging from ~2 mm long to microlites ~0.1 mm. This makes distinguishing groundmass crystals from pheno-crysts somewhat arbitrary, and categorizing the rock as aphyric or as sparsely plagioclase-olivine phyric basalt is often a question of whether a sufficient number of crystals exceeds ~1 mm in size. The problem of visually estimating the modal proportions of phases in hand specimen is exacerbated by (1) the high degree of alteration of most pieces, which tends to overemphasize the abundance of both olivine and plagioclase crystals (Figs. F9, F10) and (2) the tendency for the crystals to occur as glomerocrysts, which can lead to overestimations of size (Figs. F11, F12). In general, the abundance of plagioclase and olivine crystals larger than 1 mm is <1%. We therefore consider the rocks to be aphyric, containing rare phenocrysts of plagioclase + microphenocrysts of olivine.

Although most plagioclase crystals tend to be small and prismatic (<1 mm long), anhedral tabular crystals as large as 3 mm are observed in a few pieces (e.g., Sample 187-1164B-5R-1 [Piece 5]). Approximately 10%-20% of the phenocrysts and microphenocrysts are included in glomerocrysts or clusters (Figs. F11, F12). All plagioclase phenocrysts show albite twinning and are generally unaltered. Equant olivine microphenocrysts (<1 mm) are relatively abundant (3%-5%) and range from subhedral to euhedral. Fresh olivine is rarely observed; however, it is totally replaced by Fe oxyhydroxides and clay in most moderately to highly altered samples (see "Hole 1164B" in "Alteration").

Groundmass textures are microcrystalline, ranging from intersertal to sheaf quench crystal morphologies. Skeletal acicular to prismatic plagioclase forms ~30%-40% of the groundmass. Dark brown quench-textured mesostasis makes up ~50% of most rocks (Fig. F11), but a few samples show a slightly greater degree of clinopyroxene growth (Fig. F13). Although the groundmass is still microcrystalline, the clinopyroxene is present in bundles of bladed crystals (~10 µm wide) that are generally in optical continuity. Enhanced clinopyroxene crystal growth also occurs within miarolitic cavities, where clinopyroxene forms subhedral to euhedral crystals as long as 250 µm (Fig. F13). Similarly, Fe-Ti oxides make up ~1% of the groundmass and are generally present as minute (<2 µm) equant crystals concentrated in areas of mesostasis. However, within miarolitic cavities they may reach sizes of 50 µm.

Vesicles are small (<300 µm in diameter) and rare; most are unfilled or lined with a white to blue cryptocrystalline clay/silica. In the less highly altered areas of some pieces, they are filled with calcite or clay (see "Hole 1164B" in "Alteration"). In contrast, miarolitic cavities are common (3%-5% of the rock). They are generally filled with clear sparry calcite or clay, but in Sample 187-1164B-5R-1, 100-104 cm, they are filled with an inclusion-filled calcite that has a bladed habit and shows sweeping extinction (Fig. F14).

Chilled Margins. Chilled margins were recovered on 45 rubble clasts (i.e., 18% of the pieces); two are phyric basalts (e.g., Fig. F6) and the rest are aphyric. In spite of the overall high degree of alteration of the unit, fresh glass was preserved on a large number of pieces. Rinds of clear glass typically range from 1 to 10 mm in thickness; this is followed by 1-2 mm of glass and discrete spherulites. Inward from the glassy rind, the chilled margin typically consists of a 5-mm-wide zone of coalesced spherulites; the glass interstitial to the spherulites in this zone is replaced by palagonite in most samples. Based on the overall abundance of chilled margins among the rubble pieces, the presence of pieces with classic V shapes and radial fractures (e.g., Fig. F15), and the presence of arcuate chilled margins (e.g., Sample 187-1164B-8R-1 [Piece 19]), the basalts in Hole 1164B are inferred to have originated as a sequence of pillow basalts.

Petrography of the Breccia

There are two breccia intervals in Hole 1164B (i.e., Section 187-1164B-3R-1 [Pieces 7-12], Fig. F16; Section 4R-2 [Pieces 13-15]; Fig. F17). The sediments in these two intervals are similar in that both are poorly sorted and dominated by lithic clasts of aphyric basalt and palagonite ± glass. Angular clasts of slightly to moderately altered basalt dominate the >5-mm range. Some of these clasts have concentric alteration halos, but most do not, indicating that little postdepositional alteration has occurred (see "Alteration"). Since sediment is found adhering to several long basalt pieces (Figs. F16, F18), the maximum clast size for the deposit is significantly greater than the diameter of the core.

In spite of the similarities described above, the proportions of the different lithic clasts differ within the two breccias. In Section 187-1164B-3R-1, for example, subrounded clasts of yellow palagonite ± glass and white clay after palagonite dominate the 1-3-mm range. Indeed, clasts of palagonite may form as much as ~60% of the rock, since the matrix is a honey brown clay to silt, probably derived from the breakdown of palagonite clasts. In addition, the sediment in this interval is loosely cemented by clay and/or quartz, and some pieces in Section 187-1164B-3R-1 have drusy to botryoidal quartz lining pore spaces. There is no indication of sorting by density or size.

In contrast, yellow palagonite and white clay form <25% of the 1-3-mm range for the breccias in Section 187-1164B-4R-2. Although most have concentric rims of yellow palagonite or white clay, clasts of unaltered glass are common, as is unaltered olivine (Fig. F19). The matrix is composed predominantly of gray to white, silt- to sand-sized particles of uncertain origin (Fig. F20), but it probably originates from the breakdown of basalt, glass, and plagioclase rather than from palagonite. Thus, the breccia from this interval has a dark black appearance throughout. Sample 187-1164B-4R-2, 65-69 cm, also shows inverse size grading (Fig. F18). This piece has a subvertical, tube-shaped pocket (2.5 cm × 1 cm × 0.5 cm) in which a buff-colored clay and silt layer (1-5 mm thick) lines the pocket, grading into a 2.5-cm pocket of sand-sized lithic clasts, composed mostly of palagonite and glass.

Both breccias differ significantly from the basalt-carbonate breccias recovered previously at Sites 1157, 1160, and 1162, since carbonate is absent from the sediment in Hole 1164B. The abundance of unaltered olivine and glass and the absence of carbonate cements or large amounts of clay suggest that these breccias formed in a sediment-starved setting. They probably do not represent a significant amount of sedimentary transport and may instead have formed by autobrecciation of pillows during eruption. In contrast to the high degree of alteration of the surrounding rubble clasts, the relatively unaltered state of these sediments suggests that fluid flow through them has been negligible.