~ 5°) has not been confirmed by electron probe microanalysis (EPM). Anhedral groundmass quartz is rare and occurs only in the most evolved samples.
On the basis of macroscopic description and geophysical data, the basement section has been divided into three main lithologic units (Stephen, Kasahara, Acton, et al., 2003).
Lithologic Unit 1 (~28–63 mbsf) (Fig. F1) comprises massive basaltic flows with estimated thicknesses of ~35 m and recovery of ~53%. The color of the Unit 1 basalts is dark gray and the grain size is fine, with some gradation to slightly coarser grains toward the middle of the flows. Vesicles are rare and generally <1 cm in diameter with a circular shape. This unit composes the top of basement in Holes 1224A, 1224D, 1224E, and 1224F. The base of this unit probably coincides with a zone characterized by a density and porosity gradient at ~63 mbsf (Stephen, Kasahara, Acton, et al., 2003). Two flows have been identified in this unit on the basis of geochemistry (Lustrino et al., 2004); the contact between these two subunits may be identified as a relatively altered level particularly evident in Hole 1224D at ~48–50 mbsf. Additional evidence for two flows in Unit 1 are chilled margins and degassing structures (probably gas pipes) at the top of the lower flow (interval 200-1224D-4R-1, 11.5–21.5 cm). At least some of the vertical fractures may be related to cooling of the flow; most fractures in the basalts of Holes 1224D and 1224F are filled by clay minerals, oxyhydroxides, carbonate, and pyrite. Unit 1 samples from Hole 1224F show higher content of secondary minerals compared to Unit 1 in Hole 1224D.
Lithologic Unit 2 (~63–133 mbsf) (Fig. F1) comprises thin flows and pillow fragments that were cored in Hole 1224F only, with an estimated total thickness of ~71 m and relatively low recovery (~15%), which reflects the abundant fractures in thin flows and pillow fragments that are poorly cemented by carbonate. Most of the basaltic fragments show brownish alteration halos propagating from fractures, in many cases filled by iron oxyhydroxides. At least a dozen pillow lavas and thin flows have been identified in this unit on the basis of chilled margins (Stephen, Kasahara, Acton, et al., 2003). Two hyaloclastite breccias with volcanic glass partially altered to palagonite are cemented by carbonate material and zeolites.
Lithologic Unit 3 (~133–162 mbsf) (Fig. F1) comprises medium thickness flows alternating with thin flows and pillow lavas. These lithologies are found only in Hole 1224F at a recovery of ~21%. Many macroscopic features of this unit (e.g., relatively coarse grained flows, alteration halos, inclined fracture network, and intercalation of finer-grained basalts with pillow fragments) are common to the first two units.
The petrographic characteristics of Site 1224 basalts are reported in Stephen, Kasahara, Acton, et al. (2003). The most important features observed in thin sections are summarized here. The dominant minerals are plagioclase, clinopyroxene, and opaque minerals (both ilmenite-hematite and ulvöspinel-magnetite solid solutions), with rare olivine that is almost always iddingsitized. The presence of pigeonite, observed on the basis of onboard petrographic investigation and identified on the basis of pseudouniaxial character (interval 200-1224D-3R-1, 50–53 cm; 2V ~ 5°) has not been confirmed by electron probe microanalysis (EPM). Anhedral groundmass quartz is rare and occurs only in the most evolved samples.
Both massive and thin flows are nearly aphyric; only very few intervals are characterized by sparsely phyric lithologies (phenocryst content is always <3 vol%). Granularity ranges from holocrystalline (<10% glass) to hypocrystalline (~10%–50% glass), and the texture is isotropic, intergranular, and only rarely subophitic or intersertal. The average dimension of groundmass minerals ranges from very fine (<0.5 mm) to fine (~0.5–1 mm). Considering the age of the samples, the preservation of the basalts is surprising, with a general content of clay minerals (mostly replacing devitrified glass) <5 vol% and never exceeding 20 vol% (e.g., Paul et al., 2006).
Pillow fragments are generally aphyric, ranging from hypocrystalline to hypohyaline (~50–90% glass), with isotropic texture characterized by hyalopilitic and intersertal relationships among groundmass phases.
The upper chilled margin in contact with the overlying sediment is >90% brownish to yellow-orange volcanic glass; the change in color is a consequence of palagonitization. The microcrystalline portion of the chilled margin is composed of plagioclase, clinopyroxene, and rare iddingsite pseudomorphs of olivine. Vesicles are <1 vol% and partially filled by clay minerals.
The extremely rare phenocrysts in the basalt are euhedral tabular plagioclase, with a maximum dimension of ~1.2 mm, associated with euhedral prismatic to pseudo-octagonal clinopyroxene with a maximum dimension of ~3 mm. The intersertal to intergranular and rarely sub-ophitic groundmass is made up of the same minerals, plus opaque minerals with an average dimension of 0.1–0.2 mm. Groundmass plagioclase is often skeletal, whereas groundmass clinopyroxene is anhedral; opaque minerals are either acicular or equant. Interstitial brownish devitrified volcanic glass is nearly ubiquitous (~10 vol%) and is associated with plagioclase spherulites. Millimeter-size segregation vesicles are common as dark portions with circular shapes or as irregular pods composed of acicular to skeletal plagioclase, anhedral equant clinopyroxene, acicular opaques, and secondary clay minerals.
The mineralogy in Hole 1224D is similar to that of Hole 1224A basalts, that is, rare plagioclase and clinopyroxene phenocrysts (present also as glomerules) and plagioclase, clinopyroxene, and opaque minerals as groundmass phases with an average dimension <0.5 mm. Groundmass plagioclase is commonly present as open spherulites composed of radial crystalites and interstitial clinopyroxene. The texture is holocrystalline, hypidiomorph, and equigranular. The transition zone between the upper and lower flow of lithostratigraphic Unit 1 (Fig. F1) is an alteration zone characterized by highly altered groundmass phases. The main difference between the transition zone and the flows at top and bottom is the relative abundance of segregation vesicles, which may reach as much as 25 vol% in thin section; some vesicles contain small spherical sulfides. The abundance of subspherical cavities generally decreases with increasing depth from ~6 to ~1 vol%. They range from empty to partially or totally filled by clay minerals and/or plagioclase-clinopyroxene microlitic overgrowth, carbonate, and sulfide.
Filamentous structures in calcite-filled subspherical cavities at 51 mbsf resemble fungal hyphae (Schumann et al., 2004). These hyphae were found within sections perpendicular to the elongation axis of carbonate-filled tubular pores of basalt. These pores have a diameter ranging from ~0.5 to ~3 mm, and the filling media is calcite and aragonite. The net of fungal hyphae completely fills the whole pore space from the basalt/carbonate boundary toward the center of the pores. The cross-section dimension of these hyphae is ~5–10 µm and their length ranges from ~50 µm to several hundred micrometers; therefore, the cell septa of the hyphae are clearly visible. The number of hyphae ranges from tens to hundreds in a pore. Semiquantitative chemical analyses of the etched hyphae show a chemical composition different from the surrounding carbonate matrix. The mineral growth and small open space between the fungi and matrix indicates endolithic fungal growth after the carbonate filling. The presence of euhedral columnar crystals of pyrite in these cavities are evidence of anaerobic conditions (relatively low oxygen fugacity). According to Schumann et al. (2004), this is the first finding of eukaryotic life signs within relatively unaltered deep ocean basaltic rocks.
The top of the lower unit of this hole shows a hypohyaline border (interval 200-1224D-3R-3, 45–48 cm) and structures that may be gas pipe conduits (interval 200-1224D-4R-1, 15–17 cm) with widths varying from ~0.5 to ~5 mm and composed of the same paragenesis of groundmass but with coarser size; some of these conduits are filled by spathic carbonate.
Two-thirds of the studied samples are hyaloclastitic breccia with rounded to subrectangular yellowish to pale brownish glass fragments, from ~0.5 to ~3 cm in diameter, cemented by micritic carbonate. Only near the contact with the glass does the carbonate become spathic, with an average crystal dimension of ~0.3 mm. Often, a rim of relatively coarse grained zeolites develops between the glass fragment and carbonate cement. In some cases, the glass is totally replaced by orange palagonite, whereas in other cases the alteration is confined only to the outer rim of the shards. These are generally characterized by rare subhedral plagioclase, clinopyroxene, and iddingsitized olivine microcrysts.
The analyzed aphyric holocrystalline basalt has a texture and mineral paragenesis typical of basalts from the other holes, with <1% of plagioclase phenocrysts and hypidiomoph texture.
Hole 1224F is the only hole where all three lithostratigraphic units are present. Unit 1 basalts are similar to previously described Unit 1 basalt from the other holes. These are holocrystalline to hypocrystalline and aphyric with hypidiomorphic isotropic and intergranular groundmass composed of anhedral to subhedral clinopyroxene in intergranular to subophitic relationship with subhedral to euhedral, skeletal to tabular plagioclase, and subhedral opaque minerals. The grain size of groundmass phases varies from ~0.8 to ~0.3 mm. Gabbroic glomerules and phenocrysts are very rare. The peculiarity of Unit 1 basalts in Hole 1224F is the presence of a myrmekitic overgrowth of vermicular Na-rich plagioclase and quartz, mostly confined to the borders of groundmass or microphenocrysts of plagioclase. Another feature is relatively more alteration compared to Unit 1 basalts from the other holes. Rare iddingsitized olivine has been found in rare pillow fragments.
Unit 2 lithologies are pillow fragments and thin basaltic flows. The pillows show textural variability with holohyaline to hypohyaline, hypocrystalline, and holocrystalline types. These pillow fragments are the only places where microphenocrysts of olivine are not entirely replaced by iddingsite, whereas in the more massive (crystalline) portions of the basaltic flows, only ghosts of olivine are present. The gradation of color of the chilled margin (ranging from yellowish orange to gray toward the inner part) is mostly a result of the palagonitization of the outer rim and the increasing development of plagioclase and clinopyroxene microlites toward the core. Two hyaloclastitic breccias, found at ~76 and 129 mbsf, are composed of subrectangular glassy shards, with a major dimension ranging from ~0.5 to ~3 cm, cemented in a carbonate matrix; palagonitic alteration ranges from nearly total to partial (confined to outer rims).
Unit 3 lithologies are medium-thickness flows interbedded with thin flows and pillow fragments. The petrographic features of these rocks closely resemble those observed for Unit 1 basalts with a similar paragenesis and distribution of segregation vesicles. The only differences are the totally aphyric texture, coarser grain size of groundmass phases (average dimension of single grains > 0.5 mm), strongly fractured groundmass phases, higher content of segregation vesicles, low alteration (~2%), and low porosity (~1% cavities).
