Drilling of basement in Hole 1140A recovered submarine-erupted pillow lavas with lesser and more massive lobes, intercalated with minor dolomitized chalk and brittlely deformed pale green mudstone. Unambiguous lava pillows indicate that these lavas were erupted on top of the sediments as flows instead of emplaced as sills. The absence of overlying terrestrial sediments indicates that the basement was not subjected to subaerial weathering.
The basement rocks were subdivided into six units; five are composed primarily of basaltic flows. The smaller sedimentary pieces recovered were not recorded as separate basement units (e.g., Sample 183-1140A-31R-1 [Piece 9, 133-139 cm]); Sample 183-1140A-34R-4 [Piece 1, 0-5 cm]) except for Unit 4, which is an ~1-m-thick bed of dolomitized chalk (see "Igneous Petrology and Geochemistry"). The flows are aphanitic to medium-grained, aphyric to sparsely plagioclase-phyric basalts and mostly only sparsely vesicular. The downhole variation of alteration in the basement (Fig. F34) is recorded in the alteration and vein/structure logs for this site (see the "Supplementary Materials" contents list). Time constraints precluded detailed analyses of structural data, but orientations were measured for most planar features observed in these cores and were recorded in the vein/structure log. We identified selected secondary minerals by XRD (Table T8).
On the margins of lava pillows, vitreous glass abounds (see Fig. F34). The glassy margins (~5-10 mm) are macroscopically fresh and isotropic in thin section with only minor alteration along wispy brown clay veins. Highly altered glass is rare, and we recorded only a few occurrences of orange-brown clay with relict fragments of altered glass or hyaloclastite (e.g., Sample 183-1140A-33R-1 [Piece 7, 37-39 cm]; see Fig. F34). Most occurrences of altered glass are in Unit 5 (see "Igneous Petrology and Geochemistry"). Irregular patches (0.5-5.0 mm) of black manganese minerals (manganite and pyrolusite; Table T8) in the altered pillow margins possibly suggest an extended duration of exposure of these rocks to ocean currents.
The glassy margins are crosscut by numerous dolomite and/or calcite veins that developed concentrically to the pillow rinds both externally and within the glass itself. The outer veins are generally thick (2-3 mm), and both carbonate minerals exhibit dog-tooth sparry habits indicating mineral precipitation from fluids into voids in the pillow interstices. Commonly, angular fragments of fresh glass have spalled off the quenched rind; these pieces are now preserved in a cement of dolomite and/or calcite (Fig. F35). Carbonate veins (<2 mm) within and parallel to the glassy margin are commonly slightly offset by fine clay- or calcite-filled fractures that have developed radially from the pillow margins.
Baked, highly indurated foraminifer-bearing chalk and mudstone are commonly preserved adjacent to the pillow margins and in the interstices (Fig. F23B). Rare indenters of these sediments have apparently penetrated through the glassy margins while the basalt was still molten, resulting in internal glassy quenched zones in the pillow interiors. These zones are offset from the quenched rind by a sharp, brittle boundary (Fig. F36).
Immediately interior to the glassy margins, we observed a transition from glass to dark gray-brown, variole-rich aphanitic rocks. This zone grades into an orange-brown stained groundmass with sparse vesicles partly filled with brown clay, iron oxyhydroxides, and minor calcite. Mafic groundmass phases (clinopyroxene) are pseudomorphically replaced by brown clay minerals and iron oxyhydroxides. These oxidized halos typically extend 2-5 cm from the pillow margins, but oxidation halos are also developed within the wall rock to brown clay-filled radial fractures that penetrate the interior of the pillows. Large portions of the rocks display brown staining and oxidation. The most intense orange-brown staining is common at the edge of the oxidation halo adjacent to the gray basalt (Fig. F37).
Away from the pillow margins and vein halos, the groundmass of the crystalline interiors of the lavas are slightly to moderately altered. The fine- to medium-grained interiors of the more massive units are dark gray to greenish dark gray, reflecting alteration of the commonly prominent mesostasis to green clay (Fig. F34). Rare, large elongate pipe vesicles (1-2 cm) are partly filled with green or blue clay and coarse-grained pyrite. Pyrite is also common in the groundmass of the fine- to medium-grained basalts, outside the brown oxidation halos.
Carbonates and clay are the most abundant vein fillings, and they are in several morphologies and compositions. Green clay is most common in the gray portions of the basalts, but brown clay (± iron oxyhydroxides) is present within the oxidized halos (Fig. F37). Blue clay is abundant, commonly with stringers of pyrite-filled veins in parts of basement Units 2 and 3 (Sections 183-1140A-31R-1 through 32R-3). Open-space filled calcite and dolomite form cements in the interpillow interstices and coarse (<2 mm) veins within the glassy margins. Pale pink calcite veins with a powdery texture are also common in the pillow margins. Toward the bottom of Hole 1140A, fibrous aragonite fills rare subhorizontal veins in fine-grained basalt.
Rare, completely altered but not visibly oxidized patches within the fine- to medium-grained massive interiors of some flows are either elongate subvertical trails (~2 cm × 5-10 cm) or spheroidal (~5-10 cm). High vesicularity (~20%) suggests that these features are pools or rising fingers of late-stage magmatic segregations (see "Physical Volcanology"). The dominance of altered mesostasis and the complete absence of either groundmass minerals or phenocrysts indicates that these patches were originally glassy. The glassy regions are completely altered to black clay, whereas the generally large vesicles (~5 mm) are filled with green clay and calcite (Fig. F38). Basalts hosting these segregations are commonly more altered in the surrounding ~10 cm. Cut surfaces become flaky or desiccated on drying, indicating abundant smectite (cf. Hole 1037A, Escanaba Trough Reference Hole; Fouquet, Zierenberg, Miller, et al., 1998).
Oxidation halos are less common in the more massive fine-grained interiors of the thicker lava units except where these rocks are intercalated with ~1-m-thick beds of dolomitized and oxidized chalk (e.g., Unit 4 and surrounding rocks; Sections 183-1140A-32R-3 through 32R-4). The dolomite within these sediments is coarsely crystalline, typically as colorless rhombs. It is secondary after primary sedimentary phases and replaces or partly fills foraminifers. Dolomite within the lavas is most common in the interstices between pillow lobes, and there is a clear association between the presence of dolomite in the lavas and the close proximity of intercalated sediments. Although elucidation of the mechanism of dolomite precipitation will require postcruise geochemical investigation, it is likely that the sediment horizons have acted as channels, enabling large volumes of seawater-derived fluids access to the basement. This resulted in the precipitation of abundant, euhedral, colorless dolomite crystals in the margins of the chalks and numerous sparry dolomite ± calcite veins on the pillow margins.
The alteration at Site 1140 strongly resembles that in young mid-ocean-ridge lavas from the uppermost ocean crust. In particular, it is very similar to that described from the ocean crust reference sections penetrated in DSDP-ODP Holes 504B and 896A, located in 5.9-m.y.-old crust in the eastern equatorial Pacific Ocean (Alt et al., 1986; Teagle et al., 1996). The secondary minerals and the intensity of alteration are indistinguishable from normal young ocean crust. Only abundant dolomite is rare in the alteration of the upper ocean crust (Alt and Teagle, 1999). However, the alteration of basalts erupted in a mid-ocean ridge setting may differ from that of basalts formed on the flanks of a seamount or other submarine volcanic edifice. If the Hole 1140A flows are compositionally similar to MORB, then Hole 1140A may provide an important section of intermediate age (10 to 110 Ma) for assessing the progressive alteration of the upper oceanic crust with time.