A lithic pebble conglomerate characterized by chert and basalt pebbles within a calcareous foraminifer-rich matrix directly overlies the basement in Hole 1141A. The conglomerate is coated with a 1-cm-thick manganese crust, suggesting submarine exposure for an extended time. The six basement units below the conglomerate include gabbroic rocks (Unit 1) and subaerial basalt flows (Units 2-6) (see "Physical Volcanology" and "Igneous Petrology"). All basement units in Hole 1141A have been variably affected by secondary processes. These include subaerial weathering and fluid/rock interaction, as indicated by the abundance of clay in flow tops and secondary minerals that partly to completely replace mesostasis and fill veins, vesicles, and open spaces between breccia clasts (see the "Supplementary Materials" contents list for the alteration and vein structure logs; see also Fig. F56A). Detailed analysis of structural data was not performed, but orientations were measured for most planar features observed in these cores.
The two gabbroic pebbles comprising Unit 1 are dark green and highly to completely altered. Small orange halos are present around oxidized clinopyroxene phenocrysts, and the groundmass has been pervasively replaced by calcite. The pebbles contain one 0.2-mm calcite vein.
The basalt flow comprising Unit 2 has a curated thickness of 20 m and is completely altered to clay, although the core is still coherent and many intervals have not been disturbed by drilling. We observed distinct color variations in Unit 2 that suggest a general decrease in oxidation with depth. Various shades of red characterize the color of the upper part of the unit from the top of Section 183-1141A-14R-1 to the bottom of Section 183-1141A-15R-2. The middle portion of the flow (Interval 183-1141A-15R-3 through 15R-5, 114 cm) is light gray to brown gray, and color gradually changes to light green and greenish gray with depth toward the bottom of the unit. Although the flow has been completely altered, some primary igneous features are still discernible. Vesicles are particularly well preserved and are completely filled with dark green clay, carbonate, and zeolite (Table T10). In addition, possible late-stage, more highly vesicular segregations are present near the top of the flow (e.g., intervals 183-1141A-14R-1, 90-93 cm, and 14R-1, 126-134 cm), and some intervals have brecciated textures (e.g., interval 183-1141A-15R-5, 0-30 cm). Veins are not present.
The intensity of alteration within Unit 3 varies from complete near the top of the flow to moderate at the base. This corresponds to a large increase in competency with depth, from soft clay easily penetrated by the probe to solid rock (see "Alteration and Weathering" in the "Explanatory Notes" chapter). A brick red interval, completely altered to clay, is present in the uppermost 9 cm of the flow (interval 183-1141A-16R-2, 19-28 cm). Below this interval, color gradually changes from dark to light greenish gray and finally to light gray at the base of the flow, consistent with a decrease in oxidation with depth and similar to the alteration patterns we observed in Unit 2. Primary variolitic textures are visible, even in the completely altered basalts, and tend to be accentuated by replacement of igneous minerals with light green clay. A pronounced mesostasis is also visible in the less altered intervals and is replaced by light green clay and calcite within the halos of calcite veins. Vesicles are locally abundant (as much as 7%) and are filled with dark green clay, calcite, zeolite, and, rarely, quartz. Rare amorphous silica replaces possible late-stage magmatic segregations (e.g., interval 183-1141A-16R-CC, 40-41 cm). Calcite veins are common and are generally <0.5 mm wide, some with light brown carbonate-rich halos.
Unit 4 is variably altered fine-grained basalt with only slight to moderate change in some intervals within the interior of the flow and complete alteration near the top. Color varies from greenish gray to dusky red in the highly to completely altered flow top and gradually changes to green and greenish gray with depth through the unit. Thus, a distinctive oxidized flow top, such as those present in the uppermost parts of basement Units 1-3, appears to be less well developed in Unit 4. Primary igneous features, including glassy mesostasis and variolitic textures, have been altered to green clay, and calcite locally replaces the groundmass (e.g., Sample 183-1141A-19R-21, Piece 21). Vesicle morphology is preserved because the vesicles are completely filled with brown and green clay, calcite, and amorphous silica, and some vesicles have colloform textures (Fig. F57). We observed a trace of native copper in vesicles and the groundmass. Secondary minerals filling veins include green, brown, and red clay, calcite, zeolite, and, more rarely, quartz. Dark green alteration halos <10 mm wide are common around veins and tend to be much more highly altered to clay than adjacent regions. Red oxidation vein halos or bands, not always associated with veins or fractures, are present and are typically crosscut by late calcite veins. Light green alteration halos associated with these calcite veins replace the oxidized halos (Fig. F56A). Slickensides are numerous along some fractured surfaces, and brecciated wall rock is present in some veins.
The flow top of the basalt comprising Unit 5 is red, highly vesicular (15%), and completely altered to clay with abundant slickensides on clay-lined fractures. Color gradually changes to pinkish gray and greenish gray with depth, suggesting a decrease in oxidation. Intensity of alteration correspondingly decreases with depth, but all intervals are at least moderately altered, with clay replacing the fine-grained groundmass. The least-altered basalt is in the massive, sparsely vesicular flow interior. The flow base is highly vesicular and completely altered to red clay. Secondary minerals filling vesicles include green clay, calcite, zeolite, and (more rarely) amorphous silica. Veins are extremely common, generally <1 mm wide, and filled with calcite, zeolite, and (more rarely) green and blue clay. Zeolite is the most abundant mineral filling veins in the upper portion of the unit to the bottom of Section 183-1141A-20R-3. Below this depth, and continuing to the bottom of Unit 5, veins filled with calcite and green clay are common. We observed one fracture lined with calcite, green clay, and abundant native copper that comprises ~10% of the fracture surface (Sample 183-1141A-21R-2 [Piece 5, 39-44 cm]). Slickensides are also common, particularly along fractures lined with red clay.
The flow top of the lowermost basalt flow in Hole 1141A (Unit 6) is brecciated and completely altered to red-brown clay (Fig. F58). The brecciation may be related to primary volcanic processes, but slickensides on clay-lined fractured surfaces suggest that tectonic processes may also be involved. Unlike other basement units in Hole 1141A, color does not vary consistently with depth in Unit 6 because of the presence of complex alteration halos of different color surrounding subvertical veins. Like other basement units, the intensity of alteration and vesicularity both decrease with depth; the freshest rocks are moderately altered and sparsely vesicular. The basalt in the lowermost section of Hole 1141A (Section 183-1141A-24R-CC) is highly to completely altered to brown clay; this may be a highly altered flow base. Alteration accentuates primary igneous textures, including a prominent mesostasis altered to green and, more rarely, light red clay, variolitic textures, and late-stage magmatic segregations (Fig. F59). Vesicles are filled with green clay, calcite, and amorphous silica. Multiple generations of these secondary minerals produce colloform textures (Fig. F20). Perhaps the most noteworthy aspect of the alteration within Hole 1141A in general, and Unit 6 in particular, is the abundant and very well-developed halos that surround subvertical veins filled with quartz, calcite, and green clay. In some instances, a single quartz vein can be followed for >120 cm (e.g., Sample 183-1141A-24R-1 [Piece 2, 24-144 cm]) with multiple symmetrical alteration halos progressively altering the surrounding wall rock (Fig. F60). The color of these halos varies from dark green at the vein margin to light green, dark red, and light red grading into dark red-gray basalt. A general decrease in alteration intensity is associated with this color change. Calcite veins lacking distinctive halos crosscut the quartz-rich veins, indicating multiple generations of fracturing with quartz formation before calcite. We observed similarly complex alteration halos around networks of calcite and quartz veins that locally incorporate brecciated wall rock fragments (Fig. F61).
The alteration patterns observed in Hole 1141A suggest that the upper part of the basement (Units 2 and 3) was weathered in a subaerial environment long enough to deeply and completely weather these flows to clay. Units 4-6 also have highly or completely altered flow tops, but the presence of only moderately altered flow interiors may suggest subaerial exposure for shorter lengths of time. All the units were also affected by hydrothermal alteration at low to moderate temperatures (probably <200°C), but the relative timing of weathering and hydrothermal processes is not clear.
Six basement units have been defined in Hole 1142A, including a granule-bearing clay (Unit 4) and five volcanic units of unknown origin (Units 1, 2, 3, 5, and 6). All basement units have been variably altered by fluid/rock interaction at low to moderate temperatures, as indicated by secondary minerals that replace primary igneous phases, partly line fractures, and partly to completely fill veins and vesicles. The distribution of secondary phases is recorded in the alteration and vein structure logs for Hole 1142A (see the "Supplementary Materials" contents list) and plotted in Figure F56B.
Unit 1 is a massive basalt that is relatively fresh compared to other basement units in Hole 1142A (and Hole 1141A, which is only 0.8 km away). The flow is gray, sparsely vesicular, and only slightly altered. Calcite, zeolite, and minor green clay fill vesicles. Green and red-brown halos as much as 10 mm wide are common around veins filled with green clay and calcite.
The mixed volcanic rocks comprising Unit 2 have different compositions and textures and have been altered to varying degrees. Chocolate brown volcanic breccias completely altered to clay were recovered (Sample 183-1142A-3R-1 [Pieces 3 and 4 and 12-15]). Other lithologies present include variably altered microcrystalline to fine-grained mixed volcanic rocks. Highly altered pieces are blue green (Sample 183-1142A-3R-1 [Pieces 5 and 8-10]) and are moderately vesicular with white clay and calcite-filling vesicles. Moderately altered pieces are light gray (Sample 183-1142A-3R-1 [Pieces 6, 7, and 11]) with few or no vesicles present. The lowermost pieces recovered from this unit include dark green plagioclase-phyric basalt (Sample 183-1142A-3R-1 [Pieces 16-18]) and a light gray plagioclase-phyric volcanic rock (Sample 183-1142A-3R-1 [Pieces 19-20]). Quartz(?) phenocrysts may be present in the latter, suggesting a relatively felsic composition. All of these lowermost pieces are completely altered to clay, and the groundmass of Pieces 19 and 20 are also replaced by calcite. Sparse carbonate veins, <1 mm wide, are in the unit.
Unit 3 has been divided into three subunits that include disturbed volcanic rock (Subunit 3A) and mixed lava, and volcanic breccias (Subunits 3B and 3C, respectively). Subunit 3A is dusky red and completely altered to clay. Some units are relatively coherent, whereas others have been highly disturbed by drilling. The volcanic rocks comprising Subunit 3B are dusky red, variably brecciated, and highly to completely altered to red-brown clay. Sparse vesicles are filled with brown clay. Slickensides are present on some fractured surfaces. Subunit 3C includes moderately to highly altered volcanic rubble and dusky red volcanic rocks that are completely altered to clay. The latter are variably disturbed by drilling, although some coherent pieces are present. Primary igneous textures are still visible and are accentuated by the replacement of feldspar by light green clay and mafic minerals by red-brown clay.
The granule-bearing clay comprising Unit 4 probably represents reworked volcanic debris (see "Physical Volcanology"). Subangular to rounded matrix-supported lithic pebbles are supported by a red clay matrix and become more abundant near the bottom of the unit. The pebbles are highly altered to clay minerals of variable color.
Unit 5 is volcanic breccia that is highly to completely altered to clay and variably disturbed by drilling, particularly near the base of the unit. The breccia is clast supported and varies from greenish brown near the top of the unit to reddish brown near the base. We observed pyrite and slickensides on some fracture surfaces lined with brown clay. Flow banding is present on some clasts, and quartz was tentatively identified, implying that this unit may be relatively silicic in composition.
The lowermost unit in Hole 1142A (Unit 6) is a nonvesicular basalt flow that may be either subaerial or submarine. Color varies from red to green to gray and alteration intensity varies from slight to high, depending on the abundance of red-brown oxidation and alteration halos around veins. Calcite ± hematite and brown clay veins are associated with red-oxidized halos that penetrate the wall rock to a greater extent along mesostasis trails (Fig. F34). Calcite veins clearly crosscut and locally offset red oxidation halos (Figs. F34, F35), meaning that calcite veins formed after the rock was variably oxidized. Some calcite veins in Unit 6 are associated with multiple alteration halos of varying color and alteration intensity (Fig. F62). Highly altered basalt is present as relatively narrow, very dark greenish black alteration halos immediately adjacent to calcite veins and comprises <1% of the rock. Dark green alteration halos produce moderately to highly altered basalt with moderately altered light green and red-brown halos at even greater distances from vein margins. These halos typically grade into slightly altered gray basalt.
Unit 6 has a number of alteration features that suggest the basalts may be pillows formed in a subaqueous environment. These include semicircular red-brown oxidation halos that could reflect progressive oxidation fronts into pillow-shaped lava and narrow dark green to black intervals associated with carbonate veins that may be completely altered glassy pillow margins.
The oxidation fronts are extremely common in Unit 6. Some are not connected with veins or fractures, indicating fluid migration through the rock along grain boundaries. However, close examination of these red-brown bands indicates that many of them are simply linear alteration halos cutting through the core along veins and fractures (e.g., Fig. F35). Moreover, based on our observations of numerous subaerial and submarine basalt flows on Leg 183, oxidation fronts typically have variable geometries that appear to be unrelated to macroscopic structures. They are, instead, probably more related to the complexities of fluid flow along grain boundaries. The dark green to black intervals associated with carbonate veins perhaps provide a more compelling argument for glassy pillow margins (e.g., Figs. F37, F62). Superficially, they look very much like completely altered analogs of the fresh glassy margins recovered from the pillow basalts in Hole 1140A. Close examination of some of these zones (e.g., Fig. F62) reveals that primary igneous textures extend from the relatively unaltered basalt through the dark green to black alteration zones. If this is the case, these features simply represent intense alteration halos around calcite veins. Other examples may be more compelling (e.g., F37). Thin-section analysis, however, indicates that grain size does not decrease toward the margin of the highly altered zones, an observation that is not consistent with the presence of glassy pillow margins (see "Igneous Petrology").