ALTERATION AND WEATHERING

Twenty-two basement units have been defined in Hole 1138A, including cobbles of dacite (Unit 1), 20 igneous units (Units 3 through 22) interpreted to be various subaerial basaltic lava flows (see "Physical Volcanology" and "Igneous Petrology"), and Unit 2 of volcaniclastic origin (see "Lithostratigraphy" and "Physical Volcanology"). Fluid/rock interaction has variably altered all basement units after emplacement, as indicated by secondary minerals that partly replace primary minerals, partly to completely replace mesostasis, and partly to completely fill veins, vesicles, and open spaces between clasts within breccias. We recorded the distribution of secondary minerals in the alteration and vein/structure logs (see the "Supplementary Materials" contents list) for Hole 1138A (Fig. F69). In addition, we performed XRD analyses to identify secondary minerals (Table T14).

Basement Unit 1 is comprised of rounded isolated cobbles of pale pinkish brown to grayish green aphyric to sparsely sanidine phyric dacite. These rocks have a prominent flow banding, delineated by common streaks of groundmass altered to green clay (e.g., Sample 183-1138A-74R-1 [Piece 10, 50-56 cm]). The pieces that are uppermost in this unit, which directly underlie the dark brown clays and a thin layer of well-rounded, coarse sandstone to conglomerate (grain size 5 mm; lithostratigraphic Unit III), are completely altered to red-brown to gray to green clays. A relict igneous texture is present in some pieces (e.g., interval 183-1138A-73R-CC, 0-15 cm), however. In less-altered dacite pieces, late-stage oxidation halos (~5 mm) are common along fractures. Sparse veins, <0.5 mm wide, are filled with dark green and blue-green clay and rare calcite. Core 183-1138A-75R recovered only a single piece of hard, black aphyric cryptocrystalline igneous rock, with minor, wispy, red oxidation streaks. Its relationship to the pinkish dacite is unknown, but soft material that looks morphologically identical but is completely altered to black clay in the upper 20 cm of Section 183-1138A-76R-1 directly overlies Unit 2.

Unit 2 is a heterogeneous sequence of volcaniclastic deposits that contains multiple beds of laminated sediments with variable concentrations of pumice, lithic clasts, and ash. The grain size of the clasts and matrix varies widely, as do the degrees of grading and sorting. The rocks are generally highly altered with some glass-rich intervals, including altered ash layers and the matrix of poorly sorted sediments, completely altered to green clay. Commonly, however, the groundmass of clast-rich intervals remains gritty, indicating only the partial replacement of the finer grained fragments by clay minerals. Lithic and pumice clasts are also altered to clay, but the intensity of replacement is more variable; many pumice clasts are quite fresh and retain primary igneous textures. Also, numerous intervals have been oxidized to red-brown clay. In some cases, this oxidation occurs only in the matrix between volcanic clasts, which are altered to green clay (e.g., Sample 183-1138A-76R-1 [Piece 1, 35-45 cm]).

The alteration of the subaerial basalt flows in Hole 1138A (Units 3-22) is relatively uniform. Secondary mineralogy varies little downhole (Fig. F69), and alteration is dominated by clay and zeolite that partly to completely fill vesicles and the interclast open spaces in the volcanic breccias that dominate the recovered rock. Highly vesicular flow tops and bottoms and areas with higher vein and fracture density generally exhibit moderate degrees of alteration, whereas the least-altered rocks are the minor, massive flow interiors. Within each basement unit, secondary mineral assemblages do not vary with alteration intensity (Fig. F69). One noteworthy characteristic of the alteration within these subaerial basalt flows is the complete absence of carbonate minerals in groundmass, vesicles, and veins except for Unit 3 (Fig. F69). In Unit 3, calcite locally replaces the groundmass and plagioclase phenocrysts (e.g., Sample 183-1138A-79R-4 [Piece 2, 108-115 cm]). Calcite veins as much as 5 mm wide are common and in some cases connect large calcite-filled vesicles. Calcite composes ~3% of the unit, and this alteration gives the rock a distinctive light gray hue. The complete absence of carbonate alteration in the rest of the basement in Hole 1138A contrasts with basement recovered from other Leg 183 holes where calcite is a common secondary mineral.

Seventeen brecciated intervals have been defined within basement Units 3-22, representing both flow tops and flow bottoms (see "Physical Volcanology"). All of these intervals are highly to completely altered and vary from gray (e.g., Sample 183-1138A-87R-1 [Piece 1, 0-12 cm]) to dark red (e.g., Sample 183-1138A-82R-1 [Piece 1, 0-15 cm]), suggesting different degrees of oxidation at the top and bottom of each unit.

Most breccias consist of ~30% matrix and ~70% volcanic clasts, although locally the percentage of matrix can be as high as 75% (see Fig. F69). The clasts are variably altered to green and red-brown clay, and vesicles within clasts are filled with similar clay in addition to clinoptilolite (Fig. F29). Secondary minerals within the matrix of all breccias include clinoptilolite and smectite (Fig. F46), with amorphous silica and quartz occurring in trace amounts (<1%). The relative proportion of zeolite and clay in the matrix varies among different breccias, from ~20% to ~80% clay. Spectacular zeolite crystals, exhibiting different colors and habits, are common in large open cavities within the matrix of many breccias, although XRD analyses confirm that clinoptilolite is the only zeolite present (Table T14). Well-indurated silts fill the matrix within some brecciated intervals (e.g., Sections 183-1138A-81R-4 and 82R-2; Fig. F34). These sediments are most likely clastic debris washed into open spaces between volcanic clasts in the subaerial environment (see "Physical Volcanology"). An XRD analysis indicates that these silts are quartz rich (Table T14), suggesting that the induration may be a result of silicification, although some of the quartz may also be clastic. We also identified small amounts of clay and amorphous silica (~5%) in the sediments in thin section.

Twenty highly vesicular intervals have been defined within basalt Units 3-22 in Hole 1138A (see "Physical Volcanology"). Vesicles compose ~30% of the rock in the most vesicle-rich intervals; these intervals are typically moderately altered and variably oxidized. Vesicles are 50%-100% filled with zeolites, smectite, and, more rarely, quartz, although calcite-filled vesicles are present in Unit 3 (Table T14). Smectite lines most zeolite-filled vesicles, indicating clay formation before zeolite. Some vesicles also have green to blue-green alteration halos <1 cm wide. Similar to our observations from brecciated intervals, larger vesicles commonly contain zeolites with different colors and crystal forms. The XRD analyses indicate that clinoptilolite is the only zeolite present, although we identified analcime in the center of one vesicle lined with clinoptilolite (Table T14). Geopetal structures are rare and are characterized by zeolite and smectite divided by a subhorizontal boundary that bisects the vesicle. These structures do not have a consistent form, however, and we observed zeolite above the boundary in some vesicles and below the boundary in others.

Vein and fracture density does not exhibit large downhole variations within the more massive and vesicular portions of Units 3-22 (Fig. F69), although generally veins are less common in these cores compared to other Leg 183 sites (see "Structural Geology"). Moreover, with the exception of calcite-filled veins in Unit 3, the only vein minerals present are green and blue-green smectite and zeolite. Veins are <2 mm wide and are typically lined with smectite and filled with zeolite. Some veins have narrow (<5 mm) green alteration halos. Although zeolites of different color and morphology are present in some veins, by analogy with our XRD results from vesicles it is likely that clinoptilolite is the only zeolite present. In rare circumstances, vein mineralogy changes abruptly from zeolite to smectite where the vein crosses a color change in the wall rock from light gray to dark gray (e.g., Sample 183-1138A-85R-1 [Piece 6, 57-63 cm]). Also, some veins form fine networks in which the included small (<1 cm) wall rock fragments are completely changed to clay (e.g., Sample 183-1138A-81R-1 [Piece 14, 125-136 cm]).

In summary, the typical basalt flow in the basement of Hole 1138A exhibits a brecciated or vesicular flow top that has been moderately to completely altered to clay and zeolite, with a gradual progression to only slightly altered rock in the relatively thin massive flow interiors. Alteration of vesicular or brecciated flow bases is most commonly similar to the underlying flow top, although oxidation differs slightly across some flow contacts. In addition, smectite and zeolite fill vesicles and veins in Units 4-22. This alteration is most likely the product of both weathering and low-temperature fluid-rock reaction. The most notable difference in alteration within the basement of Hole 1138A (below Unit 3) compared to other Leg 183 holes is that zeolite is an abundant secondary mineral whereas calcite is absent. Such a difference in secondary mineralogy may be related to the relative importance of alteration in the submarine vs. subaerial environment and will be a focus of shore-based studies.