The entire 337.7-m-long sequence of lithic vitric tuff, lapilli tuff, and lapillistone cored at Site 1184 has undergone low-temperature alteration. The top of Core 192-1184A-9R is capped by a thin (3-7 mm) black ferromanganese crust. The upper 8 m (Cores 192-1184A-9R through 11R) is completely altered to red, tan, and pale brown iron oxyhydroxides and clay minerals, indicative of highly oxidizing conditions. Color changes in this oxidized zone are commonly gradational and do not seem to correspond to alteration fronts. Red to bright brick-red zones commonly occur in fine-grained areas; however, we observed no correlation between alteration and primary sedimentary structures. Background alteration in the remainder of the hole has imparted a greenish gray color to the rocks because of alteration of individual clasts and matrix to smectite. Toward the bottom of the hole, red or reddish brown lithic fragments and accretionary and armored lapilli are commonly intermixed with less altered clasts and/or surrounded by greenish gray matrix. This intermixture suggests that variable conditions were operating, possibly before deposition in a marine environment. The rocks are cemented with smectite, analcime, calcite, rare celadonite, and several zeolites, including gmelinite, chabazite, levyne, and clinoptilolite (identified by X-ray diffraction [XRD]). Several generations of late, white crosscutting veins contain calcite, analcime, apophylite, natrolite, and mordenite (identified by XRD). For a complete list of secondary minerals identified by XRD, see Table T8.
The volcaniclastic rocks are made up of the following types of clasts in decreasing order of abundance: basaltic glass shards, dark tachylitic clasts, various gray lithic clasts, and diabase clasts (see "Igneous Petrology"). The basaltic glass shards are generally nonvesicular and angular to subangular. They are most commonly totally replaced by secondary minerals, mainly yellowish brown to orange-brown, submicroscopic to scaly smectite (Fig. F59). Zeolites and/or analcime replace the centers of a few glass shards (Figs. F60, F61, F62). The most commonly observed assemblage of secondary minerals filling vesicles in individual glass fragments follows the depositional sequence (from rim to center): smectite, analcime and/or zeolites, calcite. Celadonite was tentatively identified as rare, thin rims on individual fragments (Fig. F63), participating in the replacement of basaltic glass with smectite (Fig. F64) and filling vesicles in glass (Fig. F65). Celadonite spherules displaying well-developed Liesegang banding were observed in association with fine-grained celadonite filling vesicles (Fig. F66).
Unaltered basaltic glass is present in Cores 192-1184A-11R through 13R, Core 31R, and Cores 39R through 45R. A thin layer of brown to yellowish brown smectite commonly rims individual shards of unaltered glass (Fig. F67). An interesting feature in several shards is the presence of dark, tubular to vermicular, commonly fibril-like features at the interface between the alteration rim and unaltered glass (Fig. F68). These types of features have previously been ascribed to microbially mediated alteration of basaltic glass (e.g., Fisk et al., 1998; Torsvik et al., 1998). Vesicles in unaltered glass are either empty or, more commonly, completely filled by smectite (Figs. F68, F69).
The dark brown to black tachylite clasts are commonly highly vesicular and scoriaceous. We could not determine the extent of alteration of the matrix of tachylitic clasts because the matrix is almost totally opaque in thin section. Vesicle fillings in the tachylite clasts are similar to those in vesicles within glass fragments. The lithic clasts we observed in thin section are rarely altered; however, the matrix of one diabase clast was totally replaced by smectite.
The cement between individual clasts is predominantly composed of the same minerals as those replacing glass fragments and filling cavities. The assemblage of secondary minerals is significant because the zeolites we identified commonly form as the result of low-temperature alteration of basaltic rocks in subaerial environments (e.g., Coombs et al., 1959; Walker, 1951, 1959, 1960a, 1960b) and are uncommon in oceanic basalts or sediments (Kastner, 1979; Honnorez, 1981). Coarser grained intervals commonly display patchy filling of pore spaces with calcite, analcime, and zeolites. A few occurrences of milky white to bluish chalcedony filling pore spaces were also observed (e.g., Cores 192-1184A-36R through 38R). Rare pleochroic celadonite has also been identified tentatively in the cement (Fig. F70). In two thin sections (192-1184A-42R-1, 147-150 cm, and 42R-6, 82-85 cm), we observed that the clasts were cemented by a matrix made up of fine-grained ash completely altered to smectite, and that the intergranular zeolite cement was relatively rare.
Several generations of white, hairline to >5-mm-wide veins crosscut the cores. The veins are filled with analcime ± zeolites (natrolite or mordenite) ± calcite and are lined with minor smectite and/or celadonite (identified by XRD) at the vein margins. Several of the larger (2- to >5-mm-thick) veins contain prismatic crystals formed during symmetrical open-space infilling of fractures with minor or no replacement of the wall rock; empty pore space is still present in some fractures. Pyrite is observed in a large (~1 cm thick) calcite + analcime + quartz + mordenite vein at the bottom of Section 192-1184A-46R-1. The abundance of veins appears to decrease downhole, although we did not carry out a statistical study of the veins at this site.
Halos in the groundmass adjacent to the veins are rare, diffuse, and poorly developed. If present, they commonly extend <1 cm into the groundmass and contain celadonite or Fe oxyhydroxides.
Magnetite and hematite were identified on the basis of bulk-rock XRD analysis in Sections 192-1184A-28R-4 and 29R-3, respectively. These core intervals fall within Subunit IIC, which has high magnetic susceptibility (see "Paleomagnetism"). Further work is necessary to determine where these minerals reside (i.e., clasts or matrix).
The highly oxidized nature of the upper 8 m of the basement section, along with the presence of a predominantly nonmarine zeolite assemblage in the cement and veins, suggests this sequence of rocks underwent low-temperature alteration in a subaerial environment. This type of alteration is in contrast to the low-temperature alteration commonly observed in seafloor basalts, which results from interaction between the basalts and circulating bottom seawater. The mixture of oxidized and unoxidized lithic fragments and accretionary and armored lapilli present in several sections of the core suggests they were derived from different source regions in which different alteration processes were operating. Some of the highly oxidizing alteration may have occurred before deposition in a marine environment, possibly soon after eruption.