ICP-AES data for basalts were obtained late during Leg 200. The data were only partially reduced and calibrated to standards before the ship reached San Diego, and some data from the second of two runs for Site 1224 could not be used. Thus, only partial analyses are available for plotting on samples obtained from Holes 1224D, 1224E, and 1224F. The basalts were assigned to units based on lithology because geochemical data were not available. The three lithologic units are
The discussion here is based on contents of K2O, TiO2, MgO, Ba, Zr, and LOI from the ICP-AES data. Two diagrams (Figs. F43, F44) contain additional data obtained by postcruise X-ray fluorescence analysis on samples from 133 to 155 mbsf (S. Haraguchi, pers. comm., 2002).
The basalts range from differentiated to very differentiated N-MORB (4-7 wt% MgO and 2-3.5 wt% TiO2) (Fig. F43). The highly differentiated (4 wt% MgO and 3.5 wt% TiO2) basalts plot at the most differentiated end of the N-MORB data array. The samples selected for analysis from Hole 1224D are fairly fresh and have LOI values ranging from 0 to 0.45 wt% (Fig. F45). For comparison, the palagonitized crystal vitric tuffs, siltstones, and claystones have much higher LOI values; therefore, they are more altered. Concentrations of K2O (0.11-0.27 wt%) may be slightly elevated (>0.2 wt%) in three of the ten samples analyzed (Fig. F46). All the basalts have Ba concentrations (9-18 ppm) that are consistently lower than many comparably differentiated MORB glasses from the East Pacific Rise (Fig. F47). This may indicate a greater than average depletion of the mantle sources in the basalts from Hole 1224D. Alternatively, the rocks may have experienced a slight nonoxidative alteration as these components were partially removed from the rock.
The samples from Hole 1224D can be divided into two groups based on their TiO2 concentrations. These geochemical groups correspond to (1) the lithologic Unit 1 upper flow (>2.3 wt% TiO2) and (2) the lithologic Unit 1 lower flow (<2.3 wt% TiO2). Interestingly, lithologic Unit 2 pillow lavas have TiO2 concentrations similar to the lower flow basalts from Unit 1. The lithologic Unit 3 pillow lavas have TiO2 concentrations >3.1 wt% TiO2. Therefore, the geochemical divisions (based on TiO2 contents) do not correlate directly with the lithologic units.
Analyses from Holes 1224E and 1224F indicate that the topmost flow was sampled in both holes, and the second flow was also sampled in Hole 1224F. These belong to lithologic Unit 1. Samples of lithologic Unit 2 have TiO2 contents similar to those of the lower flow of Unit 1. Apparently accumulation of this chemically uniform basalt at the site began with thin flows. These were capped with a thick flow of the same material. The samples of Holes 1224E and 1224F from Unit 2, however, are more greatly altered than those of this capping flow at the base of Unit 1, usually having as much as two to three times the amount of K2O present in samples of Hole 1224D, and in one case having >1 wt% K2O. This is in accordance with the strong contrast in extent of alteration noted in the core descriptions between holes only 15 m apart.
Samples of lithologic Unit 3 have TiO2 contents greater than in any of the basalts from the thick flows or pillows of lithologic Units 1 and 2 (Fig. F44A). Based on TiO2 contents, these are among the most differentiated basalts sampled thus far from the flanks or axis of the East Pacific Rise. Their compositions are at or about the point where oxide minerals join the liquidus, causing TiO2 contents to drop as MgO decreases and producing andesitic and ultimately rhyodacitic residual liquids (Fig. F43).
At Site 1224, reddish dark-brown clay (Core 200-1224C-1H; <28 mbsf) and basalt (Holes 1224D, 1224E, and 1224F; >28 mbsf) were cored. Some white clayey pebbles were recovered in Core 200-1224A-2X. Massive basalt, present in the upper 35 m of basement, has some altered layers and fractures. Some fractures have green clay or white carbonate deposits. They were analyzed by XRD. The samples analyzed include one clayey pebble and 25 vein materials in the basalt.
Veins are classified in hand specimen according to color: dark green, light green, dark reddish brown, light reddish brown, white, and colorless, and also by degree of crystallization: muddy (clay vein) and consolidated (crystallized).
Five distinct vein types are documented by XRD analysis: clay, carbonate, zeolite, quartz, and smectite + calcite. The samples analyzed and results are shown in Table T6. Sampling points and identification are shown in Figure F48.
Pyrite and black to reddish iron oxyhydroxides were identified in many veins by visual observation, but their volumes were too small for XRD analysis.
Clay veins are the most abundant vein type in the basement at Site 1224. Most are dark green and <1 mm thick. Some thick veins, which reach nearly 2 mm in thickness, are filled with light-green fibrous clays.
The clay veins are smectite and illite rich. The smectite is characterized by a ~13- to 14-Å peak with additional peaks of montmorillonite and saponite (Fig. F49A). Illite veins have a 10-Å peak (characteristic of the mica group) (Fig. F49B). Other peaks are for montmorillonite, a smectite-group mineral, and laumontite, a zeolite-group mineral. Smectite veins are more abundant than illite veins, based on XRD identification. Only one pure illite vein was found among analyzed samples (interval 200-1224D-3R-3, 90-92 cm). Some veins have both smectite and illite characteristics in their XRD patterns.
The carbonate veins are mainly white and crystallized. Many are lined with green clay and have carbonate in the middle. Many veins in massive flows are <1 mm thick. However, those in pillows and breccias are thicker than those in the massive flows and may reach 1 cm in width. Some veins contain fragments of altered basalt. Sample 200-1224F-6R-1, 29-34 cm, is a hyaloclastite in which altered glass fragments are cemented by calcite.
The XRD patterns of these veins have characteristic calcite peaks (Fig. F50A) and lesser peaks of smectite. Smectite peaks (in the XRD pattern) are a result of the influence of the outer clay layer. Some samples also have minor phillipsite peaks.
Some carbonate deposits have poorly consolidated clays associated with them. These are present as voids in highly fragmented pillows and in cemented brecciated altered pillow fragments.
Fibrous, colorless aragonite is present in one sample (200-1224F-4R-5, 43-47 cm) (Fig. F50B).
One sample (200-1224F-9R-1, 55-57 cm), in a poorly consolidated white vein from a highly fractured pillow, contains cemented fine altered fragments of the host rock. The XRD pattern is characterized by many phillipsite peaks. Some low-intensity peaks are those of smectite. This appears to be an altered fragment of host rock (Fig. F51).
Some thin veins in the upper massive basalt are well crystallized and dark green. They are similar to calcite-clay veins but are <1 mm thick and show dark inner cores. These are dominated by quartz with accessory smectite (Fig. F52).
Some veins combine light-green clay and white carbonate and are nearly 2 mm thick. This vein material is not well consolidated. The XRD patterns have prominent calcite peaks and low smectite peaks (Fig. F53). Based on calcite and smectite XRD patterns and visual observation, the ratio of calcite and smectite in these veins is lower than that in consolidated and layered calcite veins. There is <30% calcite in these veins.
Clayey pebbles were recovered from the sediments of Hole 1224A. They are light brown and shaped like coral. They produce the complex XRD patterns of phillipsite, a zeolite-group mineral (Fig. F54). The depth of the sample may correspond to a seismic reflection at 10 mbsf (see "Core, Physical Properties, Logging, and Seismic Correlations") (Fig. F2).
Tartarotti et al. (1996) considered the sequence of secondary mineralization in open fractures based on studies of Hole 896A in the Costa Rica Rift. They suggested a general sequence of vein formation from oldest to youngest as follows:
Visual and macroscope observations together with XRD analysis of veins suggest that this sequence also is present at Site 1224 (e.g., many thick calcite veins have distinct inner calcite and outer green smectite zones; fibrous smectite is present only in thick veins; many quartz veins have associated outer green smectite; and most vein surfaces of host rocks show greenish [smectite] or black [oxyhydroxide] colors).
Tartarotti et al. (1996) characterized clay-lined veins at Site 896 into two types: nonfibrous and fibrous veins. Nonfibrous veins are thought to represent fractures filled by minerals crystallizing in open cavities where fluid-filled spaces were available for crystal growth. Fibrous veins are interpreted as crack-seal veins in which narrow cracks propagated and then were cemented. According to this model, thin and deposit-poor veins record hydrothermal activity, whereas thick crystallized veins reflect hydrothermal activity together with shear stress in the flow unit. Veins are thicker in pillows than massive flows. Perhaps this is because pillows are more fractured, sheared, and therefore more permeable to hydrothermal fluids than massive flows.
Many vein minerals in basement at Site 1224 are stable at low temperature and pressure conditions (e.g., zeolite). Phillipsite, the main zeolite present at Site 1224, is a low-temperature member of the zeolite group (e.g., Miyashiro, 1973). Smectite is also commonly found as a product of the alteration of volcanic ashes and rocks from the seafloor. Smectite is present in most low-grade metamorphic terranes around the world.
Four vein types, smectite-illite, calcite-aragonite, quartz, and zeolite, at Site 1224 are similar to those at Site 896 at the Costa Rica Rift. Quartz veins are not present at Site 896 but are found deeper at nearby Site 504B (Alt, Kinoshita, Stokking, et al., 1993). These minerals are present in relatively lower temperature hydrothermal assemblages (probably <100°C; Laverne et al., 1996).
Truly high temperature vein assemblages, such as the actinolite and epidote veins found deeper than 2000 mbsf in Hole 504B (Alt, Kinoshita, Stokking, et al., 1993), are not present at Site 1224. The mineral laumontite in the illite vein indicates a higher zeolite facies (Miyashiro, 1973). Aragonite generally forms at a higher temperature than calcite. These minerals indicate the local influences of warm hydrothermal fluids.