The entire section of basaltic basement recovered from Hole 1183A has undergone low-temperature water-rock interactions, resulting in complete replacement of olivine and almost complete replacement of glassy mesostasis. Clinopyroxene and plagioclase only show alteration in some halos close to veins and miarolitic cavities. The overall alteration of the basalt pieces ranges from 5 to 30 vol%, estimated visually by color distribution and not taking into account the veins. Throughout this section we refer to volume percentages of the various alteration types, veins, hyaloclastite, etc.; our assumption is that the area occupied by these features on the sawed surfaces of the cores is equivalent to volume percent of the cores.
Clay minerals (tentatively identified as nontronite, saponite, and celadonite) are the most abundant secondary minerals. Secondary phyllosilicates in rocks from Hole 1183A were identified by color and hardness in hand specimen and by optical properties in thin section with occasional use of the XRD. For instance, we distinguished nontronite from saponite on the basis of absorption color: yellow-green vs. brown ish tan to colorless, respectively. Celadonite was distinguished from the other clay minerals on the basis of its pleochroism and anomalous birefringence. We compared the phyllosilicates observed in rocks from Hole 1183A with well-studied clay minerals identified in other sections of the oceanic crust (Böhlke et al., 1980; Honnorez, 1981; Alt and Honnorez, 1984; Alt et al., 1986). The identification of the phyllosilicates in Leg 192 basalts remains tentative pending shore-based analyses with XRD, electron microprobe, and scanning electron microscope. Calcite, pyrite, chalcedony, and quartz are less abundant and have more restricted distributions. No zeolite minerals were identified in basaltic samples.
Based on core descriptions and thin section observations, we have identified three types of low-temperature alteration.
All of the basalts recovered from Hole 1183A show signs of pervasive alteration. Color ranges from dark to light gray in pieces affected by pervasive alteration (Fig. F68). Most pieces also exhibit colored halos resulting from one or both of the other two alteration processes described below. We identified these colors on wet, cut surfaces of the core. Gray basalts are the most abundant, and the abundance of this color does not vary downhole. Secondary minerals in gray basalts are clay minerals (predominantly saponite with subordinate nontronite and celadonite) replacing interstitial glass and mesostasis in the groundmass. Clay minerals (celadonite, saponite, or minor nontronite) and (less commonly) Fe oxyhydroxides and calcite or (more rarely) pyrite replace euhedral olivine microphenocrysts (Figs. F69, F70, F71, F72, F73, F74, F75, F76) and, rarely, plagioclase phenocrysts. Overall alteration in pervasively altered basalts ranges from <5 to 20 vol%. Pervasive alteration results from extended interaction between basalt and seawater-derived fluid (evolved seawater) under anoxic to suboxic conditions at low temperature (probably 10°-50°C). Pervasive alteration ceases once secondary minerals fill fluid pathways (i.e., open cracks).
We observed black halos along surfaces previously exposed to seawater and, less commonly, along the margins of veins (Figs. F72, F73). Olivine phenocrysts are most commonly altered to clay minerals (including celadonite [Fig. F74] and/or nontronite [Fig. F76]) and Fe oxyhydroxides. In a few instances, pyrite is associated with celadonite in olivine pseudomorphs (Fig. F75). Fine-grained pyrite is also commonly disseminated in the groundmass of the adjacent gray rock beyond the edges of black halos. Calcite is less common in these halos than in pervasively altered basalt. The contact between black halos and the dark gray interior is very sharp both in hand specimen and in thin section (Fig. F69). This is a result of strong changes in chemical conditions across the alteration front during the formation of the black halos.
This stage of alteration results from low temperature (<60°C) interaction between basalt and seawater mixed with hydrothermal solutions. Black halos are characteristic of an early alteration process initiated during cooling of the lava within 1-2 m.y. of basalt emplacement (Böhlke et al., 1980; Honnorez, 1981; Laverne, 1987). Black halos decrease in abundance downhole but less so than the olive halos described below (Fig. F68).
Olive halos are olive-brown discolorations, in zones <1 to 5 mm thick, of the host rock adjacent to Fe oxyhydroxide-bearing veins. They are caused by Fe oxyhydroxides staining and partly replacing the primary minerals of the basalt groundmass and filling the interstitial voids in the groundmass (Fig. F77). Olive halos result from a basalt-seawater reaction corresponding to halmyrolysis or submarine weathering, which takes place at bottom seawater temperature (i.e., ~2°C), in oxidizing conditions, and generally with large water/rock ratios. The halos represent the last alteration stage, which ceases when the oceanic crust is sealed off from the overlying seawater by a sufficiently thick and comparatively impermeable sediment cover. Olive halos decrease in abundance downhole (Fig. F68) and almost disappear below 1180 mbsf, except for three occurrences in Sections 192-1183A-65R-2, 65R-3, and 67R-2.
Alteration of glass to phyllosilicates ranges from 20 to 100 vol%. Basaltic glass is present either in pillow rims or as shards in hyaloclastites. Glassy mesostasis is rarely present in the pillow-basalt groundmass. Pillow-rim glass is generally the least altered because of its low permeability. Glass shards in the hyaloclastites, because of their large reactional surface areas, are almost always completely replaced by phyllosilicates. One exception is a hyaloclastite found in Section 192-1183A-60R-1 (Piece 1) that contains pillow-rim material composed mostly of unaltered glass cemented by calcite. The association of unaltered glass clasts cemented by calcite is commonly observed in the oceanic crust, regardless of the age of the hyaloclastite or the environment in which it formed (Honnorez, 1967, 1972).
We counted 849 veins in the 80.7 m of basement penetrated in Hole 1183A. This number represents an average of 19 veins/m for the 44.8 m of basalt recovered at the site. Most of the veins result from symmetrical infilling of open cracks with minor or no replacement of the wall rock. Evidence for the successive reopening and filling of veins is often clear, particularly in the case of calcite deposition, because of the contrast in color between the carbonate and the other vein-filling secondary minerals (Fig. F73). The vast majority of the veins contain the following succession of secondary minerals, from vein walls to centers: smectite and/or celadonite, Fe oxyhydroxide (Fig. F78) or pyrite, and calcite. Disseminated pyrite grains commonly line the walls of smectite and/or celadonite veins cracked open during drilling. When observed within veins in thin section, pyrite completely fills the width of the vein. Pyrite is also associated with black halos as grains scattered in the gray basalt groundmass beyond the alteration front.
The downhole distribution of calcite and smectite (i.e., either or both nontronite and saponite) in veins (Fig. F79) does not show any systematic variation with depth. The presence of celadonite is greatly underestimated because both smectite and celadonite appear black on wet, cut core surfaces; hence the plot of downhole distribution of celadonite is misleading. Pyrite first appears at 1146 mbsf and shows a slight increase in abundance with depth. Conversely, Fe oxyhydroxides seem to be distributed unevenly, with only one occurrence above 1145 mbsf and an apparent absence between 1148 and 1156 mbsf and between 1169 and 1181 mbsf.
Rare occurrences of native copper are restricted to smectite-bearing veins in the uppermost basement cores, whereas we only observed chalcedony and quartz in the lower part of the cored basement: Section 192-1183A-66R-1 (Pieces 6 and 7), Section 66R-2 (Piece 4) (Figs. F80, F81), and Section 67R-1 (Piece 2C) (Figs. F82, F83).
Vein width does not show any downhole trend (see Fig. F84) except for the 2- to 5-mm-wide veins that seem to be more abundant above 1167 mbsf. Hairline, <1-mm- or 1-mm-wide veins are uniformly present through the entire basement section. The observed number of >5-mm-wide veins is not statistically significant. Miarolitic voids are common throughout the cores, whereas vesicles are present only rarely. The maximum dimension of the miarolitic cavities ranges from a few tenths of a millimeter to 4 mm (Fig. F85). The cavities are generally completely filled by the same secondary minerals as those observed in the veins (Fig. F86).