Although this paper is intended to allow broad dissemination of primary data pertaining to the alteration of basaltic crust drilled during Leg 192, certain preliminary observations have been documented. For a description of alteration on a site-by-site basis, the reader is referred to the Leg 192 Initial Reports volume (Mahoney, Fitton, Wallace, et al., 2001).
The entire sequence of basaltic basement rocks recovered during Leg 192 has undergone low-temperature water-rock interactions, resulting in complete replacement of olivine and almost complete replacement of glassy mesostasis. Clay minerals are the most abundant secondary minerals. We have tentatively identified saponite and celadonite based on color in hand specimen, optical properties in thin section, and intermittent XRD analyses. 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). Calcite, pyrite, chalcedony, quartz, and zeolites are less abundant and have more restricted distributions. 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 the volume percentage in the cores. Based on core descriptions and thin section observations, we have identified three major types of low-temperature alteration.
The term gray basalt refers to the normal gray color (ranging from dark to light gray) of the least-altered basalts from the inner portions of cooling units, commonly adjacent to the variously colored halos described below. Gray basalt is the most abundant alteration type. This type is characterized by signs of pervasive low-temperature alteration. Clay minerals (predominantly saponite with subordinate celadonite) commonly replace rare interstitial glass and mesostasis in the groundmass. Phyllosilicates (celadonite or saponite) also commonly replace olivine microphenocrysts. Less commonly, Fe oxyhydroxides and calcite or (more rarely) pyrite are found in olivine pseudomorphs and, rarely, plagioclase phenocrysts. Similar secondary mineral assemblages fill miarolitic cavities and rare vesicles. The overall alteration of the gray basalts ranges from 5% to 30% and averages ~20%.
The gray color results from extended interaction between basalt and seawater-derived fluid (evolved seawater) under anoxic to suboxic conditions at low temperature (probably 10°-50°C). These fluids probably reacted previously with basaltic crust (e.g., with the rock of the halo adjacent to the gray basalt [Alt et al., 1986]). The water-rock ratio during such interaction is probably low. This alteration stage ceases once secondary minerals fill fluid pathways, sealing the formation and permeability.
Centimeter-scale dusky green halos and black halos are observed along surfaces previously exposed to seawater and, less commonly, along the margins of veins. Within these halos, olivine phenocrysts are commonly altered to clay minerals (including celadonite and/or nontronite) and Fe oxyhydroxides. The dusky green halos are very similar to black halos but represent an extreme case of replacement of primary basaltic phases by higher proportions of celadonite, which impart the dusky green color. The overall alteration rarely exceeds 30% in either dusky green or black halos. The boundary beyond the edges of these halos and the adjacent gray rock is commonly marked by fine-grained disseminated pyrite and/or marcasite in the groundmass. Calcite is less common in these halos than in the gray basalt. The contact between black halos and the gray interior is very sharp both in hand specimen and in thin section. This is a result of strong changes in chemical conditions across the alteration front during the formation of the dusky green halos and black halos.
This stage of alteration results from early interaction between basalt and warm (<60°C), seawater-derived fluids (Böhlke et al., 1980; Honnorez, 1981; Laverne, 1987). Such fluids are supplied by diffuse warm springs of shimmering water that have been cooled by mixing with bottom seawater (James and Elderfield, 1996). The formation of black and dusky green halos are characteristic of an early alteration process initiated during cooling of the lava in 1-2 m.y. of basalt emplacement (Böhlke et al., 1980; Honnorez, 1981; Laverne, 1987). Further effusions of lava and/or injection of magma during diking events reactivate this alteration process.
Brown halos and olive halos are olive-brown discolorations, in zones <1 to 5 mm thick, of the host rock. They are generally parallel to or concentric with smectite ± celadonite ± calcite veins, Fe oxyhydroxide-bearing veins, or glassy pillow margins, and they surround the least-altered basalt. Their color generally ranges, from the (originally) exposed surface inward, from light yellow brown to dark yellow brown (and, more rarely, dark brown) at the exterior, to olive, and finally grading to gray or dark gray in the inner parts of the cooling unit. In contrast to the sharp contact observed between the dusky green or black halos and the gray basalt, the transition between brown halos and both black (or dusky green) halos and gray interiors is generally gradational. The various brown and olive colors result from variable proportions of Fe oxyhydroxide, tan to brown smectite, and minor calcite staining and partially to totally replacing the primary minerals of the basalt groundmass and filling the interstitial voids in the groundmass. Within these halos, olivine phenocrysts are totally replaced by Fe oxyhydroxide, smectite, and minor calcite. Miarolitic cavities and rare vesicles in the brown and olive halos are filled with similar secondary minerals. Smectite and/or Fe oxyhydroxide replace as much as 90% of the basalt groundmass in the lighter-colored halos. The overall alteration in the brown halos ranges from 30% to almost 100%.
Brown and olive halos result from basalt-seawater reaction referred to as halmyrolysis (or submarine weathering). This form of alteration takes place at bottom seawater temperatures (~2°C), under oxidizing conditions, and generally with large water/rock ratios (Honnorez, 1981, and references therein). This corresponds to passive alteration of basalt by bottom seawater circulating through the crust to depths of several hundred meters. In the most permeable basaltic formations, such as pillow lavas, hyaloclastites, and breccias, halmyrolysis may lead to intense alteration. The halos represent the last low-temperature alteration stage, which ceases when the oceanic crust is sealed off from overlying seawater by a sufficiently thick and comparatively impermeable sediment cover.
Alteration of basaltic glass to phyllosilicates ranges from 20% to 100%. Basaltic glass is present either in pillow rims or as shards in hyaloclastites, possibly associated with interpillow cavities. Glassy mesostasis is rare in the pillow interiors. Pillow-rim glass is generally the least altered because of its low permeability. Glass shards in the hyaloclastites, because of their large surface areas, are almost always completely replaced by phyllosilicates, except where cemented by micritic calcite. The association of unaltered glass clasts cemented by calcite is commonly observed in the submarine basalts, regardless of the age of the hyaloclastite or the environment in which it formed (Honnorez, 1967, 1972).
Most of the veins result from symmetrical infilling of open cracks with minor or no replacement of the wallrock. Vein widths vary from <1 mm (hairline veins) to tens of millimeters. Most veins contain the following succession of secondary minerals, from vein walls to centers: smectite and/or celadonite, Fe oxyhydroxide or pyrite, and calcite. Disseminated pyrite grains commonly line the walls of smectite and/or celadonite veins cracked open during drilling. Furthermore, where the core is fractured perpendicular to veins containing dusky green halos, bright bands of pyrite (and/or marcasite) are commonly observed at the margins of the halos, indicating the terminus of the reduction front. 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. Some veins are filled with micritic pink carbonate that contains Fe oxyhydroxide pellets and foraminifer ghosts, indicating that the veins are sediment-filled open fissures. 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.
We noticed a clear relationship between vein density and host-rock alteration color. Pervasively altered basalts that display yellow-brown or olive alteration colors are generally associated with portions of cores displaying more horizontal and subhorizontal veins. Not surprisingly, the degree of alteration is highest in the rocks with the highest permeability (e.g., fractured pillow lavas, hyaloclastites, and breccias), in which the rock color is also the lightest.
Alteration and vein logs compiled for each hole from visual core descriptions made aboard the ship during Leg 192 provide a method for estimation of downhole variations in basalt alteration and vein abundance/mineralogy. Three main alteration types have been identified: gray basalt, black or dusky green halos, and brown or olive halos. These alteration types all result from low-temperature fluid-rock interactions. Phyllosilicates are the most abundant secondary minerals, with lesser amounts of calcite, pyrite, chalcedony, quartz, Fe oxyhydroxides, and zeolites. Veins resulting from symmetrical infilling of open cracks commonly contain smectite and/or celadonite, Fe oxyhydroxide or pyrite, and calcite. Overall, alteration of basalts cored during Leg 192 is similar to that observed from other Deep Sea Drilling Project/ODP sites drilled into the upper oceanic crust.