Alteration of the basaltic basement at Site 1186 is similar to that at Site 1185 and, particularly, Site 1183. The entire section of basalt cored at Site 1186 has undergone low-temperature water-rock interactions, resulting in complete replacement of olivine and almost complete replacement of glassy mesostasis. Clinopyroxene and plagioclase generally remain unaltered. The overall alteration of the basalt, not taking into account the veins, ranges from 5 to almost 100 vol%, estimated visually by color distribution in hand specimen, and confirmed by thin section study.
In hand specimen and thin section, we observed the effects of the following three main types of low temperature alteration.
In the basalts from Hole 1186A (e.g., Cores 192-1186A-30R and 31R) dusky green halos are commonly present instead of the black halos observed at Site 1183 and in basement rocks from younger seafloor. The dusky green halos are very similar to black halos but represent an extreme case of replacement of primary basaltic phases by celadonite associated with nontronite (and probably mixed-layer phyllosilicates formed by both clay minerals) and Fe oxyhydroxide (Figs. F44, F45, F46). In a few instances, pyrite is associated with celadonite in olivine pseudomorphs. More commonly, fine-grained pyrite or rare marcasite is disseminated in the groundmass of the adjacent gray basalt beyond the edges of the dusky green halos. Calcite is less common in these halos than in the brown halos (see below) and the pervasively altered basalts. The overall alteration rarely exceeds 30% in either black or dusky green halos. Unlike Site 1183, we did not observe a decrease of these halos with depth.
Black and dusky green halos result from early interaction between basalt and warm (<60°C) seawater-derived fluids (Honnorez, 1981; Böhlke et al., 1980; Laverne, 1987). Such fluids are supplied by "diffusers," warm springs of hydrothermal solutions that have been conductively cooled by mixing with bottom seawater. On the seafloor, they occur as diffuse warm springs of shimmering water, from which they derive their name (James and Elderfield, 1996). The formation of black and dusky green halos commences during cooling of the lava once it is emplaced on the seafloor. Further effusions of lava and/or injection of magma during diking events reactivate this alteration process.
These are the most abundant halos observed in core from Hole 1186A. They are present throughout the basaltic sequence and do not appear to decrease in abundance with depth. They are generally parallel to or concentric with smectite ± celadonite ± calcite veins (Fig. F47) or with 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 gray or dark gray in the inner parts of the cooling unit. The various brown colors result from the total replacement of olivine phenocrysts by variable proportions of tan to brown smectite, Fe oxyhydroxide, and minor calcite. Miarolitic cavities and rare vesicles in the brown halos are filled with the same 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 halos are formed by halmyrolysis (i.e., alteration of basalt by bottom seawater circulating through the crust to depths of several hundred meters). Halmyrolysis takes place at seafloor temperature (i.e., <2°C), with large water-rock ratios, and generally under oxidizing conditions. In the most permeable basaltic formations, such as pillow lavas, hyaloclastites, and breccias, halmyrolysis leads to intense alteration. Halmyrolysis ceases when the oceanic crust is sealed off from the overlying water column by a less permeable and sufficiently thick sediment cover.
This term refers to the normal gray color of the least-altered basalts from the inner portions of cooling units, commonly adjacent to the variously colored halos. The least-altered basalts all exhibit the effects of pervasive low-temperature alteration. Brown smectite replaces olivine phenocrysts and the glassy groundmass and fills 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 pervasive water-rock interaction under anoxic to suboxic conditions between basalt and evolved fluids derived from seawater that reacted previously with basaltic crust (e.g., with the rock of the halo surrounded by the gray basalt [Alt et al., 1986]). The water-rock ratio during such interaction is probably low.
The contact between black halos and gray interiors is always sharp, both in hand specimen and thin section. This is a result of strong chemical gradients across the alteration front during formation of the black halos. In contrast, the transition between brown halos and both black (or dusky green) halos and gray interiors is generally gradational.
Basaltic glass is present either in pillow rims or as shards in hyaloclastites, probably associated with interpillow cavities. Glassy mesostasis is rare in the pillow interiors. Glass alteration into phyllosilicates ranges from 0% to 100%, depending on its relationship with the nearest crystalline basalt. For example, 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. Unaltered glass shards cemented by calcite are 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 750 veins in the 65.4 m of basement penetrated in Hole 1186A. This number represents an average of 19 veins/m for the 39.6 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. Petrographic 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 secondary minerals. Most veins contain the following succession of secondary minerals from vein walls to centers: smectite and/or celadonite, Fe oxyhydroxide or pyrite, and calcite. Rare chalcedony associated with calcite spherules was observed in one thin section (Fig. F48). Disseminated pyrite and/or marcasite grains are commonly found scattered in the walls of smectite veins cracked open during drilling. 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. Veins filled with pinkish micritic carbonate, interpreted as sediment at Site 1185, are also observed in Hole 1186A. The walls of these veins are commonly lined with crystalline calcite, suggesting successive filling and reopening.