Basement was cored in Holes 1149B, 1149C, and 1149D, with basement penetrations of 38.4, 28.8, and 135.1 m, respectively. At all sites the basement is made up of pillow lavas, pillow breccias, hyaloclastites, and thin flows. Alteration of basement from the three holes is generally similar, with Hole 1149D providing some documentation of variations with depth.
All basalt types are pervasively and strongly altered by oxidizing water/rock interactions. Complex multicolored alteration halos occur along fractures and other surfaces that were exposed to circulating fluids. These halos concentrically encircle kernels of host rock bounded by fractures (Fig. F47A). The halos are as wide as 2 cm and range in color from brown at the exposed surfaces to dark green for the main portion of the halos to a narrow (1-2 mm) brown band at the boundary with the host rock. The contact between the outer brown zone and the dark green halo is gradational, but that between the dark green and inner brown band is sharp, as is the contact with the host rock. Halos comprise ~34 vol% of the recovered basalt, and dark green and brown halos comprise 14 and 20 vol%, respectively. Unaltered glass is exceptional and is found only in small amounts in a few pieces (Table T6).
The host rock surrounded by fractures and halos generally exhibits dusky red to dark red colors and displays a light gray to brown mottling. The mottling is variable in intensity, ranging from complete to slight patchy replacement. In the cases of slight replacement the patches form a subdendritic pattern. In the single available thin section of this material, the mottles correspond to zones lacking primary Ti magnetite and may have formed by leaching of Fe oxide. These mottled regions appear to be present only where outer brown halos are associated with the bounding vein.
Some significant variations occur with depth in Hole 1149D. In three intervals (Sections 185-1149D-11R-1 through 11R-2, 13R-1, and 16R-1 through 16R-2), the basalt is pervasively altered and displays a cloudy, dark brown to orange-tan color (Fig. F47B). Because of the fine-grained texture and fracturing of these rocks, alteration is pervasive and no distinct halos are visible. These same cores contain hyaloclastite and pillow-rim breccias in which the basaltic glass has been totally replaced and cemented by dark saponite, green phyllosilicates (celadonitic?), and minor calcite. Section 185-1149D-11R-3 is completely made up of basaltic breccia with calcite cement. The basalt clasts exhibit the same cloudy alteration as the basalt pieces in surrounding cores. This interval in Hole 1149D likely had a high permeability that facilitated more intensive alteration at high water-rock ratios.
The distribution and abundance of veins, halos, and breccias logged in Holes 1149B, 1149C, and 1149D are shown in Figure F48 (data in Table T9; also see "Site 1149 Vein Log," also in ASCII format). All basement cores are cut by veins in varying proportions (1.5-7 vol%). The three holes exhibit the same features and are characterized by a relatively high density of veins with an average of 35 veins/m. An extreme value of 50 veins/m is recorded in Core 185-1149C-7R. Calcite and clay minerals (saponite and/or celadonite) are the dominant vein and hyaloclastite components, with lesser Fe oxyhydroxides (Fig. F49). Veins can be filled by a single phase but are most commonly filled by several different phases (Tables T9, T10). Where clay minerals and calcite are present in the same vein, calcite is generally the last phase to precipitate. Carbonate-rich veins are thicker (1.5 mm) and have wide associated alteration halos (~5.6-6.3 mm wide), whereas saponite-rich veins are thinner (0.2-0.4 mm) and are associated with narrower halos (~3.7-4.6 mm wide). The brown, orange, and reddish veins are composed of Fe oxides and hydroxides mixed with clays. Pure Fe oxyhydroxide veins are very scarce, but ~40% of the veins logged are estimated to contain between 10% and 20% of Fe oxyhydroxides, associated mostly with saponite.
Only one thin section of an alteration halo was made on ship; hence, such observations are very preliminary in character. We suggest that the alteration in cores from Site 1149 results from a succession of at least three low-temperature alteration stages that all proceed from open fissures and exposed surfaces:
Besides the abundant alteration halos, the abundance of red Fe oxyhydroxide in veins and fractures is one of the more striking features of cores recovered in Holes 1149B and 1149C (as much as 3.9 vol% of Fe oxyhydroxides). Oxidation of early secondary Fe sulfides in veins or alteration of igneous Fe oxides could contribute to this enrichment. The late calcite in saponite veins suggests that many veins were reopened after circulating fluid had changed composition. Different generations of halos are also delineated by their different colors, though a gradation of color may be present within a single alteration halo. A common chronology of veins and halos includes (1) early fracturing of basalt with filling of veins by clay minerals (mostly saponite, as well as celadonite?) and development of celadonite-bearing dark halos; (2) progressive alteration and oxidation of host basalt with the progression of a brown oxidation halo away from the fracture and into the rock; and (3) reopening of veins allowing the circulation of fluids and precipitation of carbonate and reworking of previous halos.