The entire basaltic crustal section drilled at Hole 1187A exhibits the effects of pervasive halmyrolysis or submarine weathering, the low-temperature process of oceanic basalt alteration by bottom seawater. Alteration occurred under oxidizing conditions and with high water-rock ratios, resulting in the development of light to dark yellow-brown colors in the most exposed rocks (e.g., near the outer zones of cooling units) (Fig. F24). In contrast to the other Leg 192 sites, no black or dusky green halos were observed.
The basalt in Hole 1187 appears to have undergone the greatest overall alteration of basement of any Leg 192 site. This observation is consistent with the dominantly pillowed nature of the basalts recovered in this hole; massive basalt forms only a minor component and is present only in the lower parts of Units 6 and 7 (see "Igneous Petrology"). The light to dark yellow-brown colors in the outer portions of the pillow lavas result from the complete replacement of olivine (Fig. F25) and partial to complete alteration of groundmass by brown to tan smectite (probably an Fe-rich saponite) (Fig. F26) and Fe oxyhydroxide. Smectite, calcite, and minor goethite fill miarolitic cavities (Fig. F27, F28), which are surrounded by 2- to 10-mm-wide brown halos. The primary igneous minerals in these halos are totally replaced by brown smectite and minor calcite and goethite (Fig. F29).
Unaltered glass is more abundant at this site than at any other Leg 192 site, despite the pervasive alteration of pillow margins (Fig. F14). Away from pillow margins, the color grades into dark gray within several coarser-grained pillow interiors. Although olivine phenocrysts are generally completely replaced, unaltered or only incipiently altered olivine is present in several dark gray basalt intervals interpreted as either the interiors of large pillows or massive flows (Fig. F30). In this respect, Site 1187 is unique among the Leg 192 sites.
Veins throughout Hole 1187A are mostly filled with calcite, zeolites (identified in hand specimen; probably phillipsite with analcime), smectite, Fe oxyhydroxide, and rare celadonite, pyrite, and marcasite. No zeolites were observed in thin section. Abundant subhorizontal calcite veins are present in light to dark yellow-brown pillow margins, and calcite veins radiate from pillow interiors toward the margins in well-preserved pillows.
As noted above, dusky green and black halos that surround veins at the other basement sites are absent in basalt from Site 1187. Reduction fronts containing pyrite and/or marcasite are similarly absent. Two possible explanations are that (1) neither feature ever formed or (2) both may have formed but were overprinted by the later pervasive oxidative alteration that produced the various brown-colored halos. We prefer the second explanation because black and dusky green halos are ubiquitous and form quickly, within 1-2 m.y. of basalt emplacement (Honnorez, 1981; Böhlke et al., 1980; Laverne, 1987).
As with the upper alteration zone in basement at Site 1185, we again noticed a clear relationship between vein abundance and the color of host-rock alteration. The lighter yellow-brown colors in the basalts at both sites are generally associated with the parts of cores that display the most abundant horizontal and subhorizontal veins (Fig. F31). Because the degree of alteration is a function of permeability, the abundance of veins, pervasiveness of the alteration, and development of light and dark yellow-brown alteration colors indicate that basement in Hole 1187 was initially the most permeable of any drilled during Leg 192.