Calcite veins at both Site 897 and 899 are found dominantly in the upper sections of the basement and are observed to overprint serpentine veins at Site 897 (Shipboard Scientific Party, 1994c). At Site 899, serpentine veins occur exclusively within clasts, denoting a vein-filling phase that predated fragmentation and transport of the breccia units to their final resting place. Calcite veins, although found throughout the crystalline lithologies, are concentrated in Cores 899-18R-21R, coincidentally correlating with the greatest abundance of large clasts in the core (Shipboard Scientific Party, 1994c).
There are several recurring types of calcite veins, distinguished by their macroscopic appearance. The large veins are most distinctive and take three main forms. Zoned veins are most common at Site 899, but also occur at Site 897. They display an approximately planar geometry, and are marked by subparallel bands of inclusions and color variations grading into clear, coarsely crystalline, sparry calcite near the center of the vein. These features may reach 5 cm across (Fig. 1A). Sheeted veins are so called because they constitute several sub-parallel veins with various textures, separated by thin bands of wall rock or noncarbonate phases (Fig. 1B). These features tend to develop at the margins of large clasts, possibly because of contrasts in strength between clast and breccia. They are only observed at Site 899. Complex veins include most of the larger vein types recovered from Site 897, are poorly correlated among samples, and encompass multiple vein textures in a chaotic package. One of the most distinctive textures observed in abundance in these vein types, particularly at Site 897, is a relict aragonite texture replaced by sparry calcite (Fig. 2). More pervasive at both sites are fine, clear veins. They are most common in the upper reaches of basement, where they permeate the disaggregated breccia. They often occur in subplanar, anastomosing sets, or as circumgranular features. At Site 897, similar fine calcite veins define the familiar serpentinite mesh texture near the seafloor, but revert to serpentine filling with depth (Shipboard Scientific Party, 1994b). Micrite-filled veins, common at Site 897, but rare at Site 899, display a clastic texture, dominated by microcrystalline calcite but also containing material apparently derived from local fragmentation of the serpentinite and its alteration phases. The detritus-filled fractures postdate most other vein generations with which they occur (Fig. 3). These features bear some resemblance to neptunian fractures, recognized elsewhere in extensional or collapse settings (e.g., Smart et al., 1987), an analogy that may bear on their genesis in this setting.
The high degree of fragmentation and relative rotation of unoriented cores, particularly at Site 897, confound efforts to establish an orientation distribution of the vein occurrences at both sites, useful for kinematic inferences. Casual observations, however, indicate that the vein orientations are quite variable, even within a single core, and appear to be largely controlled by preexisting rock fabrics (e.g., foliation, serpentine veins, margins of clasts). The implication is that fracturing in these heterogeneous rocks was governed largely by local stress concentrations in response to a general dilational tendency.