MICROSCOPIC DESCRIPTION OF VEINS

Petrographic examination of the vein samples revealed a varied array of calcite habits and multiple crosscutting, truncation, and replacement textures, demonstrating a complex history of vein fracture, precipitation, and sealing. Several textures recur in many veins of various types and can be correlated to reveal a progression of features useful for extracting basement history. The common occurrences are presented in approximate order of their appearance in the rocks.

Calcite Textures

Aragonite and Calcite Pseudomorphs after Aragonite

Equant, unoriented, clear sparry calcite, occurring in elongate to prismatic sections within a more varied matrix, points to the presence of relict aragonite in several complex veins from Site 897 (e.g., Fig. 2, Fig. 4). In places, these features occur as radiating stellate clusters centered on the vein wall, with preserved crystal outlines consistent with coarse, bladed aragonite (B. Kirkland-George, pers. comm., 1994). Embayed faces serve as nucleation sites for later, infilling botryoidal calcite, which also fills fractures in the now-replaced bladed crystals, demonstrating that aragonite represents the earlier phase. The presence of primary aragonite in some samples (e.g., Sample 149-897D-16R-4, 0-5 cm) supports the interpretation that these textures preserve an early phase of aragonite precipitation in open fractures. This relict aragonite texture was generally absent in veins at Site 899 with one exception: Sample 149-899B-17R-2, 91-94 cm, revealed elongate, radiating blades reminiscent of aragonite in a complex vein which was subsequently crosscut by several zoned and fibrous veins.

Botryoidal Calcite

Radiating, fibrous clusters of calcite, nucleated at the vein wall or along an earlier carbonate phase, define a common botryoidal habit observed in many vein types at both Sites 897 and 899. Much of the botryoidal calcite is highly vacuolized. The gas-filled vacuoles are on the order of 1 m across and may occur in relatively uniform concentrations or form concentric rings about the core. The most highly vacuolized samples are prominently white in reflected light. Less vacuolized examples occur, and crosscutting relationships suggest that the less vacuolized botryoidal calcite may be older (e.g., Sample 149-899B-25R-3, 21-24 cm). Typically, this phase is highly luminescent, with the intensity of luminescence correlating positively with the density of inclusions. Calcite fibers appear to have grown in fan-like arrays into open fractures, in many cases nearly sealing the vein. Postprecipitation fractures confirm the early paragenesis of the vacuolized calcite. Later fractures often localize between individual clusters, and are subsequently filled by clear sparry calcite (Fig. 5). Much of the subsequent fracturing is also localized along the contact of the botryoidal calcite with the fracture wall, truncating and displacing the radial clusters.

Zoned Sparry Calcite

The most common calcite texture observed in many veins takes the form of elongate blades, oriented perpendicular to the vein walls and growing into open fractures. The crystal facets of the dogtooth spar are still preserved at the edges of open porosity, and elsewhere result in a jagged suture where crystals growing from opposite sides of the wall meet. Blades typically coarsen inward, as more favorably oriented crystals occlude others. The calcite is highly zoned, grading from alternating bands of inclusions with variable luminescence close to the wall (Fig. 6A-C), to uniform, clear calcite displaying fine rhythmic luminescent zoning of decreasing intensity near the center (Fig. 6D-F). This calcite type dominates veins at both sites, although it is best developed in the zoned veins at Site 899. Precipitation of this sparry calcite is observed to postdate the botryoidal phases, occurring along the truncated boundaries (e.g., Sample 149-899B-18R-2, 135-139 cm), or, in places, apparently syntaxial to the botryoids (e.g., Sample 149-897D-16R-3, 17-19 cm). Fluid-filled inclusions ranging from 10 to 50 m in size are common in the clear, sparry calcites. No two-phase (fluid-gas) inclusions were observed, which is consistent with the interpreted low temperatures of precipitation.

Fibrous Calcite (Common)

This ubiquitous texture is found throughout the calcite-bearing basement cores at the two sites and occurs both in abundant, narrow veins (Fig. 7), and as a late-stage vein-filling near the margins of the larger sheeted and zoned veins. Fibrous veins are particularly pervasive near the tops of the basement units, where the primary basement mineralogy has undergone significant weathering and alteration, and they commonly show a circumgranular distribution. The texture is marked by fine, clear calcite fibers, although the center of the vein may contain abundant vacuoles, possibly correlated with earlier, highly vacuolized calcite exposed in the larger veins. The fibers are typically oriented perpendicular to the vein wall, show little change in orientation across the vein, and indicate displacive, antitaxial growth from center to wall. Luminescence varies both within and among the veins, demonstrating complicated fracture and growth histories. Timing relationships are very difficult to establish, as veins with this texture appear to have formed throughout much of the evolution of these rocks.

Massive Fibrous Calcite

Few vein samples, notably Samples 149-899B-18R-2, 3-7 cm, and 149-899B-18R-5, 58-62 cm, are marked by a related calcite texture, distinguished by massive (up to 1 cm) generations of relatively homogeneous, clear fibrous calcite. In each case, this texture is observed as a marginal phase, and suggests displacive, antitaxial growth; occurrences are bounded internally by calcites of more blocky or sparry appearance. Luminescence varies little across the domains, so it is difficult to establish their histories. In Sample 149-899B-18R-2, 3-7 cm, this extensive zone of fibrous calcite grades upward into narrow fibrous veins, which displace thin peels of a highly altered serpentine clast; this hints that the massive fibrous phase is simply a well-developed example of the common veins. However, in this sample it occurs with fibrous silica, a unique occurrence in this suite of veins.

The massive fibrous calcite in Sample 149-899B-18R-5, 58-62 cm, is more enigmatic. The calcite is intergrown with a coarse, fibrous or platy brown phyllosilicate (probably chlorite), with a fabric that suggests that the calcite is replacing an earlier vein-filling phase (Fig. 8). Fiber orientations of the calcite mimic those of the phyllosilicate; kinks in the phyllosilicate are matched by kinks in the calcite. Calcite also occurs as fine beads that grow at the edges of the phyllosilicate laths and may cut across cleavage (Fig. 8). The fibrous calcite is not sharply bounded by other phases, but rather grades toward the center of the vein into clear sparry calcite. Elsewhere, similar intimate relationships between calcite and phyllosilicate are observed, but the fibrous habit is not always evident (e.g., Sample 149-899B-20R-1, 121 cm).

Micrite-filled Veins

Numerous, narrow- to medium-width, clastic-textured veins, ranging in size from 50 m to 5 mm, crosscut earlier vein-filling phases (including other micrite-filled veins), particularly complex veins noted at Site 897 (e.g., Fig. 3). These typically contain micro-crystalline calcite, mixed with fragments of serpentinite, oxides, and pieces of preexisting vein-filling calcite (e.g., Sample 149-899B-35R-1, 32-36 cm). The relative importance of the different constituents can vary from vein to vein, from nearly pure micrite (Sample 149-897C-64R-4, 16-19 cm) to well-oxidized veins with high concentrations of wall rock (Sample 149-897D-13R-5, 57-61 cm). Luminescence of these veins is typically relatively high and uniform for the calcite component.

The origin of these veins remains equivocal. The clastic textures are reminiscent of neptunian veins, which are interpreted to form close to the seafloor in dilational or collapsed karst environments and consequently are open to marine sediment influx (Smart et al., 1987; Winterer et al., 1991). In contrast to those occurrences, however, geopetal fabrics and microfossils are not evident in our clastic veins. The granular and polymict nature of micrite-filled veins, however, does argue for transport and redeposition of clastic material in open fractures, possibly under dynamic fluid flow conditions (e.g., Hs, 1983), and the close association of these veins with zones of intense in situ brecciation suggests a likely local source for the vein-filling material. Our sampling is not complete enough to establish the exact relationship between the two clastic modes.

Other Calcite Textures

Other calcite textures observed in the basement cores may also record important events in the fracture and precipitation history, but their relative timing and evolution are much less apparent. Examples include first, clear, medium-course grained, equant calcite with uniform luminescence; second, finely bladed, sparry calcite with uniform luminescence; and third, small, radial aggregates of calcite in complex veins at Site 897. The luminescent properties of these phases tend to be very uniform, possibly because of postprecipitation replacement.

Other Vein Phases

Various noncarbonate phases occur within the calcite veins, perhaps preserving part of the history of vein formation. Several appear to predate the calcite vein filling in the breccias at Site 897 (e.g., phyllosilicates, possible chlorite and smectite, and fibrous quartz; Fig. 8, Fig. 9), whereas others apparently postdate at least some of the calcite phases (e.g., brucite; Sample 149-897D-17R-4, 110-114 cm). Other phases, such as the ubiquitous oxides present among the calcite phases, clearly are deposited synchronously and are very useful for unraveling textural relationships and timing. Further discussion of the relationships between these phases and the calcite occurrences is deferred to discussion of chemical analyses (Milliken and Morgan, this volume).

Crack-seal Textures

This textural examination has revealed striking evidence for repeated fracture, vein opening, and resealing by subsequent precipitation, yielding distinctive vein complexes and crosscutting relationships. In many tectonically active settings, such crack-seal textures preserve a clear record of incremental strain, which can be read in the context of a relatively consistent stress regime (e.g., Durney and Ramsay, 1973; Fisher and Brantley, 1992). A remarkable feature of the textures observed here is the apparent inconsistency, and consequent complexity, of these kinematic events. Certain fracture events appear to have followed a pattern, such as the repeated fracturing of existing vein complexes, which must define a plane of weakness (Fig. 10A-C). Others display fractures that crosscut each other in a poorly defined manner, suggesting alternating stress conditions (Fig. 10C-E). The complexity of these features is often not revealed under plane light, but may be clearly displayed by cathodoluminescence imaging (Fig. 10).

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