This petrographic study of calcite veins at Sites 897 and 899 has revealed two distinctive features that direct our discussion of the tectonic and environmental conditions of their formation: first, vein opening and precipitation proceeded through multiple dilational steps, with little systematic organization in terms of fracture orientation or mineral growth; second, mineralization in open veins occurred through a progression of carbonate precipitation events, from prismatic aragonite, followed by botyroidal, fibrous calcite, and, finally, by bladed sparry calcite, with the last precipitation event involving the opening and filling of clastic veins.
The multiple deformational events led to the opening and maintenance of large fractures, which argue for overall dilation of the rock mass. There is no evidence for subsequent collapse of these fractures (e.g., penetration, pressure solution fabrics; Fisher and Brantley, 1992), which might occur if the fractures were supported by high transient fluid pressures. The picture presented of discrete fracturing events, followed by long periods of carbonate precipitation, points to discontinuous, intermittent deformation. The overall picture supports a series of dilational events which took place in a setting of regional and/or local extension, consistent with near-surface brittle deformation. Perhaps these events correspond to deformation late in the rifting process, responsible for the uplift of the regional basement highs inferred to have postdated the deposition of the mass flow units found above basement at both sites (e.g., Shipboard Scientific Party, 1994a, b).
The culminating deformational event, at least in the samples examined here, corresponds to the brittle fracture and opening of the micrite-filled veins, possibly associated with the generation of the in situ breccias at Site 897. By analogy with neptunian veins observed in other extensional settings (Smart et al., 1987; Winterer et al., 1991), these features may be interpreted to reflect very late, near surface fracture of the exposed ultramafic basement (e.g., Lagabrielle and Auzende, 1982), possibly leading to gravitational instabilities: slumping, slope failure, and generation of landslides. It is not outside the realm of possibility that late dilational events, similar to those preserved by the micrite-filled veins at Site 897, were responsible for the ultimate fragmentation and collapse of the serpentinized peridotite now preserved in the breccia units at Site 899 (Gibson, Milliken, and Morgan, this volume), which may account for the relative dearth of micrite-filled veins at Site 899. In this scenario, the postdepositional veins sampled from the breccia units at Site 899 do not correlate directly with those at Site 897 (consider for example, the absence of the complex, aragonite-bearing veins at Site 899), despite the textural similarities at the two sites.
The mineralogical and morphologic progression identified here suggests that conditions of carbonate precipitation varied during filling of the veins. Conditions that favor the growth of one carbonate phase or habit at the expense of others have long been debated (e.g., Folk, 1974; Given and Wilkinson, 1985; Burton, 1993). Warm water temperatures, high Mg/Ca ratios, high salinity, and high carbonate concentrations have commonly been correlated with precipitation of aragonite; a hydrothermal origin is invoked to explain acicular aragonite observed in serpentinitic deposits from seamounts in the Mariana forearc setting (Lagabrielle et al. 1992). Chemical and thermal conditions associated with fibrous, botryoidal calcite occurrences are similar (Folk, 1974). Some would argue that botryoidal calcite actually represents a replacement texture of acicular radiating aragonite (e.g., Ross, 1991). More equant or bladed sparry calcite has been correlated with cooler, deeper marine or meteoric settings, typically with low Mg, and carbonate concentrations (Folk, 1974; Burton and Walter, 1987).
In an accompanying chemical study of the carbonate veins examined here, Milliken and Morgan (this volume) explored the correlations between carbonate morphology and the trace-elemental and isotopic signatures. Curiously, analyses designed to highlight just such chemical and thermal trends among the vein-filling phases instead demonstrate a relatively uniform fluid source, consistent with relatively low temperatures (10°-20°C), apparently representing an Early Cretaceous seawater only slightly modified by interaction with serpentinized peridotite basement (Milliken and Morgan, this volume). It is possible that some of the earlier vein-filling carbonates have been largely replaced, in the process losing their distinctive chemical, thermal, or temporal signatures, and acquiring those of the ambient fluid at the time of replacement. Replacement, however, would tend to erase primary directional growth textures, yielding more uniform, equant calcite (e.g., Bathurst, 1975; Ross, 1991). This is evidently the case for the examples of relict aragonite, which have been replaced by clear, equant calcite with very uniform luminescence. Certain occurrences of botryoidal calcite also show a more equant calcite habit; the botryoidal form is only evident from concentric bands of inclusions. Generally, however, the radiating fibrous patterns and fine inclusion and luminescent zoning in the botryoids point to the preservation of primary mineralogy. There is little evidence that the bladed, sparry calcites have undergone any replacement, as the fine details of the crystal habit and luminescent zoning are extremely well preserved (e.g., Fig. 6). These observations confirm that most of the carbonate chemistries that we have documented accurately record primary precipitation conditions.
Perhaps the conditions controlling carbonate morphology and phase depended more on precipitation rate (e.g., Given and Wilkinson, 1985). Given the intermittent history of deformation in these rocks, the width and stability of open fractures may have varied over time, directly controlling the flux of carbonate-bearing fluids through the system, and indirectly controlling the precipitation rate. Tectonically induced fractures, activating preexisting planes of weakness in the rock, may have introduced an extensive system of open veins, through which flow rates were quite high; aragonite may have been the favored phase, followed by botryoidal, fibrous calcite. Progressive narrowing of the veins, by the growth of vein-filling phases unaccompanied by successive widening events, may have favored the growth of coarse, bladed calcite. Collapse and in situ brecciation of the rock, possibly accompanied by high fluid flux, is inferred to have been one of the latest stages of deformation, at least at Site 897; the introduction of micrite into the fractures may indicate a sudden increase in precipitation rates (Folk, 1974) associated with the injection of fluids and remobilized calcite into the fractures. These features were subsequently cemented by sparry calcite.