Several conclusions can be drawn from this petrographic study of carbonate veins in serpentinized basement cores from Sites 897 and 899.
1. Veins demonstrate an extensive history of fracture and refracture, consistent with local dilation of the basement rocks in a rift-related extensional setting. The progression in vein types suggests the early formation of large, open fractures, which were sealed by various carbonate phases and subsequently refractured during renewed deformational activity. Fracture orientations appear to be largely controlled by local stress variations, perhaps associated with preexisting basement fabric, rather than regional tectonic stress conditions.
2. The last apparent fracture and vein-filling event, most pronounced at Site 897, resulted in the development of crosscutting micrite-filled veins, similar in character to neptunian veins associated with dilation and fragmentation of rocks, and sedimentary influx in a near-seafloor environment. These features appear to correlate with in situ brecciation of the rocks, which may have introduced much of the internal sediment. The micrite-filled veins are interpreted to reflect late-stage gravitational collapse of the basement rock subsequent to uplift.
3. Petrographic observations reveal a temporal progression in carbonate phase and morphology, from precipitation of aragonite in large, open fractures, followed by fibrous, botryoidal calcite, and finally by coarse, bladed sparry calcite. There is little evidence to support replacement of most of these phases. Variations in carbonate morphology, without corresponding variations in calcite chemistry, suggest that precipitation conditions and rates in the fractures may have been controlled by fluid flow rates through the veins.