Calcite veins recovered from serpentinized peridotite and serpentinite breccia basement rocks at Sites 897 and 899 reveal a complex history of fracturing and carbonate precipitation in an extensional environment related to the rifting of the North Atlantic. Crosscutting and textural relationships among the veins suggest a progression of carbonate vein-filling events in open fractures, involving the growth of acicular aragonite, subsequently altered mostly to calcite, followed by the precipitation of botryoidal, fibrous calcite, and finally by highly zoned, sparry calcite. The morphological progression may arise from changes in carbonate precipitation rate, possibly associated with temporal variations in vein width. At Site 897 (and rarely at Site 899), this sequence is crosscut by micrite-filled veins resembling neptunian fractures, composed of remobilized calcite, oxides, and serpentinite fragments, which may be derived from the in situ brecciation of the wall rock. The vein complexes, and complicated cracking, resealing, and crosscutting textures, are probably the result of a series of discrete, brittle dilational events associated with the late stages of uplift and exposure of serpentinized peridotite basement at the seafloor. The final fracture event, producing the micrite-filled veins, may have resulted from gravitational collapse of the uplifted blocks, perhaps culminating locally in complete brecciation and subsequent mass flow.

Date of initial receipt: 5 December 1994
Date of acceptance: 19 July 1995

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