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Replacement minerals in olivine record the evolution of hydrothermal alteration between 1600 and 2000 mbsf in the sheeted dike complex in Hole 504B. 1. Talc (+ magnetite) rim on olivine represents the earliest alteration. Talc probably crystallized during initial cooling of the dikes. 2. The partial breakdown of talc to "deweylite," a chaotic mixture of serpentine and Al-free stevensite, was facilitated by further cooling and a somewhat increased fluid:rock interaction in the dikes. 3. The presence of chlorite veins and the replacement of unaltered olivine cores, talc, and deweylite and of other silicates by chlorite suggest fracturing of the rocks during cooling (shrinkage cracks) and local influx of seawater into the dikes. 4. Late amphibole veins and locally extensive amphibole alteration indicate increasing temperature and the development of new sets of fractures, possibly due to the injection of fresh magma. Several generations of chlorite and amphibole veins are present in the dikes. Offset veins and the crack-seal texture within veins in the dikes suggest that the alteration cycle was probably repeated with the injection of each set of new dikes. Presently measured temperatures (195°C) at 2000 m depth in Hole 504B indicate that deweylite, which was previously considered a low-temperature mineral, can form well above its previously estimated crystallization temperature of 50°C.

Phyllosilicate Alteration of Olivine in the Lower Sheeted Dike Complex, Leg 140, Hole 504B