Ocean Drilling Program, United States

Peridotites from the boundary between the Atlantic Ocean crust and the West Iberia continental margin (west of Portugal) were drilled during Leg 149 in Holes 897C, 897D, and 899B. These peridotites have been serpentinized intensively with more than 90% of the primary phases being essentially lizardite. No talc or antigorite were detected. During an early episode of hydrothermal interaction (500 degrees to 350 degrees C), rare tremolites and chlorites formed after pyroxenes and spinel. Amphiboles are undeformed except in narrow shear zones. Serpentinization cannot be linked with this high-temperature hydrous event. Like the serpentines drilled 100 km to the north at Hole 637A, Leg 103, the great majority of serpentines from the Leg 149 peridotites have delta (super 18) O values around 10% and have large Delta (super 18) O (sub serpentine-magnetite) (12 per mil), which confirms that the serpentinization event occurred at low temperature (<200 degrees C) as a consequence of the introduction of a large amount of seawater. Calcite, which fills veins and cracks in the peridotites, precipitated from seawater at very low temperatures (19 degrees to 13 degrees C).We think the lack of deep-seated serpentinization mineralogical records (antigorite and talc) that would have formed a deep low-velocity zone (shown by seismic studies at the roof of the mantle peridotites at 5 to 7 km below the top of the sediment-free basement) may be explained by retrogression of antigorite and talc to low-temperature lizardite. Another explanation is the possibility that Leg 149 peridotites do not record a high-temperature serpentinization episode because serpentinization in this deep zone was not possible. However, the complete absence of antigorite and talc in the Hole 899B peridotites (in which the low-temperature serpentines overprinted the peridotites much less intensively) and (super 18) O/ (super 16) O ratios of the lizardites do not support the first possibility unless a complete dissolution-precipitation process affected all the antigorite and the talc. The second possibility suggests that the deep low-velocity zone would not be made of serpentines.