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° to 350°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 18O values around 10‰ and have large 18Oserpentine–magnetite (12‰), which confirms that the serpentinization event occurred at low temperature (<200°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° to 13°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 18O/16O 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.
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2Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe, 2 place Jussieu, F-75251 Paris cedex 05, France. email@example.com
3Laboratoire de Pétrologie, Département des Sciences de la Terre, Université de Nantes, 2 rue de la Houssiniére, F-44072 Nantes cedex 03, France.
4Laboratoire de Géodynamique Sous-Marine, CNRS Université Pierre et Marie Curie, BP 48, F-06230 Villefranche sur Mer, France.
Date of initial receipt: 17 January 1995
Date of acceptance: 3 November 1995
Reproduced online: 21 May 2004