Contrasting serpentinization processes in the eastern Central Alps

Stable isotope compositions have been determined for serpentinites from between Davos (Arosa-Platta nappe, Switzerland) and the Valmalenco (Italy). δD and δ 18 O values (−120 to −60 and 6–10‰, respectively) in the Arosa-Platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water. One sample of chrysotile has a δ 18 O value of 13‰ providing evidence of high W/R ratios and low formation temperature of lizardite-chrysotile in this area. In contrast, relatively high δD values (−42 to −34‰) and low δ 18 O values (4.4–7.4‰) for serpentine in the eastern part of the Valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water. The antigorite in the Valmalenco is the first reported example of continental antigorite with an ocean water signature. An amphibole sample from a metasomatically overprinted contact zone to metasediments ( δD =-36‰) indicates that the metasomatic event also took place in the presence of ocean water. Lower δD values (−93 to −60‰) of serpentines in the western part of the Valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism. Low water/rock ratios during regional metamorphism (and metasomatism) have to be assumed for both regions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47349/1/410_2004_Article_BF00371940.pd