Contrasting serpentinization processes in the eastern Central Alps |
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| Authors: | Dorothee J M Burkhard James R O'Neil |
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| Institution: | (1) Mineralogical and Petrological Institute, University of Heidelberg, D-6900 Heidelberg, Federal Republic of Germany;(2) US Geological Survey, 94025 Menlo Park, CA, USA;(3) Present address: Department of Geology, University of Melbourne, 3052 Parkville, Victoria, Australia;(4) Present address: Department of Geological Sciences, University of Michigan, 48109-1063 Ann Arbor, MI, USA |
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| Abstract: | Stable isotope compositions have been determined for serpentinites from between Davos (Arosa-Platta nappe, Switzerland) and the Valmalenco (Italy).  D and
18O 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
18O value of 13 providing evidence of high W/R ratios and low formation temperature of lizardite-chrysotile in this area. In contrast, relatively high D values (–42 to –34) and low
18O 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 (D=-36) indicates that the metasomatic event also took place in the presence of ocean water. Lower D 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. |
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