HR: 1340h
AN: T53C-1459    [Abstracts]
TI: Chemical Modeling of Metamorphic Vein Mineralization in the Vanoise Metabauxites (French Alps)
AU: * Verlaguet, A
EM: verlaguet@ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure 24 rue Lhomond, Paris, 75005 France
AU: Goffe, B
EM: goffe@ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure 24 rue Lhomond, Paris, 75005 France
AU: Brunet, F
EM: brunet@ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure 24 rue Lhomond, Paris, 75005 France
AB: The aim of this study is to determine the chemical processes that lead to crystallization and segregation of Al-bearing minerals in veins in the Vanoise metabauxites (Western Alps). The studied bauxitic lenses (10m per 1m) represent karstic deposits in a calcareous Dogger formation. The absence of carbonate minerals in the metabauxites indicates that the system remained closed to external fluid. These rocks were metamorphosed in the blueschist facies (370°C, 1.1 GPa) and they are slightly deformed. Several generations of synmetamorphic veins containing different Al-bearing minerals (chloritoid, cookeite, pyrophyllite, chlorite) were identified.
Comparing the modal composition of the metabauxites with their protolith (low-metamorphosed bauxite equivalent from the external Alps) shows that the volume of fluid released by dehydration reactions during prograde metamorphism equals 13-16 % of the initial rock volume. The evolution of the fluid composition and mineral solubility along the P-T path is modeled (SUPCRT92 calculations) in order to quantify the amount of dissolved and crystallized minerals in response to fluid-mineral equilibration.
The apex metamorphic minerals (chloritoid, cookeite, pyrophyllite) are formed by dehydration reactions, with a positive reaction volume (1.5-4 %). Thus veins of chloritoid, cookeite and pyrophyllite could be the result of metamorphic reactions. However, only one of the reaction products crystallized in veins (monomineralic generations) and the segregation mechanism is still poorly understood.
We recently showed (experiments in the Al₂O₃-SiO₂-H₂O ± K₂O systems, coupled to thermodynamic modeling) that in the course of its re-equilibration with minerals (reaction path) the fluid first becomes saturated with respect to low-soluble phases (e.g. aluminosilicates) before it reaches silica saturation. Therefore fluid-mineral equilibration is an efficient way to mobilize insoluble elements and could take part in Al-mineral segregation in veins. Mineral textures are studied by SEM (BSE and EDS analysis) to evaluate the relative contribution of (1) dehydrating fluid - mineral equilibration and (2) metamorphic reactions to vein mineralization.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: Fall Meeting 2005