HR: 0800h
AN: V21A-0591    [Abstracts]
TI: Transient Chemical Equilibration Between Dehydration Fluid and Host Rock: an Efficient way to Mobilize Insoluble Elements
AU: * Verlaguet, A
EM: verlaguet@geologie.ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France, Metropolitan
AU: Brunet, F
EM: brunet@geologie.ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France, Metropolitan
AU: Goffe, B
EM: goffe@geologie.ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France, Metropolitan
AB: During prograde metamorphism, the fluid released by dehydration reactions is out of equilibrium with respect to the host rock. On its way to chemical equilibration with the rock, the released fluid will get saturated with respect to successive mineral phases (thus likely to crystallize) as predicted by reaction path modeling (Helgeson 1968, 1979). In order to quantify to which extent elements can be mobilized during fluid-rock equilibration and stored in secondary crystallizations, we coupled thermodynamic modeling of reaction path with an experimental study. We carried out closed system isothermal experiments on two simple chemical systems (Al$_{2}$O$_{3}$-SiO$_{2}$-H$_{2}$O and K$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$-H$_{2}$O): ground natural minerals (quartz, kyanite $\pm$ muscovite) and pure water were encapsulated and placed in an Internally Heated Pressure Vessel, at 7 kbar and $350\deg$C or $550\deg$C. We used a tube-in-tube set-up: initial minerals were confined into an inner capsule, which was perforated so that only fluid could circulate towards the external tube. In the external tube, we observed secondary crystallization of: at $350\deg$C, diaspore, kaolinite and pyrophyllite in the ASH system and diaspore, kaolinite and muscovite in the KASH system; at $550\deg$C, kyanite and quartz in the ASH system and Al-rich phase and muscovite in the KASH system. The nature and amount of these secondary phases are in agreement with the sequence predicted by reaction path calculations. In view of its very low solubility in pure water, aluminum was often considered as an immobile element. However, the volume of Al-rich crystallizations in these experiments shows that Al was strongly mobilized during transient fluid-mineral equilibration. Calculated amount of aluminum that crystallizes along the reaction path is 1000 times greater than the aqueous Al concentration in the fluid itself. Thus in natural rocks, transient equilibration between dehydration fluid and host rock is a potential efficient mechanism for mobilizing insoluble elements such as Al. This amount of Al potentially mobilized could contribute to aluminosilicate crystallization in metamorphic veins. Fluid-rock ratios calculated from the abundance of aluminosilicates in veins, considering Al as immobile, should take into account this potential mobilization. Moreover, mass transfer in the experimental closed system was probably largely effected by diffusion. Diffusion thus deserves consideration as a potential mass transfer mechanism in natural rocks. Finally, the tube-in-tube set-up is a way to explore reaction path beyond the P-T range of thermodynamic data application.
DE: 3630 Experimental mineralogy and petrology
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting