Formation of kyanite-quartz veins of the Alpe Sponda, Central Alps, Switzerland: implications for Al transport during regional metamorphism

In this study, we have investigated the formation of quartz-kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P-T conditions of the quartz-kyanite veins, based on conventional geothermometry (garnet-biotite, white mica solvus and quartz-kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 +/- 3 degrees C at 5.0 +/- 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reaction-path simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the vein-forming fluids mixed Al-Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz-kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium- to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection-dominanted fluid-rock systems in the upper and middle crust.