Fluid-induced mineral composition adjustments during exhumation: the case of Alpine stilbite
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Stilbite is locally present as a very late mineral on fractures and fissures of granitic basement in the Central Swiss Alps. Stilbite samples from the Gotthard rail base tunnel provide evidence that they originally formed as a K-absent variety at depth. However, all stilbite samples from surface outcrops above the tunnel display significant potassium concentrations. Interestingly, water from fractures in the tunnel (at 50 °C) is oversaturated with respect to stilbite and essentially potassium-free whereas waters from high-Alpine brooks above the tunnel (and at other high-Alpine areas) have unusually high K/Na ratios. The data suggest that stilbite that may actively form on fissures at tunnel level as a K-absent variety by precipitation from water. Older stilbite that originally formed as coatings on fracture walls was gradually exhumed and uplifted and finally reached the today’s erosion surface about 2,000 m above the tunnel. However, the stilbite reaches the erosion surface as a K-rich variety as a result of interaction of the original low-K stilbite with surface water and near-surface groundwater. This leads to the conclusion that minerals once formed at depth may significantly change their composition once they reach the ground water zone on their way to the erosion surface. In the case of the stilbite, if surface outcrops would have been the only source of samples and data, the K-rich composition could have been mistaken for the composition of the mineral when it formed, which is not the case. Late-stage compositional readjustments may be difficult to discern in samples from surface outcrops. The provided data show that original mineral compositions may be adjusted by late-stage water–rock interaction in a highly selective way.
KeywordsStilbite Petrology Fluid–rock interaction Zeolites Alpine minerals
AlpTransit Gotthard AG is thanked for providing access to the tunnel and providing water samples. We would like to thank P. Amacher who provided high-quality mineral specimens from the Gotthard NEAT tunnel. We thank B. Hoffmann and P. Vollenweider from the Natural History Museum Bern for access to the NMBE mineral collection. Special thanks to the Friedrich Rinne foundation for the financial support. We thank three anonymous reviewers for their detailed and constructive comments and C. Ballhaus for his editorial efforts and the editorial handling of the paper.
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